Move iOS and macOS implementations into new apple module (#3756)

Move iOS and macOS implementations to a shared folder called `apple`, to allow
us to reduce the code-duplication between these platforms in the future.

The folder structure is now:
- `src/platform_impl/apple/`
  - `appkit/`
  - `uikit/`
  - `example_shared_file.rs`
  - `mod.rs`

* Add preliminary support for tvOS, watchOS and visionOS
* Reduce duplication in Cargo.toml when specifying dependencies

src/platform_impl/apple/uikit/app_delegate.rs

@@ -0,0 +1,60 @@
+use objc2::{declare_class, mutability, ClassType, DeclaredClass};
+use objc2_foundation::{MainThreadMarker, NSObject};
+use objc2_ui_kit::UIApplication;
+
+use super::app_state::{self, send_occluded_event_for_all_windows, EventWrapper};
+use crate::event::Event;
+
+declare_class!(
+    pub struct AppDelegate;
+
+    unsafe impl ClassType for AppDelegate {
+        type Super = NSObject;
+        type Mutability = mutability::InteriorMutable;
+        const NAME: &'static str = "WinitApplicationDelegate";
+    }
+
+    impl DeclaredClass for AppDelegate {}
+
+    // UIApplicationDelegate protocol
+    unsafe impl AppDelegate {
+        #[method(application:didFinishLaunchingWithOptions:)]
+        fn did_finish_launching(&self, _application: &UIApplication, _: *mut NSObject) -> bool {
+            app_state::did_finish_launching(MainThreadMarker::new().unwrap());
+            true
+        }
+
+        #[method(applicationDidBecomeActive:)]
+        fn did_become_active(&self, _application: &UIApplication) {
+            let mtm = MainThreadMarker::new().unwrap();
+            app_state::handle_nonuser_event(mtm, EventWrapper::StaticEvent(Event::Resumed))
+        }
+
+        #[method(applicationWillResignActive:)]
+        fn will_resign_active(&self, _application: &UIApplication) {
+            let mtm = MainThreadMarker::new().unwrap();
+            app_state::handle_nonuser_event(mtm, EventWrapper::StaticEvent(Event::Suspended))
+        }
+
+        #[method(applicationWillEnterForeground:)]
+        fn will_enter_foreground(&self, application: &UIApplication) {
+            send_occluded_event_for_all_windows(application, false);
+        }
+
+        #[method(applicationDidEnterBackground:)]
+        fn did_enter_background(&self, application: &UIApplication) {
+            send_occluded_event_for_all_windows(application, true);
+        }
+
+        #[method(applicationWillTerminate:)]
+        fn will_terminate(&self, application: &UIApplication) {
+            app_state::terminated(application);
+        }
+
+        #[method(applicationDidReceiveMemoryWarning:)]
+        fn did_receive_memory_warning(&self, _application: &UIApplication) {
+            let mtm = MainThreadMarker::new().unwrap();
+            app_state::handle_nonuser_event(mtm, EventWrapper::StaticEvent(Event::MemoryWarning))
+        }
+    }
+);

src/platform_impl/apple/uikit/app_state.rs

@@ -0,0 +1,919 @@
+#![deny(unused_results)]
+
+use std::cell::{RefCell, RefMut};
+use std::collections::HashSet;
+use std::os::raw::c_void;
+use std::sync::{Arc, Mutex, OnceLock};
+use std::time::Instant;
+use std::{fmt, mem, ptr};
+
+use core_foundation::base::CFRelease;
+use core_foundation::date::CFAbsoluteTimeGetCurrent;
+use core_foundation::runloop::{
+    kCFRunLoopCommonModes, CFRunLoopAddTimer, CFRunLoopGetMain, CFRunLoopRef, CFRunLoopTimerCreate,
+    CFRunLoopTimerInvalidate, CFRunLoopTimerRef, CFRunLoopTimerSetNextFireDate,
+};
+use objc2::rc::Retained;
+use objc2::runtime::AnyObject;
+use objc2::{msg_send, sel};
+use objc2_foundation::{
+    CGRect, CGSize, MainThreadMarker, NSInteger, NSObjectProtocol, NSOperatingSystemVersion,
+    NSProcessInfo,
+};
+use objc2_ui_kit::{UIApplication, UICoordinateSpace, UIView, UIWindow};
+
+use super::window::WinitUIWindow;
+use crate::dpi::PhysicalSize;
+use crate::event::{Event, InnerSizeWriter, StartCause, WindowEvent};
+use crate::event_loop::{ActiveEventLoop as RootActiveEventLoop, ControlFlow};
+use crate::window::WindowId as RootWindowId;
+
+macro_rules! bug {
+    ($($msg:tt)*) => {
+        panic!("winit iOS bug, file an issue: {}", format!($($msg)*))
+    };
+}
+
+macro_rules! bug_assert {
+    ($test:expr, $($msg:tt)*) => {
+        assert!($test, "winit iOS bug, file an issue: {}", format!($($msg)*))
+    };
+}
+
+pub(crate) struct EventLoopHandler {
+    #[allow(clippy::type_complexity)]
+    pub(crate) handler: Box<dyn FnMut(Event, &RootActiveEventLoop)>,
+    pub(crate) event_loop: RootActiveEventLoop,
+}
+
+impl fmt::Debug for EventLoopHandler {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("EventLoopHandler")
+            .field("handler", &"...")
+            .field("event_loop", &self.event_loop)
+            .finish()
+    }
+}
+
+impl EventLoopHandler {
+    fn handle_event(&mut self, event: Event) {
+        (self.handler)(event, &self.event_loop)
+    }
+}
+
+#[derive(Debug)]
+pub(crate) enum EventWrapper {
+    StaticEvent(Event),
+    ScaleFactorChanged(ScaleFactorChanged),
+}
+
+#[derive(Debug)]
+pub struct ScaleFactorChanged {
+    pub(super) window: Retained<WinitUIWindow>,
+    pub(super) suggested_size: PhysicalSize<u32>,
+    pub(super) scale_factor: f64,
+}
+
+enum UserCallbackTransitionResult<'a> {
+    Success {
+        handler: EventLoopHandler,
+        active_control_flow: ControlFlow,
+        processing_redraws: bool,
+    },
+    ReentrancyPrevented {
+        queued_events: &'a mut Vec<EventWrapper>,
+    },
+}
+
+impl Event {
+    fn is_redraw(&self) -> bool {
+        matches!(self, Event::WindowEvent { event: WindowEvent::RedrawRequested, .. })
+    }
+}
+
+// this is the state machine for the app lifecycle
+#[derive(Debug)]
+#[must_use = "dropping `AppStateImpl` without inspecting it is probably a bug"]
+enum AppStateImpl {
+    NotLaunched {
+        queued_windows: Vec<Retained<WinitUIWindow>>,
+        queued_events: Vec<EventWrapper>,
+        queued_gpu_redraws: HashSet<Retained<WinitUIWindow>>,
+    },
+    Launching {
+        queued_windows: Vec<Retained<WinitUIWindow>>,
+        queued_events: Vec<EventWrapper>,
+        queued_handler: EventLoopHandler,
+        queued_gpu_redraws: HashSet<Retained<WinitUIWindow>>,
+    },
+    ProcessingEvents {
+        handler: EventLoopHandler,
+        queued_gpu_redraws: HashSet<Retained<WinitUIWindow>>,
+        active_control_flow: ControlFlow,
+    },
+    // special state to deal with reentrancy and prevent mutable aliasing.
+    InUserCallback {
+        queued_events: Vec<EventWrapper>,
+        queued_gpu_redraws: HashSet<Retained<WinitUIWindow>>,
+    },
+    ProcessingRedraws {
+        handler: EventLoopHandler,
+        active_control_flow: ControlFlow,
+    },
+    Waiting {
+        waiting_handler: EventLoopHandler,
+        start: Instant,
+    },
+    PollFinished {
+        waiting_handler: EventLoopHandler,
+    },
+    Terminated,
+}
+
+pub(crate) struct AppState {
+    // This should never be `None`, except for briefly during a state transition.
+    app_state: Option<AppStateImpl>,
+    control_flow: ControlFlow,
+    waker: EventLoopWaker,
+}
+
+impl AppState {
+    pub(crate) fn get_mut(_mtm: MainThreadMarker) -> RefMut<'static, AppState> {
+        // basically everything in UIKit requires the main thread, so it's pointless to use the
+        // std::sync APIs.
+        // must be mut because plain `static` requires `Sync`
+        static mut APP_STATE: RefCell<Option<AppState>> = RefCell::new(None);
+
+        let mut guard = unsafe { APP_STATE.borrow_mut() };
+        if guard.is_none() {
+            #[inline(never)]
+            #[cold]
+            fn init_guard(guard: &mut RefMut<'static, Option<AppState>>) {
+                let waker = EventLoopWaker::new(unsafe { CFRunLoopGetMain() });
+                **guard = Some(AppState {
+                    app_state: Some(AppStateImpl::NotLaunched {
+                        queued_windows: Vec::new(),
+                        queued_events: Vec::new(),
+                        queued_gpu_redraws: HashSet::new(),
+                    }),
+                    control_flow: ControlFlow::default(),
+                    waker,
+                });
+            }
+            init_guard(&mut guard);
+        }
+        RefMut::map(guard, |state| state.as_mut().unwrap())
+    }
+
+    fn state(&self) -> &AppStateImpl {
+        match &self.app_state {
+            Some(ref state) => state,
+            None => bug!("`AppState` previously failed a state transition"),
+        }
+    }
+
+    fn state_mut(&mut self) -> &mut AppStateImpl {
+        match &mut self.app_state {
+            Some(ref mut state) => state,
+            None => bug!("`AppState` previously failed a state transition"),
+        }
+    }
+
+    fn take_state(&mut self) -> AppStateImpl {
+        match self.app_state.take() {
+            Some(state) => state,
+            None => bug!("`AppState` previously failed a state transition"),
+        }
+    }
+
+    fn set_state(&mut self, new_state: AppStateImpl) {
+        bug_assert!(
+            self.app_state.is_none(),
+            "attempted to set an `AppState` without calling `take_state` first {:?}",
+            self.app_state
+        );
+        self.app_state = Some(new_state)
+    }
+
+    fn replace_state(&mut self, new_state: AppStateImpl) -> AppStateImpl {
+        match &mut self.app_state {
+            Some(ref mut state) => mem::replace(state, new_state),
+            None => bug!("`AppState` previously failed a state transition"),
+        }
+    }
+
+    fn has_launched(&self) -> bool {
+        !matches!(self.state(), AppStateImpl::NotLaunched { .. } | AppStateImpl::Launching { .. })
+    }
+
+    fn has_terminated(&self) -> bool {
+        matches!(self.state(), AppStateImpl::Terminated)
+    }
+
+    fn will_launch_transition(&mut self, queued_handler: EventLoopHandler) {
+        let (queued_windows, queued_events, queued_gpu_redraws) = match self.take_state() {
+            AppStateImpl::NotLaunched { queued_windows, queued_events, queued_gpu_redraws } => {
+                (queued_windows, queued_events, queued_gpu_redraws)
+            },
+            s => bug!("unexpected state {:?}", s),
+        };
+        self.set_state(AppStateImpl::Launching {
+            queued_windows,
+            queued_events,
+            queued_handler,
+            queued_gpu_redraws,
+        });
+    }
+
+    fn did_finish_launching_transition(
+        &mut self,
+    ) -> (Vec<Retained<WinitUIWindow>>, Vec<EventWrapper>) {
+        let (windows, events, handler, queued_gpu_redraws) = match self.take_state() {
+            AppStateImpl::Launching {
+                queued_windows,
+                queued_events,
+                queued_handler,
+                queued_gpu_redraws,
+            } => (queued_windows, queued_events, queued_handler, queued_gpu_redraws),
+            s => bug!("unexpected state {:?}", s),
+        };
+        self.set_state(AppStateImpl::ProcessingEvents {
+            handler,
+            active_control_flow: self.control_flow,
+            queued_gpu_redraws,
+        });
+        (windows, events)
+    }
+
+    fn wakeup_transition(&mut self) -> Option<EventWrapper> {
+        // before `AppState::did_finish_launching` is called, pretend there is no running
+        // event loop.
+        if !self.has_launched() || self.has_terminated() {
+            return None;
+        }
+
+        let (handler, event) = match (self.control_flow, self.take_state()) {
+            (ControlFlow::Poll, AppStateImpl::PollFinished { waiting_handler }) => {
+                (waiting_handler, EventWrapper::StaticEvent(Event::NewEvents(StartCause::Poll)))
+            },
+            (ControlFlow::Wait, AppStateImpl::Waiting { waiting_handler, start }) => (
+                waiting_handler,
+                EventWrapper::StaticEvent(Event::NewEvents(StartCause::WaitCancelled {
+                    start,
+                    requested_resume: None,
+                })),
+            ),
+            (
+                ControlFlow::WaitUntil(requested_resume),
+                AppStateImpl::Waiting { waiting_handler, start },
+            ) => {
+                let event = if Instant::now() >= requested_resume {
+                    EventWrapper::StaticEvent(Event::NewEvents(StartCause::ResumeTimeReached {
+                        start,
+                        requested_resume,
+                    }))
+                } else {
+                    EventWrapper::StaticEvent(Event::NewEvents(StartCause::WaitCancelled {
+                        start,
+                        requested_resume: Some(requested_resume),
+                    }))
+                };
+                (waiting_handler, event)
+            },
+            s => bug!("`EventHandler` unexpectedly woke up {:?}", s),
+        };
+
+        self.set_state(AppStateImpl::ProcessingEvents {
+            handler,
+            queued_gpu_redraws: Default::default(),
+            active_control_flow: self.control_flow,
+        });
+        Some(event)
+    }
+
+    fn try_user_callback_transition(&mut self) -> UserCallbackTransitionResult<'_> {
+        // If we're not able to process an event due to recursion or `Init` not having been sent out
+        // yet, then queue the events up.
+        match self.state_mut() {
+            &mut AppStateImpl::Launching { ref mut queued_events, .. }
+            | &mut AppStateImpl::NotLaunched { ref mut queued_events, .. }
+            | &mut AppStateImpl::InUserCallback { ref mut queued_events, .. } => {
+                // A lifetime cast: early returns are not currently handled well with NLL, but
+                // polonius handles them well. This transmute is a safe workaround.
+                return unsafe {
+                    mem::transmute::<
+                        UserCallbackTransitionResult<'_>,
+                        UserCallbackTransitionResult<'_>,
+                    >(UserCallbackTransitionResult::ReentrancyPrevented {
+                        queued_events,
+                    })
+                };
+            },
+
+            &mut AppStateImpl::ProcessingEvents { .. }
+            | &mut AppStateImpl::ProcessingRedraws { .. } => {},
+
+            s @ &mut AppStateImpl::PollFinished { .. }
+            | s @ &mut AppStateImpl::Waiting { .. }
+            | s @ &mut AppStateImpl::Terminated => {
+                bug!("unexpected attempted to process an event {:?}", s)
+            },
+        }
+
+        let (handler, queued_gpu_redraws, active_control_flow, processing_redraws) = match self
+            .take_state()
+        {
+            AppStateImpl::Launching { .. }
+            | AppStateImpl::NotLaunched { .. }
+            | AppStateImpl::InUserCallback { .. } => unreachable!(),
+            AppStateImpl::ProcessingEvents { handler, queued_gpu_redraws, active_control_flow } => {
+                (handler, queued_gpu_redraws, active_control_flow, false)
+            },
+            AppStateImpl::ProcessingRedraws { handler, active_control_flow } => {
+                (handler, Default::default(), active_control_flow, true)
+            },
+            AppStateImpl::PollFinished { .. }
+            | AppStateImpl::Waiting { .. }
+            | AppStateImpl::Terminated => unreachable!(),
+        };
+        self.set_state(AppStateImpl::InUserCallback {
+            queued_events: Vec::new(),
+            queued_gpu_redraws,
+        });
+        UserCallbackTransitionResult::Success { handler, active_control_flow, processing_redraws }
+    }
+
+    fn main_events_cleared_transition(&mut self) -> HashSet<Retained<WinitUIWindow>> {
+        let (handler, queued_gpu_redraws, active_control_flow) = match self.take_state() {
+            AppStateImpl::ProcessingEvents { handler, queued_gpu_redraws, active_control_flow } => {
+                (handler, queued_gpu_redraws, active_control_flow)
+            },
+            s => bug!("unexpected state {:?}", s),
+        };
+        self.set_state(AppStateImpl::ProcessingRedraws { handler, active_control_flow });
+        queued_gpu_redraws
+    }
+
+    fn events_cleared_transition(&mut self) {
+        if !self.has_launched() || self.has_terminated() {
+            return;
+        }
+        let (waiting_handler, old) = match self.take_state() {
+            AppStateImpl::ProcessingRedraws { handler, active_control_flow } => {
+                (handler, active_control_flow)
+            },
+            s => bug!("unexpected state {:?}", s),
+        };
+
+        let new = self.control_flow;
+        match (old, new) {
+            (ControlFlow::Wait, ControlFlow::Wait) => {
+                let start = Instant::now();
+                self.set_state(AppStateImpl::Waiting { waiting_handler, start });
+            },
+            (ControlFlow::WaitUntil(old_instant), ControlFlow::WaitUntil(new_instant))
+                if old_instant == new_instant =>
+            {
+                let start = Instant::now();
+                self.set_state(AppStateImpl::Waiting { waiting_handler, start });
+            },
+            (_, ControlFlow::Wait) => {
+                let start = Instant::now();
+                self.set_state(AppStateImpl::Waiting { waiting_handler, start });
+                self.waker.stop()
+            },
+            (_, ControlFlow::WaitUntil(new_instant)) => {
+                let start = Instant::now();
+                self.set_state(AppStateImpl::Waiting { waiting_handler, start });
+                self.waker.start_at(new_instant)
+            },
+            // Unlike on macOS, handle Poll to Poll transition here to call the waker
+            (_, ControlFlow::Poll) => {
+                self.set_state(AppStateImpl::PollFinished { waiting_handler });
+                self.waker.start()
+            },
+        }
+    }
+
+    fn terminated_transition(&mut self) -> EventLoopHandler {
+        match self.replace_state(AppStateImpl::Terminated) {
+            AppStateImpl::ProcessingEvents { handler, .. } => handler,
+            s => bug!("`LoopExiting` happened while not processing events {:?}", s),
+        }
+    }
+
+    pub(crate) fn set_control_flow(&mut self, control_flow: ControlFlow) {
+        self.control_flow = control_flow;
+    }
+
+    pub(crate) fn control_flow(&self) -> ControlFlow {
+        self.control_flow
+    }
+}
+
+pub(crate) fn set_key_window(mtm: MainThreadMarker, window: &Retained<WinitUIWindow>) {
+    let mut this = AppState::get_mut(mtm);
+    match this.state_mut() {
+        &mut AppStateImpl::NotLaunched { ref mut queued_windows, .. } => {
+            return queued_windows.push(window.clone())
+        },
+        &mut AppStateImpl::ProcessingEvents { .. }
+        | &mut AppStateImpl::InUserCallback { .. }
+        | &mut AppStateImpl::ProcessingRedraws { .. } => {},
+        s @ &mut AppStateImpl::Launching { .. }
+        | s @ &mut AppStateImpl::Waiting { .. }
+        | s @ &mut AppStateImpl::PollFinished { .. } => bug!("unexpected state {:?}", s),
+        &mut AppStateImpl::Terminated => {
+            panic!("Attempt to create a `Window` after the app has terminated")
+        },
+    }
+    drop(this);
+    window.makeKeyAndVisible();
+}
+
+pub(crate) fn queue_gl_or_metal_redraw(mtm: MainThreadMarker, window: Retained<WinitUIWindow>) {
+    let mut this = AppState::get_mut(mtm);
+    match this.state_mut() {
+        &mut AppStateImpl::NotLaunched { ref mut queued_gpu_redraws, .. }
+        | &mut AppStateImpl::Launching { ref mut queued_gpu_redraws, .. }
+        | &mut AppStateImpl::ProcessingEvents { ref mut queued_gpu_redraws, .. }
+        | &mut AppStateImpl::InUserCallback { ref mut queued_gpu_redraws, .. } => {
+            let _ = queued_gpu_redraws.insert(window);
+        },
+        s @ &mut AppStateImpl::ProcessingRedraws { .. }
+        | s @ &mut AppStateImpl::Waiting { .. }
+        | s @ &mut AppStateImpl::PollFinished { .. } => bug!("unexpected state {:?}", s),
+        &mut AppStateImpl::Terminated => {
+            panic!("Attempt to create a `Window` after the app has terminated")
+        },
+    }
+}
+
+pub(crate) fn will_launch(mtm: MainThreadMarker, queued_handler: EventLoopHandler) {
+    AppState::get_mut(mtm).will_launch_transition(queued_handler)
+}
+
+pub fn did_finish_launching(mtm: MainThreadMarker) {
+    let mut this = AppState::get_mut(mtm);
+    let windows = match this.state_mut() {
+        AppStateImpl::Launching { queued_windows, .. } => mem::take(queued_windows),
+        s => bug!("unexpected state {:?}", s),
+    };
+
+    this.waker.start();
+
+    // have to drop RefMut because the window setup code below can trigger new events
+    drop(this);
+
+    for window in windows {
+        // Do a little screen dance here to account for windows being created before
+        // `UIApplicationMain` is called. This fixes visual issues such as being
+        // offcenter and sized incorrectly. Additionally, to fix orientation issues, we
+        // gotta reset the `rootViewController`.
+        //
+        // relevant iOS log:
+        // ```
+        // [ApplicationLifecycle] Windows were created before application initialization
+        // completed. This may result in incorrect visual appearance.
+        // ```
+        let screen = window.screen();
+        let _: () = unsafe { msg_send![&window, setScreen: ptr::null::<AnyObject>()] };
+        window.setScreen(&screen);
+
+        let controller = window.rootViewController();
+        window.setRootViewController(None);
+        window.setRootViewController(controller.as_deref());
+
+        window.makeKeyAndVisible();
+    }
+
+    let (windows, events) = AppState::get_mut(mtm).did_finish_launching_transition();
+
+    let events = std::iter::once(EventWrapper::StaticEvent(Event::NewEvents(StartCause::Init)))
+        .chain(events);
+    handle_nonuser_events(mtm, events);
+
+    // the above window dance hack, could possibly trigger new windows to be created.
+    // we can just set those windows up normally, as they were created after didFinishLaunching
+    for window in windows {
+        window.makeKeyAndVisible();
+    }
+}
+
+// AppState::did_finish_launching handles the special transition `Init`
+pub fn handle_wakeup_transition(mtm: MainThreadMarker) {
+    let mut this = AppState::get_mut(mtm);
+    let wakeup_event = match this.wakeup_transition() {
+        None => return,
+        Some(wakeup_event) => wakeup_event,
+    };
+    drop(this);
+
+    handle_nonuser_event(mtm, wakeup_event)
+}
+
+pub(crate) fn handle_nonuser_event(mtm: MainThreadMarker, event: EventWrapper) {
+    handle_nonuser_events(mtm, std::iter::once(event))
+}
+
+pub(crate) fn handle_nonuser_events<I: IntoIterator<Item = EventWrapper>>(
+    mtm: MainThreadMarker,
+    events: I,
+) {
+    let mut this = AppState::get_mut(mtm);
+    if this.has_terminated() {
+        return;
+    }
+
+    let (mut handler, active_control_flow, processing_redraws) =
+        match this.try_user_callback_transition() {
+            UserCallbackTransitionResult::ReentrancyPrevented { queued_events } => {
+                queued_events.extend(events);
+                return;
+            },
+            UserCallbackTransitionResult::Success {
+                handler,
+                active_control_flow,
+                processing_redraws,
+            } => (handler, active_control_flow, processing_redraws),
+        };
+    drop(this);
+
+    for wrapper in events {
+        match wrapper {
+            EventWrapper::StaticEvent(event) => {
+                if !processing_redraws && event.is_redraw() {
+                    tracing::info!("processing `RedrawRequested` during the main event loop");
+                } else if processing_redraws && !event.is_redraw() {
+                    tracing::warn!(
+                        "processing non `RedrawRequested` event after the main event loop: {:#?}",
+                        event
+                    );
+                }
+                handler.handle_event(event)
+            },
+            EventWrapper::ScaleFactorChanged(event) => handle_hidpi_proxy(&mut handler, event),
+        }
+    }
+
+    loop {
+        let mut this = AppState::get_mut(mtm);
+        let queued_events = match this.state_mut() {
+            &mut AppStateImpl::InUserCallback { ref mut queued_events, queued_gpu_redraws: _ } => {
+                mem::take(queued_events)
+            },
+            s => bug!("unexpected state {:?}", s),
+        };
+        if queued_events.is_empty() {
+            let queued_gpu_redraws = match this.take_state() {
+                AppStateImpl::InUserCallback { queued_events: _, queued_gpu_redraws } => {
+                    queued_gpu_redraws
+                },
+                _ => unreachable!(),
+            };
+            this.app_state = Some(if processing_redraws {
+                bug_assert!(
+                    queued_gpu_redraws.is_empty(),
+                    "redraw queued while processing redraws"
+                );
+                AppStateImpl::ProcessingRedraws { handler, active_control_flow }
+            } else {
+                AppStateImpl::ProcessingEvents { handler, queued_gpu_redraws, active_control_flow }
+            });
+            break;
+        }
+        drop(this);
+
+        for wrapper in queued_events {
+            match wrapper {
+                EventWrapper::StaticEvent(event) => {
+                    if !processing_redraws && event.is_redraw() {
+                        tracing::info!("processing `RedrawRequested` during the main event loop");
+                    } else if processing_redraws && !event.is_redraw() {
+                        tracing::warn!(
+                            "processing non-`RedrawRequested` event after the main event loop: \
+                             {:#?}",
+                            event
+                        );
+                    }
+                    handler.handle_event(event)
+                },
+                EventWrapper::ScaleFactorChanged(event) => handle_hidpi_proxy(&mut handler, event),
+            }
+        }
+    }
+}
+
+fn handle_user_events(mtm: MainThreadMarker) {
+    let mut this = AppState::get_mut(mtm);
+    let (mut handler, active_control_flow, processing_redraws) =
+        match this.try_user_callback_transition() {
+            UserCallbackTransitionResult::ReentrancyPrevented { .. } => {
+                bug!("unexpected attempted to process an event")
+            },
+            UserCallbackTransitionResult::Success {
+                handler,
+                active_control_flow,
+                processing_redraws,
+            } => (handler, active_control_flow, processing_redraws),
+        };
+    if processing_redraws {
+        bug!("user events attempted to be sent out while `ProcessingRedraws`");
+    }
+    drop(this);
+
+    handler.handle_event(Event::UserWakeUp);
+
+    loop {
+        let mut this = AppState::get_mut(mtm);
+        let queued_events = match this.state_mut() {
+            &mut AppStateImpl::InUserCallback { ref mut queued_events, queued_gpu_redraws: _ } => {
+                mem::take(queued_events)
+            },
+            s => bug!("unexpected state {:?}", s),
+        };
+        if queued_events.is_empty() {
+            let queued_gpu_redraws = match this.take_state() {
+                AppStateImpl::InUserCallback { queued_events: _, queued_gpu_redraws } => {
+                    queued_gpu_redraws
+                },
+                _ => unreachable!(),
+            };
+            this.app_state = Some(AppStateImpl::ProcessingEvents {
+                handler,
+                queued_gpu_redraws,
+                active_control_flow,
+            });
+            break;
+        }
+        drop(this);
+
+        for wrapper in queued_events {
+            match wrapper {
+                EventWrapper::StaticEvent(event) => handler.handle_event(event),
+                EventWrapper::ScaleFactorChanged(event) => handle_hidpi_proxy(&mut handler, event),
+            }
+        }
+
+        handler.handle_event(Event::UserWakeUp);
+    }
+}
+
+pub(crate) fn send_occluded_event_for_all_windows(application: &UIApplication, occluded: bool) {
+    let mtm = MainThreadMarker::from(application);
+
+    let mut events = Vec::new();
+    #[allow(deprecated)]
+    for window in application.windows().iter() {
+        if window.is_kind_of::<WinitUIWindow>() {
+            // SAFETY: We just checked that the window is a `winit` window
+            let window = unsafe {
+                let ptr: *const UIWindow = window;
+                let ptr: *const WinitUIWindow = ptr.cast();
+                &*ptr
+            };
+            events.push(EventWrapper::StaticEvent(Event::WindowEvent {
+                window_id: RootWindowId(window.id()),
+                event: WindowEvent::Occluded(occluded),
+            }));
+        }
+    }
+    handle_nonuser_events(mtm, events);
+}
+
+pub fn handle_main_events_cleared(mtm: MainThreadMarker) {
+    let mut this = AppState::get_mut(mtm);
+    if !this.has_launched() || this.has_terminated() {
+        return;
+    }
+    match this.state_mut() {
+        AppStateImpl::ProcessingEvents { .. } => {},
+        _ => bug!("`ProcessingRedraws` happened unexpectedly"),
+    };
+    drop(this);
+
+    handle_user_events(mtm);
+
+    let mut this = AppState::get_mut(mtm);
+    let redraw_events: Vec<EventWrapper> = this
+        .main_events_cleared_transition()
+        .into_iter()
+        .map(|window| {
+            EventWrapper::StaticEvent(Event::WindowEvent {
+                window_id: RootWindowId(window.id()),
+                event: WindowEvent::RedrawRequested,
+            })
+        })
+        .collect();
+    drop(this);
+
+    handle_nonuser_events(mtm, redraw_events);
+    handle_nonuser_event(mtm, EventWrapper::StaticEvent(Event::AboutToWait));
+}
+
+pub fn handle_events_cleared(mtm: MainThreadMarker) {
+    AppState::get_mut(mtm).events_cleared_transition();
+}
+
+pub(crate) fn terminated(application: &UIApplication) {
+    let mtm = MainThreadMarker::from(application);
+
+    let mut events = Vec::new();
+    #[allow(deprecated)]
+    for window in application.windows().iter() {
+        if window.is_kind_of::<WinitUIWindow>() {
+            // SAFETY: We just checked that the window is a `winit` window
+            let window = unsafe {
+                let ptr: *const UIWindow = window;
+                let ptr: *const WinitUIWindow = ptr.cast();
+                &*ptr
+            };
+            events.push(EventWrapper::StaticEvent(Event::WindowEvent {
+                window_id: RootWindowId(window.id()),
+                event: WindowEvent::Destroyed,
+            }));
+        }
+    }
+    handle_nonuser_events(mtm, events);
+
+    let mut this = AppState::get_mut(mtm);
+    let mut handler = this.terminated_transition();
+    drop(this);
+
+    handler.handle_event(Event::LoopExiting)
+}
+
+fn handle_hidpi_proxy(handler: &mut EventLoopHandler, event: ScaleFactorChanged) {
+    let ScaleFactorChanged { suggested_size, scale_factor, window } = event;
+    let new_inner_size = Arc::new(Mutex::new(suggested_size));
+    let event = Event::WindowEvent {
+        window_id: RootWindowId(window.id()),
+        event: WindowEvent::ScaleFactorChanged {
+            scale_factor,
+            inner_size_writer: InnerSizeWriter::new(Arc::downgrade(&new_inner_size)),
+        },
+    };
+    handler.handle_event(event);
+    let (view, screen_frame) = get_view_and_screen_frame(&window);
+    let physical_size = *new_inner_size.lock().unwrap();
+    drop(new_inner_size);
+    let logical_size = physical_size.to_logical(scale_factor);
+    let size = CGSize::new(logical_size.width, logical_size.height);
+    let new_frame: CGRect = CGRect::new(screen_frame.origin, size);
+    view.setFrame(new_frame);
+}
+
+fn get_view_and_screen_frame(window: &WinitUIWindow) -> (Retained<UIView>, CGRect) {
+    let view_controller = window.rootViewController().unwrap();
+    let view = view_controller.view().unwrap();
+    let bounds = window.bounds();
+    let screen = window.screen();
+    let screen_space = screen.coordinateSpace();
+    let screen_frame = window.convertRect_toCoordinateSpace(bounds, &screen_space);
+    (view, screen_frame)
+}
+
+struct EventLoopWaker {
+    timer: CFRunLoopTimerRef,
+}
+
+impl Drop for EventLoopWaker {
+    fn drop(&mut self) {
+        unsafe {
+            CFRunLoopTimerInvalidate(self.timer);
+            CFRelease(self.timer as _);
+        }
+    }
+}
+
+impl EventLoopWaker {
+    fn new(rl: CFRunLoopRef) -> EventLoopWaker {
+        extern "C" fn wakeup_main_loop(_timer: CFRunLoopTimerRef, _info: *mut c_void) {}
+        unsafe {
+            // Create a timer with a 0.1µs interval (1ns does not work) to mimic polling.
+            // It is initially setup with a first fire time really far into the
+            // future, but that gets changed to fire immediately in did_finish_launching
+            let timer = CFRunLoopTimerCreate(
+                ptr::null_mut(),
+                f64::MAX,
+                0.000_000_1,
+                0,
+                0,
+                wakeup_main_loop,
+                ptr::null_mut(),
+            );
+            CFRunLoopAddTimer(rl, timer, kCFRunLoopCommonModes);
+
+            EventLoopWaker { timer }
+        }
+    }
+
+    fn stop(&mut self) {
+        unsafe { CFRunLoopTimerSetNextFireDate(self.timer, f64::MAX) }
+    }
+
+    fn start(&mut self) {
+        unsafe { CFRunLoopTimerSetNextFireDate(self.timer, f64::MIN) }
+    }
+
+    fn start_at(&mut self, instant: Instant) {
+        let now = Instant::now();
+        if now >= instant {
+            self.start();
+        } else {
+            unsafe {
+                let current = CFAbsoluteTimeGetCurrent();
+                let duration = instant - now;
+                let fsecs =
+                    duration.subsec_nanos() as f64 / 1_000_000_000.0 + duration.as_secs() as f64;
+                CFRunLoopTimerSetNextFireDate(self.timer, current + fsecs)
+            }
+        }
+    }
+}
+
+macro_rules! os_capabilities {
+    (
+        $(
+            $(#[$attr:meta])*
+            $error_name:ident: $objc_call:literal,
+            $name:ident: $major:literal-$minor:literal
+        ),*
+        $(,)*
+    ) => {
+        #[derive(Clone, Debug)]
+        pub struct OSCapabilities {
+            $(
+                pub $name: bool,
+            )*
+
+            os_version: NSOperatingSystemVersion,
+        }
+
+        impl OSCapabilities {
+            fn from_os_version(os_version: NSOperatingSystemVersion) -> Self {
+                $(let $name = meets_requirements(os_version, $major, $minor);)*
+                Self { $($name,)* os_version, }
+            }
+        }
+
+        impl OSCapabilities {$(
+            $(#[$attr])*
+            pub fn $error_name(&self, extra_msg: &str) {
+                tracing::warn!(
+                    concat!("`", $objc_call, "` requires iOS {}.{}+. This device is running iOS {}.{}.{}. {}"),
+                    $major, $minor, self.os_version.majorVersion, self.os_version.minorVersion, self.os_version.patchVersion,
+                    extra_msg
+                )
+            }
+        )*}
+    };
+}
+
+os_capabilities! {
+    /// <https://developer.apple.com/documentation/uikit/uiview/2891103-safeareainsets?language=objc>
+    #[allow(unused)] // error message unused
+    safe_area_err_msg: "-[UIView safeAreaInsets]",
+    safe_area: 11-0,
+    /// <https://developer.apple.com/documentation/uikit/uiviewcontroller/2887509-setneedsupdateofhomeindicatoraut?language=objc>
+    home_indicator_hidden_err_msg: "-[UIViewController setNeedsUpdateOfHomeIndicatorAutoHidden]",
+    home_indicator_hidden: 11-0,
+    /// <https://developer.apple.com/documentation/uikit/uiviewcontroller/2887507-setneedsupdateofscreenedgesdefer?language=objc>
+    defer_system_gestures_err_msg: "-[UIViewController setNeedsUpdateOfScreenEdgesDeferringSystem]",
+    defer_system_gestures: 11-0,
+    /// <https://developer.apple.com/documentation/uikit/uiscreen/2806814-maximumframespersecond?language=objc>
+    maximum_frames_per_second_err_msg: "-[UIScreen maximumFramesPerSecond]",
+    maximum_frames_per_second: 10-3,
+    /// <https://developer.apple.com/documentation/uikit/uitouch/1618110-force?language=objc>
+    #[allow(unused)] // error message unused
+    force_touch_err_msg: "-[UITouch force]",
+    force_touch: 9-0,
+}
+
+fn meets_requirements(
+    version: NSOperatingSystemVersion,
+    required_major: NSInteger,
+    required_minor: NSInteger,
+) -> bool {
+    (version.majorVersion, version.minorVersion) >= (required_major, required_minor)
+}
+
+fn get_version() -> NSOperatingSystemVersion {
+    let process_info = NSProcessInfo::processInfo();
+    let atleast_ios_8 = process_info.respondsToSelector(sel!(operatingSystemVersion));
+    // Winit requires atleast iOS 8 because no one has put the time into supporting earlier os
+    // versions. Older iOS versions are increasingly difficult to test. For example, Xcode 11 does
+    // not support debugging on devices with an iOS version of less than 8. Another example, in
+    // order to use an iOS simulator older than iOS 8, you must download an older version of Xcode
+    // (<9), and at least Xcode 7 has been tested to not even run on macOS 10.15 - Xcode 8 might?
+    //
+    // The minimum required iOS version is likely to grow in the future.
+    assert!(atleast_ios_8, "`winit` requires iOS version 8 or greater");
+    process_info.operatingSystemVersion()
+}
+
+pub fn os_capabilities() -> OSCapabilities {
+    // Cache the version lookup for efficiency
+    static OS_CAPABILITIES: OnceLock<OSCapabilities> = OnceLock::new();
+    OS_CAPABILITIES.get_or_init(|| OSCapabilities::from_os_version(get_version())).clone()
+}

src/platform_impl/apple/uikit/event_loop.rs

@@ -0,0 +1,391 @@
+use std::collections::VecDeque;
+use std::ffi::{c_char, c_int, c_void};
+use std::marker::PhantomData;
+use std::ptr::{self, NonNull};
+use std::sync::atomic::{AtomicBool, Ordering as AtomicOrdering};
+use std::sync::Arc;
+
+use core_foundation::base::{CFIndex, CFRelease};
+use core_foundation::runloop::{
+    kCFRunLoopAfterWaiting, kCFRunLoopBeforeWaiting, kCFRunLoopCommonModes, kCFRunLoopDefaultMode,
+    kCFRunLoopExit, CFRunLoopActivity, CFRunLoopAddObserver, CFRunLoopAddSource, CFRunLoopGetMain,
+    CFRunLoopObserverCreate, CFRunLoopObserverRef, CFRunLoopSourceContext, CFRunLoopSourceCreate,
+    CFRunLoopSourceInvalidate, CFRunLoopSourceRef, CFRunLoopSourceSignal, CFRunLoopWakeUp,
+};
+use objc2::rc::Retained;
+use objc2::{msg_send_id, ClassType};
+use objc2_foundation::{MainThreadMarker, NSString};
+use objc2_ui_kit::{UIApplication, UIApplicationMain, UIDevice, UIScreen, UIUserInterfaceIdiom};
+
+use super::app_state::EventLoopHandler;
+use crate::application::ApplicationHandler;
+use crate::error::EventLoopError;
+use crate::event::Event;
+use crate::event_loop::{ActiveEventLoop as RootActiveEventLoop, ControlFlow, DeviceEvents};
+use crate::platform::ios::Idiom;
+use crate::window::{CustomCursor, CustomCursorSource};
+
+use super::app_delegate::AppDelegate;
+use super::app_state::AppState;
+use super::{app_state, monitor, MonitorHandle};
+
+#[derive(Debug)]
+pub struct ActiveEventLoop {
+    pub(super) mtm: MainThreadMarker,
+}
+
+impl ActiveEventLoop {
+    pub fn create_custom_cursor(&self, source: CustomCursorSource) -> CustomCursor {
+        let _ = source.inner;
+        CustomCursor { inner: super::PlatformCustomCursor }
+    }
+
+    pub fn available_monitors(&self) -> VecDeque<MonitorHandle> {
+        monitor::uiscreens(self.mtm)
+    }
+
+    pub fn primary_monitor(&self) -> Option<MonitorHandle> {
+        #[allow(deprecated)]
+        Some(MonitorHandle::new(UIScreen::mainScreen(self.mtm)))
+    }
+
+    #[inline]
+    pub fn listen_device_events(&self, _allowed: DeviceEvents) {}
+
+    #[cfg(feature = "rwh_05")]
+    #[inline]
+    pub fn raw_display_handle_rwh_05(&self) -> rwh_05::RawDisplayHandle {
+        rwh_05::RawDisplayHandle::UiKit(rwh_05::UiKitDisplayHandle::empty())
+    }
+
+    #[cfg(feature = "rwh_06")]
+    #[inline]
+    pub fn raw_display_handle_rwh_06(
+        &self,
+    ) -> Result<rwh_06::RawDisplayHandle, rwh_06::HandleError> {
+        Ok(rwh_06::RawDisplayHandle::UiKit(rwh_06::UiKitDisplayHandle::new()))
+    }
+
+    pub(crate) fn set_control_flow(&self, control_flow: ControlFlow) {
+        AppState::get_mut(self.mtm).set_control_flow(control_flow)
+    }
+
+    pub(crate) fn control_flow(&self) -> ControlFlow {
+        AppState::get_mut(self.mtm).control_flow()
+    }
+
+    pub(crate) fn exit(&self) {
+        // https://developer.apple.com/library/archive/qa/qa1561/_index.html
+        // it is not possible to quit an iOS app gracefully and programmatically
+        tracing::warn!("`ControlFlow::Exit` ignored on iOS");
+    }
+
+    pub(crate) fn exiting(&self) -> bool {
+        false
+    }
+
+    pub(crate) fn owned_display_handle(&self) -> OwnedDisplayHandle {
+        OwnedDisplayHandle
+    }
+}
+
+#[derive(Clone)]
+pub(crate) struct OwnedDisplayHandle;
+
+impl OwnedDisplayHandle {
+    #[cfg(feature = "rwh_05")]
+    #[inline]
+    pub fn raw_display_handle_rwh_05(&self) -> rwh_05::RawDisplayHandle {
+        rwh_05::UiKitDisplayHandle::empty().into()
+    }
+
+    #[cfg(feature = "rwh_06")]
+    #[inline]
+    pub fn raw_display_handle_rwh_06(
+        &self,
+    ) -> Result<rwh_06::RawDisplayHandle, rwh_06::HandleError> {
+        Ok(rwh_06::UiKitDisplayHandle::new().into())
+    }
+}
+
+fn map_user_event<A: ApplicationHandler>(
+    app: &mut A,
+    proxy_wake_up: Arc<AtomicBool>,
+) -> impl FnMut(Event, &RootActiveEventLoop) + '_ {
+    move |event, window_target| match event {
+        Event::NewEvents(cause) => app.new_events(window_target, cause),
+        Event::WindowEvent { window_id, event } => {
+            app.window_event(window_target, window_id, event)
+        },
+        Event::DeviceEvent { device_id, event } => {
+            app.device_event(window_target, device_id, event)
+        },
+        Event::UserWakeUp => {
+            if proxy_wake_up.swap(false, AtomicOrdering::Relaxed) {
+                app.proxy_wake_up(window_target);
+            }
+        },
+        Event::Suspended => app.suspended(window_target),
+        Event::Resumed => app.resumed(window_target),
+        Event::AboutToWait => app.about_to_wait(window_target),
+        Event::LoopExiting => app.exiting(window_target),
+        Event::MemoryWarning => app.memory_warning(window_target),
+    }
+}
+
+pub struct EventLoop {
+    mtm: MainThreadMarker,
+    proxy_wake_up: Arc<AtomicBool>,
+    window_target: RootActiveEventLoop,
+}
+
+#[derive(Default, Debug, Copy, Clone, PartialEq, Eq, Hash)]
+pub(crate) struct PlatformSpecificEventLoopAttributes {}
+
+impl EventLoop {
+    pub(crate) fn new(
+        _: &PlatformSpecificEventLoopAttributes,
+    ) -> Result<EventLoop, EventLoopError> {
+        let mtm = MainThreadMarker::new()
+            .expect("On iOS, `EventLoop` must be created on the main thread");
+
+        static mut SINGLETON_INIT: bool = false;
+        unsafe {
+            assert!(
+                !SINGLETON_INIT,
+                "Only one `EventLoop` is supported on iOS. `EventLoopProxy` might be helpful"
+            );
+            SINGLETON_INIT = true;
+        }
+
+        // this line sets up the main run loop before `UIApplicationMain`
+        setup_control_flow_observers();
+
+        let proxy_wake_up = Arc::new(AtomicBool::new(false));
+
+        Ok(EventLoop {
+            mtm,
+            proxy_wake_up,
+            window_target: RootActiveEventLoop { p: ActiveEventLoop { mtm }, _marker: PhantomData },
+        })
+    }
+
+    pub fn run_app<A: ApplicationHandler>(self, app: &mut A) -> ! {
+        let application: Option<Retained<UIApplication>> =
+            unsafe { msg_send_id![UIApplication::class(), sharedApplication] };
+        assert!(
+            application.is_none(),
+            "\
+                `EventLoop` cannot be `run` after a call to `UIApplicationMain` on iOS\nNote: \
+             `EventLoop::run_app` calls `UIApplicationMain` on iOS",
+        );
+
+        let handler = map_user_event(app, self.proxy_wake_up.clone());
+
+        let handler = unsafe {
+            std::mem::transmute::<
+                Box<dyn FnMut(Event, &RootActiveEventLoop)>,
+                Box<dyn FnMut(Event, &RootActiveEventLoop)>,
+            >(Box::new(handler))
+        };
+
+        let handler = EventLoopHandler { handler, event_loop: self.window_target };
+
+        app_state::will_launch(self.mtm, handler);
+
+        // Ensure application delegate is initialized
+        let _ = AppDelegate::class();
+
+        extern "C" {
+            // These functions are in crt_externs.h.
+            fn _NSGetArgc() -> *mut c_int;
+            fn _NSGetArgv() -> *mut *mut *mut c_char;
+        }
+
+        unsafe {
+            UIApplicationMain(
+                *_NSGetArgc(),
+                NonNull::new(*_NSGetArgv()).unwrap(),
+                None,
+                Some(&NSString::from_str(AppDelegate::NAME)),
+            )
+        };
+        unreachable!()
+    }
+
+    pub fn create_proxy(&self) -> EventLoopProxy {
+        EventLoopProxy::new(self.proxy_wake_up.clone())
+    }
+
+    pub fn window_target(&self) -> &RootActiveEventLoop {
+        &self.window_target
+    }
+}
+
+// EventLoopExtIOS
+impl EventLoop {
+    pub fn idiom(&self) -> Idiom {
+        match UIDevice::currentDevice(self.mtm).userInterfaceIdiom() {
+            UIUserInterfaceIdiom::Unspecified => Idiom::Unspecified,
+            UIUserInterfaceIdiom::Phone => Idiom::Phone,
+            UIUserInterfaceIdiom::Pad => Idiom::Pad,
+            UIUserInterfaceIdiom::TV => Idiom::TV,
+            UIUserInterfaceIdiom::CarPlay => Idiom::CarPlay,
+            _ => Idiom::Unspecified,
+        }
+    }
+}
+
+pub struct EventLoopProxy {
+    proxy_wake_up: Arc<AtomicBool>,
+    source: CFRunLoopSourceRef,
+}
+
+unsafe impl Send for EventLoopProxy {}
+unsafe impl Sync for EventLoopProxy {}
+
+impl Clone for EventLoopProxy {
+    fn clone(&self) -> EventLoopProxy {
+        EventLoopProxy::new(self.proxy_wake_up.clone())
+    }
+}
+
+impl Drop for EventLoopProxy {
+    fn drop(&mut self) {
+        unsafe {
+            CFRunLoopSourceInvalidate(self.source);
+            CFRelease(self.source as _);
+        }
+    }
+}
+
+impl EventLoopProxy {
+    fn new(proxy_wake_up: Arc<AtomicBool>) -> EventLoopProxy {
+        unsafe {
+            // just wake up the eventloop
+            extern "C" fn event_loop_proxy_handler(_: *const c_void) {}
+
+            // adding a Source to the main CFRunLoop lets us wake it up and
+            // process user events through the normal OS EventLoop mechanisms.
+            let rl = CFRunLoopGetMain();
+            let mut context = CFRunLoopSourceContext {
+                version: 0,
+                info: ptr::null_mut(),
+                retain: None,
+                release: None,
+                copyDescription: None,
+                equal: None,
+                hash: None,
+                schedule: None,
+                cancel: None,
+                perform: event_loop_proxy_handler,
+            };
+            let source = CFRunLoopSourceCreate(ptr::null_mut(), CFIndex::MAX - 1, &mut context);
+            CFRunLoopAddSource(rl, source, kCFRunLoopCommonModes);
+            CFRunLoopWakeUp(rl);
+
+            EventLoopProxy { proxy_wake_up, source }
+        }
+    }
+
+    pub fn wake_up(&self) {
+        self.proxy_wake_up.store(true, AtomicOrdering::Relaxed);
+        unsafe {
+            // let the main thread know there's a new event
+            CFRunLoopSourceSignal(self.source);
+            let rl = CFRunLoopGetMain();
+            CFRunLoopWakeUp(rl);
+        }
+    }
+}
+
+fn setup_control_flow_observers() {
+    unsafe {
+        // begin is queued with the highest priority to ensure it is processed before other
+        // observers
+        extern "C" fn control_flow_begin_handler(
+            _: CFRunLoopObserverRef,
+            activity: CFRunLoopActivity,
+            _: *mut c_void,
+        ) {
+            let mtm = MainThreadMarker::new().unwrap();
+            #[allow(non_upper_case_globals)]
+            match activity {
+                kCFRunLoopAfterWaiting => app_state::handle_wakeup_transition(mtm),
+                _ => unreachable!(),
+            }
+        }
+
+        // Core Animation registers its `CFRunLoopObserver` that performs drawing operations in
+        // `CA::Transaction::ensure_implicit` with a priority of `0x1e8480`. We set the main_end
+        // priority to be 0, in order to send AboutToWait before RedrawRequested. This value was
+        // chosen conservatively to guard against apple using different priorities for their redraw
+        // observers in different OS's or on different devices. If it so happens that it's too
+        // conservative, the main symptom would be non-redraw events coming in after `AboutToWait`.
+        //
+        // The value of `0x1e8480` was determined by inspecting stack traces and the associated
+        // registers for every `CFRunLoopAddObserver` call on an iPad Air 2 running iOS 11.4.
+        //
+        // Also tested to be `0x1e8480` on iPhone 8, iOS 13 beta 4.
+        extern "C" fn control_flow_main_end_handler(
+            _: CFRunLoopObserverRef,
+            activity: CFRunLoopActivity,
+            _: *mut c_void,
+        ) {
+            let mtm = MainThreadMarker::new().unwrap();
+            #[allow(non_upper_case_globals)]
+            match activity {
+                kCFRunLoopBeforeWaiting => app_state::handle_main_events_cleared(mtm),
+                kCFRunLoopExit => {}, // may happen when running on macOS
+                _ => unreachable!(),
+            }
+        }
+
+        // end is queued with the lowest priority to ensure it is processed after other observers
+        extern "C" fn control_flow_end_handler(
+            _: CFRunLoopObserverRef,
+            activity: CFRunLoopActivity,
+            _: *mut c_void,
+        ) {
+            let mtm = MainThreadMarker::new().unwrap();
+            #[allow(non_upper_case_globals)]
+            match activity {
+                kCFRunLoopBeforeWaiting => app_state::handle_events_cleared(mtm),
+                kCFRunLoopExit => {}, // may happen when running on macOS
+                _ => unreachable!(),
+            }
+        }
+
+        let main_loop = CFRunLoopGetMain();
+
+        let begin_observer = CFRunLoopObserverCreate(
+            ptr::null_mut(),
+            kCFRunLoopAfterWaiting,
+            1, // repeat = true
+            CFIndex::MIN,
+            control_flow_begin_handler,
+            ptr::null_mut(),
+        );
+        CFRunLoopAddObserver(main_loop, begin_observer, kCFRunLoopDefaultMode);
+
+        let main_end_observer = CFRunLoopObserverCreate(
+            ptr::null_mut(),
+            kCFRunLoopExit | kCFRunLoopBeforeWaiting,
+            1, // repeat = true
+            0, // see comment on `control_flow_main_end_handler`
+            control_flow_main_end_handler,
+            ptr::null_mut(),
+        );
+        CFRunLoopAddObserver(main_loop, main_end_observer, kCFRunLoopDefaultMode);
+
+        let end_observer = CFRunLoopObserverCreate(
+            ptr::null_mut(),
+            kCFRunLoopExit | kCFRunLoopBeforeWaiting,
+            1, // repeat = true
+            CFIndex::MAX,
+            control_flow_end_handler,
+            ptr::null_mut(),
+        );
+        CFRunLoopAddObserver(main_loop, end_observer, kCFRunLoopDefaultMode);
+    }
+}

src/platform_impl/apple/uikit/mod.rs

@@ -0,0 +1,52 @@
+#![allow(clippy::let_unit_value)]
+
+mod app_delegate;
+mod app_state;
+mod event_loop;
+mod monitor;
+mod view;
+mod view_controller;
+mod window;
+
+use std::fmt;
+
+use crate::event::DeviceId as RootDeviceId;
+
+pub(crate) use self::event_loop::{
+    ActiveEventLoop, EventLoop, EventLoopProxy, OwnedDisplayHandle,
+    PlatformSpecificEventLoopAttributes,
+};
+pub(crate) use self::monitor::{MonitorHandle, VideoModeHandle};
+pub(crate) use self::window::{PlatformSpecificWindowAttributes, Window, WindowId};
+pub(crate) use crate::cursor::{
+    NoCustomCursor as PlatformCustomCursor, NoCustomCursor as PlatformCustomCursorSource,
+};
+pub(crate) use crate::icon::NoIcon as PlatformIcon;
+pub(crate) use crate::platform_impl::Fullscreen;
+
+/// There is no way to detect which device that performed a certain event in
+/// UIKit (i.e. you can't differentiate between different external keyboards,
+/// or whether it was the main touchscreen, assistive technologies, or some
+/// other pointer device that caused a touch event).
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct DeviceId;
+
+impl DeviceId {
+    pub const unsafe fn dummy() -> Self {
+        DeviceId
+    }
+}
+
+pub(crate) const DEVICE_ID: RootDeviceId = RootDeviceId(DeviceId);
+
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct KeyEventExtra {}
+
+#[derive(Debug)]
+pub enum OsError {}
+
+impl fmt::Display for OsError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "os error")
+    }
+}

src/platform_impl/apple/uikit/monitor.rs

@@ -0,0 +1,244 @@
+#![allow(clippy::unnecessary_cast)]
+
+use std::collections::{BTreeSet, VecDeque};
+use std::{fmt, hash, ptr};
+
+use objc2::mutability::IsRetainable;
+use objc2::rc::Retained;
+use objc2::Message;
+use objc2_foundation::{run_on_main, MainThreadBound, MainThreadMarker, NSInteger};
+use objc2_ui_kit::{UIScreen, UIScreenMode};
+
+use super::app_state;
+use crate::dpi::{PhysicalPosition, PhysicalSize};
+use crate::monitor::VideoModeHandle as RootVideoModeHandle;
+
+// Workaround for `MainThreadBound` implementing almost no traits
+#[derive(Debug)]
+struct MainThreadBoundDelegateImpls<T>(MainThreadBound<Retained<T>>);
+
+impl<T: IsRetainable + Message> Clone for MainThreadBoundDelegateImpls<T> {
+    fn clone(&self) -> Self {
+        Self(run_on_main(|mtm| MainThreadBound::new(Retained::clone(self.0.get(mtm)), mtm)))
+    }
+}
+
+impl<T: IsRetainable + Message> hash::Hash for MainThreadBoundDelegateImpls<T> {
+    fn hash<H: hash::Hasher>(&self, state: &mut H) {
+        // SAFETY: Marker only used to get the pointer
+        let mtm = unsafe { MainThreadMarker::new_unchecked() };
+        Retained::as_ptr(self.0.get(mtm)).hash(state);
+    }
+}
+
+impl<T: IsRetainable + Message> PartialEq for MainThreadBoundDelegateImpls<T> {
+    fn eq(&self, other: &Self) -> bool {
+        // SAFETY: Marker only used to get the pointer
+        let mtm = unsafe { MainThreadMarker::new_unchecked() };
+        Retained::as_ptr(self.0.get(mtm)) == Retained::as_ptr(other.0.get(mtm))
+    }
+}
+
+impl<T: IsRetainable + Message> Eq for MainThreadBoundDelegateImpls<T> {}
+
+#[derive(Debug, PartialEq, Eq, Hash, Clone)]
+pub struct VideoModeHandle {
+    pub(crate) size: (u32, u32),
+    pub(crate) bit_depth: u16,
+    pub(crate) refresh_rate_millihertz: u32,
+    screen_mode: MainThreadBoundDelegateImpls<UIScreenMode>,
+    pub(crate) monitor: MonitorHandle,
+}
+
+impl VideoModeHandle {
+    fn new(
+        uiscreen: Retained<UIScreen>,
+        screen_mode: Retained<UIScreenMode>,
+        mtm: MainThreadMarker,
+    ) -> VideoModeHandle {
+        let refresh_rate_millihertz = refresh_rate_millihertz(&uiscreen);
+        let size = screen_mode.size();
+        VideoModeHandle {
+            size: (size.width as u32, size.height as u32),
+            bit_depth: 32,
+            refresh_rate_millihertz,
+            screen_mode: MainThreadBoundDelegateImpls(MainThreadBound::new(screen_mode, mtm)),
+            monitor: MonitorHandle::new(uiscreen),
+        }
+    }
+
+    pub fn size(&self) -> PhysicalSize<u32> {
+        self.size.into()
+    }
+
+    pub fn bit_depth(&self) -> u16 {
+        self.bit_depth
+    }
+
+    pub fn refresh_rate_millihertz(&self) -> u32 {
+        self.refresh_rate_millihertz
+    }
+
+    pub fn monitor(&self) -> MonitorHandle {
+        self.monitor.clone()
+    }
+
+    pub(super) fn screen_mode(&self, mtm: MainThreadMarker) -> &Retained<UIScreenMode> {
+        self.screen_mode.0.get(mtm)
+    }
+}
+
+pub struct MonitorHandle {
+    ui_screen: MainThreadBound<Retained<UIScreen>>,
+}
+
+impl Clone for MonitorHandle {
+    fn clone(&self) -> Self {
+        run_on_main(|mtm| Self {
+            ui_screen: MainThreadBound::new(self.ui_screen.get(mtm).clone(), mtm),
+        })
+    }
+}
+
+impl hash::Hash for MonitorHandle {
+    fn hash<H: hash::Hasher>(&self, state: &mut H) {
+        (self as *const Self).hash(state);
+    }
+}
+
+impl PartialEq for MonitorHandle {
+    fn eq(&self, other: &Self) -> bool {
+        ptr::eq(self, other)
+    }
+}
+
+impl Eq for MonitorHandle {}
+
+impl PartialOrd for MonitorHandle {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for MonitorHandle {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        // TODO: Make a better ordering
+        (self as *const Self).cmp(&(other as *const Self))
+    }
+}
+
+impl fmt::Debug for MonitorHandle {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("MonitorHandle")
+            .field("name", &self.name())
+            .field("size", &self.size())
+            .field("position", &self.position())
+            .field("scale_factor", &self.scale_factor())
+            .field("refresh_rate_millihertz", &self.refresh_rate_millihertz())
+            .finish_non_exhaustive()
+    }
+}
+
+impl MonitorHandle {
+    pub(crate) fn new(ui_screen: Retained<UIScreen>) -> Self {
+        // Holding `Retained<UIScreen>` implies we're on the main thread.
+        let mtm = MainThreadMarker::new().unwrap();
+        Self { ui_screen: MainThreadBound::new(ui_screen, mtm) }
+    }
+
+    pub fn name(&self) -> Option<String> {
+        run_on_main(|mtm| {
+            #[allow(deprecated)]
+            let main = UIScreen::mainScreen(mtm);
+            if *self.ui_screen(mtm) == main {
+                Some("Primary".to_string())
+            } else if Some(self.ui_screen(mtm)) == main.mirroredScreen().as_ref() {
+                Some("Mirrored".to_string())
+            } else {
+                #[allow(deprecated)]
+                UIScreen::screens(mtm)
+                    .iter()
+                    .position(|rhs| rhs == &**self.ui_screen(mtm))
+                    .map(|idx| idx.to_string())
+            }
+        })
+    }
+
+    pub fn size(&self) -> PhysicalSize<u32> {
+        let bounds = self.ui_screen.get_on_main(|ui_screen| ui_screen.nativeBounds());
+        PhysicalSize::new(bounds.size.width as u32, bounds.size.height as u32)
+    }
+
+    pub fn position(&self) -> PhysicalPosition<i32> {
+        let bounds = self.ui_screen.get_on_main(|ui_screen| ui_screen.nativeBounds());
+        (bounds.origin.x as f64, bounds.origin.y as f64).into()
+    }
+
+    pub fn scale_factor(&self) -> f64 {
+        self.ui_screen.get_on_main(|ui_screen| ui_screen.nativeScale()) as f64
+    }
+
+    pub fn refresh_rate_millihertz(&self) -> Option<u32> {
+        Some(self.ui_screen.get_on_main(|ui_screen| refresh_rate_millihertz(ui_screen)))
+    }
+
+    pub fn video_modes(&self) -> impl Iterator<Item = VideoModeHandle> {
+        run_on_main(|mtm| {
+            let ui_screen = self.ui_screen(mtm);
+            // Use Ord impl of RootVideoModeHandle
+
+            let modes: BTreeSet<_> = ui_screen
+                .availableModes()
+                .into_iter()
+                .map(|mode| RootVideoModeHandle {
+                    video_mode: VideoModeHandle::new(ui_screen.clone(), mode, mtm),
+                })
+                .collect();
+
+            modes.into_iter().map(|mode| mode.video_mode)
+        })
+    }
+
+    pub(crate) fn ui_screen(&self, mtm: MainThreadMarker) -> &Retained<UIScreen> {
+        self.ui_screen.get(mtm)
+    }
+
+    pub fn preferred_video_mode(&self) -> VideoModeHandle {
+        run_on_main(|mtm| {
+            VideoModeHandle::new(
+                self.ui_screen(mtm).clone(),
+                self.ui_screen(mtm).preferredMode().unwrap(),
+                mtm,
+            )
+        })
+    }
+}
+
+fn refresh_rate_millihertz(uiscreen: &UIScreen) -> u32 {
+    let refresh_rate_millihertz: NSInteger = {
+        let os_capabilities = app_state::os_capabilities();
+        if os_capabilities.maximum_frames_per_second {
+            uiscreen.maximumFramesPerSecond()
+        } else {
+            // https://developer.apple.com/library/archive/technotes/tn2460/_index.html
+            // https://en.wikipedia.org/wiki/IPad_Pro#Model_comparison
+            //
+            // All iOS devices support 60 fps, and on devices where `maximumFramesPerSecond` is not
+            // supported, they are all guaranteed to have 60hz refresh rates. This does not
+            // correctly handle external displays. ProMotion displays support 120fps, but they were
+            // introduced at the same time as the `maximumFramesPerSecond` API.
+            //
+            // FIXME: earlier OSs could calculate the refresh rate using
+            // `-[CADisplayLink duration]`.
+            os_capabilities.maximum_frames_per_second_err_msg("defaulting to 60 fps");
+            60
+        }
+    };
+
+    refresh_rate_millihertz as u32 * 1000
+}
+
+pub fn uiscreens(mtm: MainThreadMarker) -> VecDeque<MonitorHandle> {
+    #[allow(deprecated)]
+    UIScreen::screens(mtm).into_iter().map(MonitorHandle::new).collect()
+}

src/platform_impl/apple/uikit/view.rs

@@ -0,0 +1,515 @@
+#![allow(clippy::unnecessary_cast)]
+use std::cell::{Cell, RefCell};
+
+use objc2::rc::Retained;
+use objc2::runtime::{NSObjectProtocol, ProtocolObject};
+use objc2::{declare_class, msg_send, msg_send_id, mutability, sel, ClassType, DeclaredClass};
+use objc2_foundation::{CGFloat, CGPoint, CGRect, MainThreadMarker, NSObject, NSSet};
+use objc2_ui_kit::{
+    UICoordinateSpace, UIEvent, UIForceTouchCapability, UIGestureRecognizer,
+    UIGestureRecognizerDelegate, UIGestureRecognizerState, UIPanGestureRecognizer,
+    UIPinchGestureRecognizer, UIResponder, UIRotationGestureRecognizer, UITapGestureRecognizer,
+    UITouch, UITouchPhase, UITouchType, UITraitEnvironment, UIView,
+};
+
+use super::app_state::{self, EventWrapper};
+use super::window::WinitUIWindow;
+use super::DEVICE_ID;
+use crate::dpi::PhysicalPosition;
+use crate::event::{Event, Force, Touch, TouchPhase, WindowEvent};
+use crate::window::{WindowAttributes, WindowId as RootWindowId};
+
+pub struct WinitViewState {
+    pinch_gesture_recognizer: RefCell<Option<Retained<UIPinchGestureRecognizer>>>,
+    doubletap_gesture_recognizer: RefCell<Option<Retained<UITapGestureRecognizer>>>,
+    rotation_gesture_recognizer: RefCell<Option<Retained<UIRotationGestureRecognizer>>>,
+    pan_gesture_recognizer: RefCell<Option<Retained<UIPanGestureRecognizer>>>,
+
+    // for iOS delta references the start of the Gesture
+    rotation_last_delta: Cell<CGFloat>,
+    pinch_last_delta: Cell<CGFloat>,
+    pan_last_delta: Cell<CGPoint>,
+}
+
+declare_class!(
+    pub(crate) struct WinitView;
+
+    unsafe impl ClassType for WinitView {
+        #[inherits(UIResponder, NSObject)]
+        type Super = UIView;
+        type Mutability = mutability::MainThreadOnly;
+        const NAME: &'static str = "WinitUIView";
+    }
+
+    impl DeclaredClass for WinitView {
+        type Ivars = WinitViewState;
+    }
+
+    unsafe impl WinitView {
+        #[method(drawRect:)]
+        fn draw_rect(&self, rect: CGRect) {
+            let mtm = MainThreadMarker::new().unwrap();
+            let window = self.window().unwrap();
+            app_state::handle_nonuser_event(
+                mtm,
+                EventWrapper::StaticEvent(Event::WindowEvent {
+                    window_id: RootWindowId(window.id()),
+                    event: WindowEvent::RedrawRequested,
+                }),
+            );
+            let _: () = unsafe { msg_send![super(self), drawRect: rect] };
+        }
+
+        #[method(layoutSubviews)]
+        fn layout_subviews(&self) {
+            let mtm = MainThreadMarker::new().unwrap();
+            let _: () = unsafe { msg_send![super(self), layoutSubviews] };
+
+            let window = self.window().unwrap();
+            let window_bounds = window.bounds();
+            let screen = window.screen();
+            let screen_space = screen.coordinateSpace();
+            let screen_frame = self.convertRect_toCoordinateSpace(window_bounds, &screen_space);
+            let scale_factor = screen.scale();
+            let size = crate::dpi::LogicalSize {
+                width: screen_frame.size.width as f64,
+                height: screen_frame.size.height as f64,
+            }
+            .to_physical(scale_factor as f64);
+
+            // If the app is started in landscape, the view frame and window bounds can be mismatched.
+            // The view frame will be in portrait and the window bounds in landscape. So apply the
+            // window bounds to the view frame to make it consistent.
+            let view_frame = self.frame();
+            if view_frame != window_bounds {
+                self.setFrame(window_bounds);
+            }
+
+            app_state::handle_nonuser_event(
+                mtm,
+                EventWrapper::StaticEvent(Event::WindowEvent {
+                    window_id: RootWindowId(window.id()),
+                    event: WindowEvent::Resized(size),
+                }),
+            );
+        }
+
+        #[method(setContentScaleFactor:)]
+        fn set_content_scale_factor(&self, untrusted_scale_factor: CGFloat) {
+            let mtm = MainThreadMarker::new().unwrap();
+            let _: () =
+                unsafe { msg_send![super(self), setContentScaleFactor: untrusted_scale_factor] };
+
+            // `window` is null when `setContentScaleFactor` is invoked prior to `[UIWindow
+            // makeKeyAndVisible]` at window creation time (either manually or internally by
+            // UIKit when the `UIView` is first created), in which case we send no events here
+            let window = match self.window() {
+                Some(window) => window,
+                None => return,
+            };
+            // `setContentScaleFactor` may be called with a value of 0, which means "reset the
+            // content scale factor to a device-specific default value", so we can't use the
+            // parameter here. We can query the actual factor using the getter
+            let scale_factor = self.contentScaleFactor();
+            assert!(
+                !scale_factor.is_nan()
+                    && scale_factor.is_finite()
+                    && scale_factor.is_sign_positive()
+                    && scale_factor > 0.0,
+                "invalid scale_factor set on UIView",
+            );
+            let scale_factor = scale_factor as f64;
+            let bounds = self.bounds();
+            let screen = window.screen();
+            let screen_space = screen.coordinateSpace();
+            let screen_frame = self.convertRect_toCoordinateSpace(bounds, &screen_space);
+            let size = crate::dpi::LogicalSize {
+                width: screen_frame.size.width as f64,
+                height: screen_frame.size.height as f64,
+            };
+            let window_id = RootWindowId(window.id());
+            app_state::handle_nonuser_events(
+                mtm,
+                std::iter::once(EventWrapper::ScaleFactorChanged(
+                    app_state::ScaleFactorChanged {
+                        window,
+                        scale_factor,
+                        suggested_size: size.to_physical(scale_factor),
+                    },
+                ))
+                .chain(std::iter::once(EventWrapper::StaticEvent(
+                    Event::WindowEvent {
+                        window_id,
+                        event: WindowEvent::Resized(size.to_physical(scale_factor)),
+                    },
+                ))),
+            );
+        }
+
+        #[method(touchesBegan:withEvent:)]
+        fn touches_began(&self, touches: &NSSet<UITouch>, _event: Option<&UIEvent>) {
+            self.handle_touches(touches)
+        }
+
+        #[method(touchesMoved:withEvent:)]
+        fn touches_moved(&self, touches: &NSSet<UITouch>, _event: Option<&UIEvent>) {
+            self.handle_touches(touches)
+        }
+
+        #[method(touchesEnded:withEvent:)]
+        fn touches_ended(&self, touches: &NSSet<UITouch>, _event: Option<&UIEvent>) {
+            self.handle_touches(touches)
+        }
+
+        #[method(touchesCancelled:withEvent:)]
+        fn touches_cancelled(&self, touches: &NSSet<UITouch>, _event: Option<&UIEvent>) {
+            self.handle_touches(touches)
+        }
+
+        #[method(pinchGesture:)]
+        fn pinch_gesture(&self, recognizer: &UIPinchGestureRecognizer) {
+            let window = self.window().unwrap();
+
+            let (phase, delta) = match recognizer.state() {
+                UIGestureRecognizerState::Began => {
+                    self.ivars().pinch_last_delta.set(recognizer.scale());
+                    (TouchPhase::Started, 0.0)
+                }
+                UIGestureRecognizerState::Changed => {
+                    let last_scale: f64 = self.ivars().pinch_last_delta.replace(recognizer.scale());
+                    (TouchPhase::Moved, recognizer.scale() - last_scale)
+                }
+                UIGestureRecognizerState::Ended => {
+                    let last_scale: f64 = self.ivars().pinch_last_delta.replace(0.0);
+                    (TouchPhase::Moved, recognizer.scale() - last_scale)
+                }
+                UIGestureRecognizerState::Cancelled | UIGestureRecognizerState::Failed => {
+                    self.ivars().rotation_last_delta.set(0.0);
+                    // Pass -delta so that action is reversed
+                    (TouchPhase::Cancelled, -recognizer.scale())
+                }
+                state => panic!("unexpected recognizer state: {:?}", state),
+            };
+
+            let gesture_event = EventWrapper::StaticEvent(Event::WindowEvent {
+                window_id: RootWindowId(window.id()),
+                event: WindowEvent::PinchGesture {
+                    device_id: DEVICE_ID,
+                    delta: delta as f64,
+                    phase,
+                },
+            });
+
+            let mtm = MainThreadMarker::new().unwrap();
+            app_state::handle_nonuser_event(mtm, gesture_event);
+        }
+
+        #[method(doubleTapGesture:)]
+        fn double_tap_gesture(&self, recognizer: &UITapGestureRecognizer) {
+            let window = self.window().unwrap();
+
+            if recognizer.state() == UIGestureRecognizerState::Ended {
+                let gesture_event = EventWrapper::StaticEvent(Event::WindowEvent {
+                    window_id: RootWindowId(window.id()),
+                    event: WindowEvent::DoubleTapGesture {
+                        device_id: DEVICE_ID,
+                    },
+                });
+
+                let mtm = MainThreadMarker::new().unwrap();
+                app_state::handle_nonuser_event(mtm, gesture_event);
+            }
+        }
+
+        #[method(rotationGesture:)]
+        fn rotation_gesture(&self, recognizer: &UIRotationGestureRecognizer) {
+            let window = self.window().unwrap();
+
+            let (phase, delta) = match recognizer.state() {
+                UIGestureRecognizerState::Began => {
+                    self.ivars().rotation_last_delta.set(0.0);
+
+                    (TouchPhase::Started, 0.0)
+                }
+                UIGestureRecognizerState::Changed => {
+                    let last_rotation = self.ivars().rotation_last_delta.replace(recognizer.rotation());
+
+                    (TouchPhase::Moved, recognizer.rotation() - last_rotation)
+                }
+                UIGestureRecognizerState::Ended => {
+                    let last_rotation = self.ivars().rotation_last_delta.replace(0.0);
+
+                    (TouchPhase::Ended, recognizer.rotation() - last_rotation)
+                }
+                UIGestureRecognizerState::Cancelled | UIGestureRecognizerState::Failed => {
+                    self.ivars().rotation_last_delta.set(0.0);
+
+                    // Pass -delta so that action is reversed
+                    (TouchPhase::Cancelled, -recognizer.rotation())
+                }
+                state => panic!("unexpected recognizer state: {:?}", state),
+            };
+
+            // Make delta negative to match macos, convert to degrees
+            let gesture_event = EventWrapper::StaticEvent(Event::WindowEvent {
+                window_id: RootWindowId(window.id()),
+                event: WindowEvent::RotationGesture {
+                    device_id: DEVICE_ID,
+                    delta: -delta.to_degrees() as _,
+                    phase,
+                },
+            });
+
+            let mtm = MainThreadMarker::new().unwrap();
+            app_state::handle_nonuser_event(mtm, gesture_event);
+        }
+
+        #[method(panGesture:)]
+        fn pan_gesture(&self, recognizer: &UIPanGestureRecognizer) {
+            let window = self.window().unwrap();
+
+            let translation = recognizer.translationInView(Some(self));
+
+            let (phase, dx, dy) = match recognizer.state() {
+                UIGestureRecognizerState::Began => {
+                    self.ivars().pan_last_delta.set(translation);
+
+                    (TouchPhase::Started, 0.0, 0.0)
+                }
+                UIGestureRecognizerState::Changed => {
+                    let last_pan: CGPoint = self.ivars().pan_last_delta.replace(translation);
+
+                    let dx = translation.x - last_pan.x;
+                    let dy = translation.y - last_pan.y;
+
+                    (TouchPhase::Moved, dx, dy)
+                }
+                UIGestureRecognizerState::Ended => {
+                    let last_pan: CGPoint = self.ivars().pan_last_delta.replace(CGPoint{x:0.0, y:0.0});
+
+                    let dx = translation.x - last_pan.x;
+                    let dy = translation.y - last_pan.y;
+
+                    (TouchPhase::Ended, dx, dy)
+                }
+                UIGestureRecognizerState::Cancelled | UIGestureRecognizerState::Failed => {
+                    let last_pan: CGPoint = self.ivars().pan_last_delta.replace(CGPoint{x:0.0, y:0.0});
+
+                    // Pass -delta so that action is reversed
+                    (TouchPhase::Cancelled, -last_pan.x, -last_pan.y)
+                }
+                state => panic!("unexpected recognizer state: {:?}", state),
+            };
+
+
+            let gesture_event = EventWrapper::StaticEvent(Event::WindowEvent {
+                window_id: RootWindowId(window.id()),
+                event: WindowEvent::PanGesture {
+                    device_id: DEVICE_ID,
+                    delta: PhysicalPosition::new(dx as _, dy as _),
+                    phase,
+                },
+            });
+
+            let mtm = MainThreadMarker::new().unwrap();
+            app_state::handle_nonuser_event(mtm, gesture_event);
+        }
+    }
+
+    unsafe impl NSObjectProtocol for WinitView {}
+
+    unsafe impl UIGestureRecognizerDelegate for WinitView {
+        #[method(gestureRecognizer:shouldRecognizeSimultaneouslyWithGestureRecognizer:)]
+        fn should_recognize_simultaneously(&self, _gesture_recognizer: &UIGestureRecognizer, _other_gesture_recognizer: &UIGestureRecognizer) -> bool {
+            true
+        }
+    }
+);
+
+impl WinitView {
+    pub(crate) fn new(
+        mtm: MainThreadMarker,
+        window_attributes: &WindowAttributes,
+        frame: CGRect,
+    ) -> Retained<Self> {
+        let this = mtm.alloc().set_ivars(WinitViewState {
+            pinch_gesture_recognizer: RefCell::new(None),
+            doubletap_gesture_recognizer: RefCell::new(None),
+            rotation_gesture_recognizer: RefCell::new(None),
+            pan_gesture_recognizer: RefCell::new(None),
+
+            rotation_last_delta: Cell::new(0.0),
+            pinch_last_delta: Cell::new(0.0),
+            pan_last_delta: Cell::new(CGPoint { x: 0.0, y: 0.0 }),
+        });
+        let this: Retained<Self> = unsafe { msg_send_id![super(this), initWithFrame: frame] };
+
+        this.setMultipleTouchEnabled(true);
+
+        if let Some(scale_factor) = window_attributes.platform_specific.scale_factor {
+            this.setContentScaleFactor(scale_factor as _);
+        }
+
+        this
+    }
+
+    fn window(&self) -> Option<Retained<WinitUIWindow>> {
+        // SAFETY: `WinitView`s are always installed in a `WinitUIWindow`
+        (**self).window().map(|window| unsafe { Retained::cast(window) })
+    }
+
+    pub(crate) fn recognize_pinch_gesture(&self, should_recognize: bool) {
+        let mtm = MainThreadMarker::from(self);
+        if should_recognize {
+            if self.ivars().pinch_gesture_recognizer.borrow().is_none() {
+                let pinch = unsafe {
+                    UIPinchGestureRecognizer::initWithTarget_action(
+                        mtm.alloc(),
+                        Some(self),
+                        Some(sel!(pinchGesture:)),
+                    )
+                };
+                pinch.setDelegate(Some(ProtocolObject::from_ref(self)));
+                self.addGestureRecognizer(&pinch);
+                self.ivars().pinch_gesture_recognizer.replace(Some(pinch));
+            }
+        } else if let Some(recognizer) = self.ivars().pinch_gesture_recognizer.take() {
+            self.removeGestureRecognizer(&recognizer);
+        }
+    }
+
+    pub(crate) fn recognize_pan_gesture(
+        &self,
+        should_recognize: bool,
+        minimum_number_of_touches: u8,
+        maximum_number_of_touches: u8,
+    ) {
+        let mtm = MainThreadMarker::from(self);
+        if should_recognize {
+            if self.ivars().pan_gesture_recognizer.borrow().is_none() {
+                let pan = unsafe {
+                    UIPanGestureRecognizer::initWithTarget_action(
+                        mtm.alloc(),
+                        Some(self),
+                        Some(sel!(panGesture:)),
+                    )
+                };
+                pan.setDelegate(Some(ProtocolObject::from_ref(self)));
+                pan.setMinimumNumberOfTouches(minimum_number_of_touches as _);
+                pan.setMaximumNumberOfTouches(maximum_number_of_touches as _);
+                self.addGestureRecognizer(&pan);
+                self.ivars().pan_gesture_recognizer.replace(Some(pan));
+            }
+        } else if let Some(recognizer) = self.ivars().pan_gesture_recognizer.take() {
+            self.removeGestureRecognizer(&recognizer);
+        }
+    }
+
+    pub(crate) fn recognize_doubletap_gesture(&self, should_recognize: bool) {
+        let mtm = MainThreadMarker::from(self);
+        if should_recognize {
+            if self.ivars().doubletap_gesture_recognizer.borrow().is_none() {
+                let tap = unsafe {
+                    UITapGestureRecognizer::initWithTarget_action(
+                        mtm.alloc(),
+                        Some(self),
+                        Some(sel!(doubleTapGesture:)),
+                    )
+                };
+                tap.setDelegate(Some(ProtocolObject::from_ref(self)));
+                tap.setNumberOfTapsRequired(2);
+                tap.setNumberOfTouchesRequired(1);
+                self.addGestureRecognizer(&tap);
+                self.ivars().doubletap_gesture_recognizer.replace(Some(tap));
+            }
+        } else if let Some(recognizer) = self.ivars().doubletap_gesture_recognizer.take() {
+            self.removeGestureRecognizer(&recognizer);
+        }
+    }
+
+    pub(crate) fn recognize_rotation_gesture(&self, should_recognize: bool) {
+        let mtm = MainThreadMarker::from(self);
+        if should_recognize {
+            if self.ivars().rotation_gesture_recognizer.borrow().is_none() {
+                let rotation = unsafe {
+                    UIRotationGestureRecognizer::initWithTarget_action(
+                        mtm.alloc(),
+                        Some(self),
+                        Some(sel!(rotationGesture:)),
+                    )
+                };
+                rotation.setDelegate(Some(ProtocolObject::from_ref(self)));
+                self.addGestureRecognizer(&rotation);
+                self.ivars().rotation_gesture_recognizer.replace(Some(rotation));
+            }
+        } else if let Some(recognizer) = self.ivars().rotation_gesture_recognizer.take() {
+            self.removeGestureRecognizer(&recognizer);
+        }
+    }
+
+    fn handle_touches(&self, touches: &NSSet<UITouch>) {
+        let window = self.window().unwrap();
+        let mut touch_events = Vec::new();
+        let os_supports_force = app_state::os_capabilities().force_touch;
+        for touch in touches {
+            let logical_location = touch.locationInView(None);
+            let touch_type = touch.r#type();
+            let force = if os_supports_force {
+                let trait_collection = self.traitCollection();
+                let touch_capability = trait_collection.forceTouchCapability();
+                // Both the OS _and_ the device need to be checked for force touch support.
+                if touch_capability == UIForceTouchCapability::Available
+                    || touch_type == UITouchType::Pencil
+                {
+                    let force = touch.force();
+                    let max_possible_force = touch.maximumPossibleForce();
+                    let altitude_angle: Option<f64> = if touch_type == UITouchType::Pencil {
+                        let angle = touch.altitudeAngle();
+                        Some(angle as _)
+                    } else {
+                        None
+                    };
+                    Some(Force::Calibrated {
+                        force: force as _,
+                        max_possible_force: max_possible_force as _,
+                        altitude_angle,
+                    })
+                } else {
+                    None
+                }
+            } else {
+                None
+            };
+            let touch_id = touch as *const UITouch as u64;
+            let phase = touch.phase();
+            let phase = match phase {
+                UITouchPhase::Began => TouchPhase::Started,
+                UITouchPhase::Moved => TouchPhase::Moved,
+                // 2 is UITouchPhase::Stationary and is not expected here
+                UITouchPhase::Ended => TouchPhase::Ended,
+                UITouchPhase::Cancelled => TouchPhase::Cancelled,
+                _ => panic!("unexpected touch phase: {phase:?}"),
+            };
+
+            let physical_location = {
+                let scale_factor = self.contentScaleFactor();
+                PhysicalPosition::from_logical::<(f64, f64), f64>(
+                    (logical_location.x as _, logical_location.y as _),
+                    scale_factor as f64,
+                )
+            };
+            touch_events.push(EventWrapper::StaticEvent(Event::WindowEvent {
+                window_id: RootWindowId(window.id()),
+                event: WindowEvent::Touch(Touch {
+                    device_id: DEVICE_ID,
+                    id: touch_id,
+                    location: physical_location,
+                    force,
+                    phase,
+                }),
+            }));
+        }
+        let mtm = MainThreadMarker::new().unwrap();
+        app_state::handle_nonuser_events(mtm, touch_events);
+    }
+}

src/platform_impl/apple/uikit/view_controller.rs

@@ -0,0 +1,176 @@
+use std::cell::Cell;
+
+use objc2::rc::Retained;
+use objc2::{declare_class, msg_send_id, mutability, ClassType, DeclaredClass};
+use objc2_foundation::{MainThreadMarker, NSObject};
+use objc2_ui_kit::{
+    UIDevice, UIInterfaceOrientationMask, UIRectEdge, UIResponder, UIStatusBarStyle,
+    UIUserInterfaceIdiom, UIView, UIViewController,
+};
+
+use super::app_state::{self};
+use crate::platform::ios::{ScreenEdge, StatusBarStyle, ValidOrientations};
+use crate::window::WindowAttributes;
+
+pub struct ViewControllerState {
+    prefers_status_bar_hidden: Cell<bool>,
+    preferred_status_bar_style: Cell<UIStatusBarStyle>,
+    prefers_home_indicator_auto_hidden: Cell<bool>,
+    supported_orientations: Cell<UIInterfaceOrientationMask>,
+    preferred_screen_edges_deferring_system_gestures: Cell<UIRectEdge>,
+}
+
+declare_class!(
+    pub(crate) struct WinitViewController;
+
+    unsafe impl ClassType for WinitViewController {
+        #[inherits(UIResponder, NSObject)]
+        type Super = UIViewController;
+        type Mutability = mutability::MainThreadOnly;
+        const NAME: &'static str = "WinitUIViewController";
+    }
+
+    impl DeclaredClass for WinitViewController {
+        type Ivars = ViewControllerState;
+    }
+
+    unsafe impl WinitViewController {
+        #[method(shouldAutorotate)]
+        fn should_autorotate(&self) -> bool {
+            true
+        }
+
+        #[method(prefersStatusBarHidden)]
+        fn prefers_status_bar_hidden(&self) -> bool {
+            self.ivars().prefers_status_bar_hidden.get()
+        }
+
+        #[method(preferredStatusBarStyle)]
+        fn preferred_status_bar_style(&self) -> UIStatusBarStyle {
+            self.ivars().preferred_status_bar_style.get()
+        }
+
+        #[method(prefersHomeIndicatorAutoHidden)]
+        fn prefers_home_indicator_auto_hidden(&self) -> bool {
+            self.ivars().prefers_home_indicator_auto_hidden.get()
+        }
+
+        #[method(supportedInterfaceOrientations)]
+        fn supported_orientations(&self) -> UIInterfaceOrientationMask {
+            self.ivars().supported_orientations.get()
+        }
+
+        #[method(preferredScreenEdgesDeferringSystemGestures)]
+        fn preferred_screen_edges_deferring_system_gestures(&self) -> UIRectEdge {
+            self.ivars()
+                .preferred_screen_edges_deferring_system_gestures
+                .get()
+        }
+    }
+);
+
+impl WinitViewController {
+    pub(crate) fn set_prefers_status_bar_hidden(&self, val: bool) {
+        self.ivars().prefers_status_bar_hidden.set(val);
+        self.setNeedsStatusBarAppearanceUpdate();
+    }
+
+    pub(crate) fn set_preferred_status_bar_style(&self, val: StatusBarStyle) {
+        let val = match val {
+            StatusBarStyle::Default => UIStatusBarStyle::Default,
+            StatusBarStyle::LightContent => UIStatusBarStyle::LightContent,
+            StatusBarStyle::DarkContent => UIStatusBarStyle::DarkContent,
+        };
+        self.ivars().preferred_status_bar_style.set(val);
+        self.setNeedsStatusBarAppearanceUpdate();
+    }
+
+    pub(crate) fn set_prefers_home_indicator_auto_hidden(&self, val: bool) {
+        self.ivars().prefers_home_indicator_auto_hidden.set(val);
+        let os_capabilities = app_state::os_capabilities();
+        if os_capabilities.home_indicator_hidden {
+            self.setNeedsUpdateOfHomeIndicatorAutoHidden();
+        } else {
+            os_capabilities.home_indicator_hidden_err_msg("ignoring")
+        }
+    }
+
+    pub(crate) fn set_preferred_screen_edges_deferring_system_gestures(&self, val: ScreenEdge) {
+        let val = {
+            assert_eq!(val.bits() & !ScreenEdge::ALL.bits(), 0, "invalid `ScreenEdge`");
+            UIRectEdge(val.bits().into())
+        };
+        self.ivars().preferred_screen_edges_deferring_system_gestures.set(val);
+        let os_capabilities = app_state::os_capabilities();
+        if os_capabilities.defer_system_gestures {
+            self.setNeedsUpdateOfScreenEdgesDeferringSystemGestures();
+        } else {
+            os_capabilities.defer_system_gestures_err_msg("ignoring")
+        }
+    }
+
+    pub(crate) fn set_supported_interface_orientations(
+        &self,
+        mtm: MainThreadMarker,
+        valid_orientations: ValidOrientations,
+    ) {
+        let mask = match (valid_orientations, UIDevice::currentDevice(mtm).userInterfaceIdiom()) {
+            (ValidOrientations::LandscapeAndPortrait, UIUserInterfaceIdiom::Phone) => {
+                UIInterfaceOrientationMask::AllButUpsideDown
+            },
+            (ValidOrientations::LandscapeAndPortrait, _) => UIInterfaceOrientationMask::All,
+            (ValidOrientations::Landscape, _) => UIInterfaceOrientationMask::Landscape,
+            (ValidOrientations::Portrait, UIUserInterfaceIdiom::Phone) => {
+                UIInterfaceOrientationMask::Portrait
+            },
+            (ValidOrientations::Portrait, _) => {
+                UIInterfaceOrientationMask::Portrait
+                    | UIInterfaceOrientationMask::PortraitUpsideDown
+            },
+        };
+        self.ivars().supported_orientations.set(mask);
+        #[allow(deprecated)]
+        UIViewController::attemptRotationToDeviceOrientation(mtm);
+    }
+
+    pub(crate) fn new(
+        mtm: MainThreadMarker,
+        window_attributes: &WindowAttributes,
+        view: &UIView,
+    ) -> Retained<Self> {
+        // These are set properly below, we just to set them to something in the meantime.
+        let this = mtm.alloc().set_ivars(ViewControllerState {
+            prefers_status_bar_hidden: Cell::new(false),
+            preferred_status_bar_style: Cell::new(UIStatusBarStyle::Default),
+            prefers_home_indicator_auto_hidden: Cell::new(false),
+            supported_orientations: Cell::new(UIInterfaceOrientationMask::All),
+            preferred_screen_edges_deferring_system_gestures: Cell::new(UIRectEdge::empty()),
+        });
+        let this: Retained<Self> = unsafe { msg_send_id![super(this), init] };
+
+        this.set_prefers_status_bar_hidden(
+            window_attributes.platform_specific.prefers_status_bar_hidden,
+        );
+
+        this.set_preferred_status_bar_style(
+            window_attributes.platform_specific.preferred_status_bar_style,
+        );
+
+        this.set_supported_interface_orientations(
+            mtm,
+            window_attributes.platform_specific.valid_orientations,
+        );
+
+        this.set_prefers_home_indicator_auto_hidden(
+            window_attributes.platform_specific.prefers_home_indicator_hidden,
+        );
+
+        this.set_preferred_screen_edges_deferring_system_gestures(
+            window_attributes.platform_specific.preferred_screen_edges_deferring_system_gestures,
+        );
+
+        this.setView(Some(view));
+
+        this
+    }
+}

src/platform_impl/apple/uikit/window.rs

@@ -0,0 +1,736 @@
+#![allow(clippy::unnecessary_cast)]
+
+use std::collections::VecDeque;
+
+use objc2::rc::Retained;
+use objc2::runtime::{AnyObject, NSObject};
+use objc2::{class, declare_class, msg_send, msg_send_id, mutability, ClassType, DeclaredClass};
+use objc2_foundation::{
+    CGFloat, CGPoint, CGRect, CGSize, MainThreadBound, MainThreadMarker, NSObjectProtocol,
+};
+use objc2_ui_kit::{
+    UIApplication, UICoordinateSpace, UIResponder, UIScreen, UIScreenOverscanCompensation,
+    UIViewController, UIWindow,
+};
+use tracing::{debug, warn};
+
+use super::app_state::EventWrapper;
+use super::view::WinitView;
+use super::view_controller::WinitViewController;
+use super::{app_state, monitor, ActiveEventLoop, Fullscreen, MonitorHandle};
+use crate::cursor::Cursor;
+use crate::dpi::{LogicalPosition, LogicalSize, PhysicalPosition, PhysicalSize, Position, Size};
+use crate::error::{ExternalError, NotSupportedError, OsError as RootOsError};
+use crate::event::{Event, WindowEvent};
+use crate::icon::Icon;
+use crate::platform::ios::{ScreenEdge, StatusBarStyle, ValidOrientations};
+use crate::window::{
+    CursorGrabMode, ImePurpose, ResizeDirection, Theme, UserAttentionType, WindowAttributes,
+    WindowButtons, WindowId as RootWindowId, WindowLevel,
+};
+
+declare_class!(
+    #[derive(Debug, PartialEq, Eq, Hash)]
+    pub(crate) struct WinitUIWindow;
+
+    unsafe impl ClassType for WinitUIWindow {
+        #[inherits(UIResponder, NSObject)]
+        type Super = UIWindow;
+        type Mutability = mutability::MainThreadOnly;
+        const NAME: &'static str = "WinitUIWindow";
+    }
+
+    impl DeclaredClass for WinitUIWindow {}
+
+    unsafe impl WinitUIWindow {
+        #[method(becomeKeyWindow)]
+        fn become_key_window(&self) {
+            let mtm = MainThreadMarker::new().unwrap();
+            app_state::handle_nonuser_event(
+                mtm,
+                EventWrapper::StaticEvent(Event::WindowEvent {
+                    window_id: RootWindowId(self.id()),
+                    event: WindowEvent::Focused(true),
+                }),
+            );
+            let _: () = unsafe { msg_send![super(self), becomeKeyWindow] };
+        }
+
+        #[method(resignKeyWindow)]
+        fn resign_key_window(&self) {
+            let mtm = MainThreadMarker::new().unwrap();
+            app_state::handle_nonuser_event(
+                mtm,
+                EventWrapper::StaticEvent(Event::WindowEvent {
+                    window_id: RootWindowId(self.id()),
+                    event: WindowEvent::Focused(false),
+                }),
+            );
+            let _: () = unsafe { msg_send![super(self), resignKeyWindow] };
+        }
+    }
+);
+
+impl WinitUIWindow {
+    pub(crate) fn new(
+        mtm: MainThreadMarker,
+        window_attributes: &WindowAttributes,
+        frame: CGRect,
+        view_controller: &UIViewController,
+    ) -> Retained<Self> {
+        let this: Retained<Self> = unsafe { msg_send_id![mtm.alloc(), initWithFrame: frame] };
+
+        this.setRootViewController(Some(view_controller));
+
+        match window_attributes.fullscreen.clone().map(Into::into) {
+            Some(Fullscreen::Exclusive(ref video_mode)) => {
+                let monitor = video_mode.monitor();
+                let screen = monitor.ui_screen(mtm);
+                screen.setCurrentMode(Some(video_mode.screen_mode(mtm)));
+                this.setScreen(screen);
+            },
+            Some(Fullscreen::Borderless(Some(ref monitor))) => {
+                let screen = monitor.ui_screen(mtm);
+                this.setScreen(screen);
+            },
+            _ => (),
+        }
+
+        this
+    }
+
+    pub(crate) fn id(&self) -> WindowId {
+        (self as *const Self as usize as u64).into()
+    }
+}
+
+pub struct Inner {
+    window: Retained<WinitUIWindow>,
+    view_controller: Retained<WinitViewController>,
+    view: Retained<WinitView>,
+    gl_or_metal_backed: bool,
+}
+
+impl Inner {
+    pub fn set_title(&self, _title: &str) {
+        debug!("`Window::set_title` is ignored on iOS")
+    }
+
+    pub fn set_transparent(&self, _transparent: bool) {
+        debug!("`Window::set_transparent` is ignored on iOS")
+    }
+
+    pub fn set_blur(&self, _blur: bool) {
+        debug!("`Window::set_blur` is ignored on iOS")
+    }
+
+    pub fn set_visible(&self, visible: bool) {
+        self.window.setHidden(!visible)
+    }
+
+    pub fn is_visible(&self) -> Option<bool> {
+        warn!("`Window::is_visible` is ignored on iOS");
+        None
+    }
+
+    pub fn request_redraw(&self) {
+        if self.gl_or_metal_backed {
+            let mtm = MainThreadMarker::new().unwrap();
+            // `setNeedsDisplay` does nothing on UIViews which are directly backed by CAEAGLLayer or
+            // CAMetalLayer. Ordinarily the OS sets up a bunch of UIKit state before
+            // calling drawRect: on a UIView, but when using raw or gl/metal for drawing
+            // this work is completely avoided.
+            //
+            // The docs for `setNeedsDisplay` don't mention `CAMetalLayer`; however, this has been
+            // confirmed via testing.
+            //
+            // https://developer.apple.com/documentation/uikit/uiview/1622437-setneedsdisplay?language=objc
+            app_state::queue_gl_or_metal_redraw(mtm, self.window.clone());
+        } else {
+            self.view.setNeedsDisplay();
+        }
+    }
+
+    pub fn pre_present_notify(&self) {}
+
+    pub fn inner_position(&self) -> Result<PhysicalPosition<i32>, NotSupportedError> {
+        let safe_area = self.safe_area_screen_space();
+        let position =
+            LogicalPosition { x: safe_area.origin.x as f64, y: safe_area.origin.y as f64 };
+        let scale_factor = self.scale_factor();
+        Ok(position.to_physical(scale_factor))
+    }
+
+    pub fn outer_position(&self) -> Result<PhysicalPosition<i32>, NotSupportedError> {
+        let screen_frame = self.screen_frame();
+        let position =
+            LogicalPosition { x: screen_frame.origin.x as f64, y: screen_frame.origin.y as f64 };
+        let scale_factor = self.scale_factor();
+        Ok(position.to_physical(scale_factor))
+    }
+
+    pub fn set_outer_position(&self, physical_position: Position) {
+        let scale_factor = self.scale_factor();
+        let position = physical_position.to_logical::<f64>(scale_factor);
+        let screen_frame = self.screen_frame();
+        let new_screen_frame = CGRect {
+            origin: CGPoint { x: position.x as _, y: position.y as _ },
+            size: screen_frame.size,
+        };
+        let bounds = self.rect_from_screen_space(new_screen_frame);
+        self.window.setBounds(bounds);
+    }
+
+    pub fn inner_size(&self) -> PhysicalSize<u32> {
+        let scale_factor = self.scale_factor();
+        let safe_area = self.safe_area_screen_space();
+        let size = LogicalSize {
+            width: safe_area.size.width as f64,
+            height: safe_area.size.height as f64,
+        };
+        size.to_physical(scale_factor)
+    }
+
+    pub fn outer_size(&self) -> PhysicalSize<u32> {
+        let scale_factor = self.scale_factor();
+        let screen_frame = self.screen_frame();
+        let size = LogicalSize {
+            width: screen_frame.size.width as f64,
+            height: screen_frame.size.height as f64,
+        };
+        size.to_physical(scale_factor)
+    }
+
+    pub fn request_inner_size(&self, _size: Size) -> Option<PhysicalSize<u32>> {
+        Some(self.inner_size())
+    }
+
+    pub fn set_min_inner_size(&self, _dimensions: Option<Size>) {
+        warn!("`Window::set_min_inner_size` is ignored on iOS")
+    }
+
+    pub fn set_max_inner_size(&self, _dimensions: Option<Size>) {
+        warn!("`Window::set_max_inner_size` is ignored on iOS")
+    }
+
+    pub fn resize_increments(&self) -> Option<PhysicalSize<u32>> {
+        None
+    }
+
+    #[inline]
+    pub fn set_resize_increments(&self, _increments: Option<Size>) {
+        warn!("`Window::set_resize_increments` is ignored on iOS")
+    }
+
+    pub fn set_resizable(&self, _resizable: bool) {
+        warn!("`Window::set_resizable` is ignored on iOS")
+    }
+
+    pub fn is_resizable(&self) -> bool {
+        warn!("`Window::is_resizable` is ignored on iOS");
+        false
+    }
+
+    #[inline]
+    pub fn set_enabled_buttons(&self, _buttons: WindowButtons) {
+        warn!("`Window::set_enabled_buttons` is ignored on iOS");
+    }
+
+    #[inline]
+    pub fn enabled_buttons(&self) -> WindowButtons {
+        warn!("`Window::enabled_buttons` is ignored on iOS");
+        WindowButtons::all()
+    }
+
+    pub fn scale_factor(&self) -> f64 {
+        self.view.contentScaleFactor() as _
+    }
+
+    pub fn set_cursor(&self, _cursor: Cursor) {
+        debug!("`Window::set_cursor` ignored on iOS")
+    }
+
+    pub fn set_cursor_position(&self, _position: Position) -> Result<(), ExternalError> {
+        Err(ExternalError::NotSupported(NotSupportedError::new()))
+    }
+
+    pub fn set_cursor_grab(&self, _: CursorGrabMode) -> Result<(), ExternalError> {
+        Err(ExternalError::NotSupported(NotSupportedError::new()))
+    }
+
+    pub fn set_cursor_visible(&self, _visible: bool) {
+        debug!("`Window::set_cursor_visible` is ignored on iOS")
+    }
+
+    pub fn drag_window(&self) -> Result<(), ExternalError> {
+        Err(ExternalError::NotSupported(NotSupportedError::new()))
+    }
+
+    pub fn drag_resize_window(&self, _direction: ResizeDirection) -> Result<(), ExternalError> {
+        Err(ExternalError::NotSupported(NotSupportedError::new()))
+    }
+
+    #[inline]
+    pub fn show_window_menu(&self, _position: Position) {}
+
+    pub fn set_cursor_hittest(&self, _hittest: bool) -> Result<(), ExternalError> {
+        Err(ExternalError::NotSupported(NotSupportedError::new()))
+    }
+
+    pub fn set_minimized(&self, _minimized: bool) {
+        warn!("`Window::set_minimized` is ignored on iOS")
+    }
+
+    pub fn is_minimized(&self) -> Option<bool> {
+        warn!("`Window::is_minimized` is ignored on iOS");
+        None
+    }
+
+    pub fn set_maximized(&self, _maximized: bool) {
+        warn!("`Window::set_maximized` is ignored on iOS")
+    }
+
+    pub fn is_maximized(&self) -> bool {
+        warn!("`Window::is_maximized` is ignored on iOS");
+        false
+    }
+
+    pub(crate) fn set_fullscreen(&self, monitor: Option<Fullscreen>) {
+        let mtm = MainThreadMarker::new().unwrap();
+        let uiscreen = match &monitor {
+            Some(Fullscreen::Exclusive(video_mode)) => {
+                let uiscreen = video_mode.monitor.ui_screen(mtm);
+                uiscreen.setCurrentMode(Some(video_mode.screen_mode(mtm)));
+                uiscreen.clone()
+            },
+            Some(Fullscreen::Borderless(Some(monitor))) => monitor.ui_screen(mtm).clone(),
+            Some(Fullscreen::Borderless(None)) => {
+                self.current_monitor_inner().ui_screen(mtm).clone()
+            },
+            None => {
+                warn!("`Window::set_fullscreen(None)` ignored on iOS");
+                return;
+            },
+        };
+
+        // this is pretty slow on iOS, so avoid doing it if we can
+        let current = self.window.screen();
+        if uiscreen != current {
+            self.window.setScreen(&uiscreen);
+        }
+
+        let bounds = uiscreen.bounds();
+        self.window.setFrame(bounds);
+
+        // For external displays, we must disable overscan compensation or
+        // the displayed image will have giant black bars surrounding it on
+        // each side
+        uiscreen.setOverscanCompensation(UIScreenOverscanCompensation::None);
+    }
+
+    pub(crate) fn fullscreen(&self) -> Option<Fullscreen> {
+        let mtm = MainThreadMarker::new().unwrap();
+        let monitor = self.current_monitor_inner();
+        let uiscreen = monitor.ui_screen(mtm);
+        let screen_space_bounds = self.screen_frame();
+        let screen_bounds = uiscreen.bounds();
+
+        // TODO: track fullscreen instead of relying on brittle float comparisons
+        if screen_space_bounds.origin.x == screen_bounds.origin.x
+            && screen_space_bounds.origin.y == screen_bounds.origin.y
+            && screen_space_bounds.size.width == screen_bounds.size.width
+            && screen_space_bounds.size.height == screen_bounds.size.height
+        {
+            Some(Fullscreen::Borderless(Some(monitor)))
+        } else {
+            None
+        }
+    }
+
+    pub fn set_decorations(&self, _decorations: bool) {}
+
+    pub fn is_decorated(&self) -> bool {
+        true
+    }
+
+    pub fn set_window_level(&self, _level: WindowLevel) {
+        warn!("`Window::set_window_level` is ignored on iOS")
+    }
+
+    pub fn set_window_icon(&self, _icon: Option<Icon>) {
+        warn!("`Window::set_window_icon` is ignored on iOS")
+    }
+
+    pub fn set_ime_cursor_area(&self, _position: Position, _size: Size) {
+        warn!("`Window::set_ime_cursor_area` is ignored on iOS")
+    }
+
+    pub fn set_ime_allowed(&self, _allowed: bool) {
+        warn!("`Window::set_ime_allowed` is ignored on iOS")
+    }
+
+    pub fn set_ime_purpose(&self, _purpose: ImePurpose) {
+        warn!("`Window::set_ime_allowed` is ignored on iOS")
+    }
+
+    pub fn focus_window(&self) {
+        warn!("`Window::set_focus` is ignored on iOS")
+    }
+
+    pub fn request_user_attention(&self, _request_type: Option<UserAttentionType>) {
+        warn!("`Window::request_user_attention` is ignored on iOS")
+    }
+
+    // Allow directly accessing the current monitor internally without unwrapping.
+    fn current_monitor_inner(&self) -> MonitorHandle {
+        MonitorHandle::new(self.window.screen())
+    }
+
+    pub fn current_monitor(&self) -> Option<MonitorHandle> {
+        Some(self.current_monitor_inner())
+    }
+
+    pub fn available_monitors(&self) -> VecDeque<MonitorHandle> {
+        monitor::uiscreens(MainThreadMarker::new().unwrap())
+    }
+
+    pub fn primary_monitor(&self) -> Option<MonitorHandle> {
+        #[allow(deprecated)]
+        Some(MonitorHandle::new(UIScreen::mainScreen(MainThreadMarker::new().unwrap())))
+    }
+
+    pub fn id(&self) -> WindowId {
+        self.window.id()
+    }
+
+    #[cfg(feature = "rwh_04")]
+    pub fn raw_window_handle_rwh_04(&self) -> rwh_04::RawWindowHandle {
+        let mut window_handle = rwh_04::UiKitHandle::empty();
+        window_handle.ui_window = Retained::as_ptr(&self.window) as _;
+        window_handle.ui_view = Retained::as_ptr(&self.view) as _;
+        window_handle.ui_view_controller = Retained::as_ptr(&self.view_controller) as _;
+        rwh_04::RawWindowHandle::UiKit(window_handle)
+    }
+
+    #[cfg(feature = "rwh_05")]
+    pub fn raw_window_handle_rwh_05(&self) -> rwh_05::RawWindowHandle {
+        let mut window_handle = rwh_05::UiKitWindowHandle::empty();
+        window_handle.ui_window = Retained::as_ptr(&self.window) as _;
+        window_handle.ui_view = Retained::as_ptr(&self.view) as _;
+        window_handle.ui_view_controller = Retained::as_ptr(&self.view_controller) as _;
+        rwh_05::RawWindowHandle::UiKit(window_handle)
+    }
+
+    #[cfg(feature = "rwh_05")]
+    pub fn raw_display_handle_rwh_05(&self) -> rwh_05::RawDisplayHandle {
+        rwh_05::RawDisplayHandle::UiKit(rwh_05::UiKitDisplayHandle::empty())
+    }
+
+    #[cfg(feature = "rwh_06")]
+    pub fn raw_window_handle_rwh_06(&self) -> rwh_06::RawWindowHandle {
+        let mut window_handle = rwh_06::UiKitWindowHandle::new({
+            let ui_view = Retained::as_ptr(&self.view) as _;
+            std::ptr::NonNull::new(ui_view).expect("Retained<T> should never be null")
+        });
+        window_handle.ui_view_controller =
+            std::ptr::NonNull::new(Retained::as_ptr(&self.view_controller) as _);
+        rwh_06::RawWindowHandle::UiKit(window_handle)
+    }
+
+    pub fn theme(&self) -> Option<Theme> {
+        warn!("`Window::theme` is ignored on iOS");
+        None
+    }
+
+    pub fn set_content_protected(&self, _protected: bool) {}
+
+    pub fn has_focus(&self) -> bool {
+        self.window.isKeyWindow()
+    }
+
+    #[inline]
+    pub fn set_theme(&self, _theme: Option<Theme>) {
+        warn!("`Window::set_theme` is ignored on iOS");
+    }
+
+    pub fn title(&self) -> String {
+        warn!("`Window::title` is ignored on iOS");
+        String::new()
+    }
+
+    pub fn reset_dead_keys(&self) {
+        // Noop
+    }
+}
+
+pub struct Window {
+    inner: MainThreadBound<Inner>,
+}
+
+impl Window {
+    pub(crate) fn new(
+        event_loop: &ActiveEventLoop,
+        window_attributes: WindowAttributes,
+    ) -> Result<Window, RootOsError> {
+        let mtm = event_loop.mtm;
+
+        if window_attributes.min_inner_size.is_some() {
+            warn!("`WindowAttributes::min_inner_size` is ignored on iOS");
+        }
+        if window_attributes.max_inner_size.is_some() {
+            warn!("`WindowAttributes::max_inner_size` is ignored on iOS");
+        }
+
+        // TODO: transparency, visible
+
+        #[allow(deprecated)]
+        let main_screen = UIScreen::mainScreen(mtm);
+        let fullscreen = window_attributes.fullscreen.clone().map(Into::into);
+        let screen = match fullscreen {
+            Some(Fullscreen::Exclusive(ref video_mode)) => video_mode.monitor.ui_screen(mtm),
+            Some(Fullscreen::Borderless(Some(ref monitor))) => monitor.ui_screen(mtm),
+            Some(Fullscreen::Borderless(None)) | None => &main_screen,
+        };
+
+        let screen_bounds = screen.bounds();
+
+        let frame = match window_attributes.inner_size {
+            Some(dim) => {
+                let scale_factor = screen.scale();
+                let size = dim.to_logical::<f64>(scale_factor as f64);
+                CGRect {
+                    origin: screen_bounds.origin,
+                    size: CGSize { width: size.width as _, height: size.height as _ },
+                }
+            },
+            None => screen_bounds,
+        };
+
+        let view = WinitView::new(mtm, &window_attributes, frame);
+
+        let gl_or_metal_backed =
+            view.isKindOfClass(class!(CAMetalLayer)) || view.isKindOfClass(class!(CAEAGLLayer));
+
+        let view_controller = WinitViewController::new(mtm, &window_attributes, &view);
+        let window = WinitUIWindow::new(mtm, &window_attributes, frame, &view_controller);
+
+        app_state::set_key_window(mtm, &window);
+
+        // Like the Windows and macOS backends, we send a `ScaleFactorChanged` and `Resized`
+        // event on window creation if the DPI factor != 1.0
+        let scale_factor = view.contentScaleFactor();
+        let scale_factor = scale_factor as f64;
+        if scale_factor != 1.0 {
+            let bounds = view.bounds();
+            let screen = window.screen();
+            let screen_space = screen.coordinateSpace();
+            let screen_frame = view.convertRect_toCoordinateSpace(bounds, &screen_space);
+            let size = LogicalSize {
+                width: screen_frame.size.width as f64,
+                height: screen_frame.size.height as f64,
+            };
+            let window_id = RootWindowId(window.id());
+            app_state::handle_nonuser_events(
+                mtm,
+                std::iter::once(EventWrapper::ScaleFactorChanged(app_state::ScaleFactorChanged {
+                    window: window.clone(),
+                    scale_factor,
+                    suggested_size: size.to_physical(scale_factor),
+                }))
+                .chain(std::iter::once(EventWrapper::StaticEvent(
+                    Event::WindowEvent {
+                        window_id,
+                        event: WindowEvent::Resized(size.to_physical(scale_factor)),
+                    },
+                ))),
+            );
+        }
+
+        let inner = Inner { window, view_controller, view, gl_or_metal_backed };
+        Ok(Window { inner: MainThreadBound::new(inner, mtm) })
+    }
+
+    pub(crate) fn maybe_queue_on_main(&self, f: impl FnOnce(&Inner) + Send + 'static) {
+        // For now, don't actually do queuing, since it may be less predictable
+        self.maybe_wait_on_main(f)
+    }
+
+    pub(crate) fn maybe_wait_on_main<R: Send>(&self, f: impl FnOnce(&Inner) -> R + Send) -> R {
+        self.inner.get_on_main(|inner| f(inner))
+    }
+
+    #[cfg(feature = "rwh_06")]
+    #[inline]
+    pub(crate) fn raw_window_handle_rwh_06(
+        &self,
+    ) -> Result<rwh_06::RawWindowHandle, rwh_06::HandleError> {
+        if let Some(mtm) = MainThreadMarker::new() {
+            Ok(self.inner.get(mtm).raw_window_handle_rwh_06())
+        } else {
+            Err(rwh_06::HandleError::Unavailable)
+        }
+    }
+
+    #[cfg(feature = "rwh_06")]
+    #[inline]
+    pub(crate) fn raw_display_handle_rwh_06(
+        &self,
+    ) -> Result<rwh_06::RawDisplayHandle, rwh_06::HandleError> {
+        Ok(rwh_06::RawDisplayHandle::UiKit(rwh_06::UiKitDisplayHandle::new()))
+    }
+}
+
+// WindowExtIOS
+impl Inner {
+    pub fn set_scale_factor(&self, scale_factor: f64) {
+        assert!(
+            dpi::validate_scale_factor(scale_factor),
+            "`WindowExtIOS::set_scale_factor` received an invalid hidpi factor"
+        );
+        let scale_factor = scale_factor as CGFloat;
+        self.view.setContentScaleFactor(scale_factor);
+    }
+
+    pub fn set_valid_orientations(&self, valid_orientations: ValidOrientations) {
+        self.view_controller.set_supported_interface_orientations(
+            MainThreadMarker::new().unwrap(),
+            valid_orientations,
+        );
+    }
+
+    pub fn set_prefers_home_indicator_hidden(&self, hidden: bool) {
+        self.view_controller.set_prefers_home_indicator_auto_hidden(hidden);
+    }
+
+    pub fn set_preferred_screen_edges_deferring_system_gestures(&self, edges: ScreenEdge) {
+        self.view_controller.set_preferred_screen_edges_deferring_system_gestures(edges);
+    }
+
+    pub fn set_prefers_status_bar_hidden(&self, hidden: bool) {
+        self.view_controller.set_prefers_status_bar_hidden(hidden);
+    }
+
+    pub fn set_preferred_status_bar_style(&self, status_bar_style: StatusBarStyle) {
+        self.view_controller.set_preferred_status_bar_style(status_bar_style);
+    }
+
+    pub fn recognize_pinch_gesture(&self, should_recognize: bool) {
+        self.view.recognize_pinch_gesture(should_recognize);
+    }
+
+    pub fn recognize_pan_gesture(
+        &self,
+        should_recognize: bool,
+        minimum_number_of_touches: u8,
+        maximum_number_of_touches: u8,
+    ) {
+        self.view.recognize_pan_gesture(
+            should_recognize,
+            minimum_number_of_touches,
+            maximum_number_of_touches,
+        );
+    }
+
+    pub fn recognize_doubletap_gesture(&self, should_recognize: bool) {
+        self.view.recognize_doubletap_gesture(should_recognize);
+    }
+
+    pub fn recognize_rotation_gesture(&self, should_recognize: bool) {
+        self.view.recognize_rotation_gesture(should_recognize);
+    }
+}
+
+impl Inner {
+    fn screen_frame(&self) -> CGRect {
+        self.rect_to_screen_space(self.window.bounds())
+    }
+
+    fn rect_to_screen_space(&self, rect: CGRect) -> CGRect {
+        let screen_space = self.window.screen().coordinateSpace();
+        self.window.convertRect_toCoordinateSpace(rect, &screen_space)
+    }
+
+    fn rect_from_screen_space(&self, rect: CGRect) -> CGRect {
+        let screen_space = self.window.screen().coordinateSpace();
+        self.window.convertRect_fromCoordinateSpace(rect, &screen_space)
+    }
+
+    fn safe_area_screen_space(&self) -> CGRect {
+        let bounds = self.window.bounds();
+        if app_state::os_capabilities().safe_area {
+            let safe_area = self.window.safeAreaInsets();
+            let safe_bounds = CGRect {
+                origin: CGPoint {
+                    x: bounds.origin.x + safe_area.left,
+                    y: bounds.origin.y + safe_area.top,
+                },
+                size: CGSize {
+                    width: bounds.size.width - safe_area.left - safe_area.right,
+                    height: bounds.size.height - safe_area.top - safe_area.bottom,
+                },
+            };
+            self.rect_to_screen_space(safe_bounds)
+        } else {
+            let screen_frame = self.rect_to_screen_space(bounds);
+            let status_bar_frame = {
+                let app = UIApplication::sharedApplication(MainThreadMarker::new().unwrap());
+                #[allow(deprecated)]
+                app.statusBarFrame()
+            };
+            let (y, height) = if screen_frame.origin.y > status_bar_frame.size.height {
+                (screen_frame.origin.y, screen_frame.size.height)
+            } else {
+                let y = status_bar_frame.size.height;
+                let height = screen_frame.size.height
+                    - (status_bar_frame.size.height - screen_frame.origin.y);
+                (y, height)
+            };
+            CGRect {
+                origin: CGPoint { x: screen_frame.origin.x, y },
+                size: CGSize { width: screen_frame.size.width, height },
+            }
+        }
+    }
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct WindowId {
+    window: *mut WinitUIWindow,
+}
+
+impl WindowId {
+    pub const unsafe fn dummy() -> Self {
+        WindowId { window: std::ptr::null_mut() }
+    }
+}
+
+impl From<WindowId> for u64 {
+    fn from(window_id: WindowId) -> Self {
+        window_id.window as u64
+    }
+}
+
+impl From<u64> for WindowId {
+    fn from(raw_id: u64) -> Self {
+        Self { window: raw_id as _ }
+    }
+}
+
+unsafe impl Send for WindowId {}
+unsafe impl Sync for WindowId {}
+
+impl From<&AnyObject> for WindowId {
+    fn from(window: &AnyObject) -> WindowId {
+        WindowId { window: window as *const _ as _ }
+    }
+}
+
+#[derive(Clone, Debug, Default)]
+pub struct PlatformSpecificWindowAttributes {
+    pub scale_factor: Option<f64>,
+    pub valid_orientations: ValidOrientations,
+    pub prefers_home_indicator_hidden: bool,
+    pub prefers_status_bar_hidden: bool,
+    pub preferred_status_bar_style: StatusBarStyle,
+    pub preferred_screen_edges_deferring_system_gestures: ScreenEdge,
+}