winit-web: return immediately from run_app on web

This avoids using JavaScript exceptions to support `EventLoop::run_app`
on the web, which is a huge hack, and doesn't work with the Exception
Handling Proposal for WebAssembly:
https://github.com/WebAssembly/exception-handling

This needs the application handler passed to `run_app` to be `'static`,
but that works better on iOS too anyhow (since you can't accidentally
forget to pass in state that then wouldn't be dropped when terminating).

winit-web/src/event_loop/mod.rs

@@ -39,32 +39,9 @@ impl EventLoop {
         EVENT_LOOP_CREATED.store(false, Ordering::Relaxed);
     }
 
-    pub fn run_app<A: ApplicationHandler>(self, app: A) -> ! {
-        let app = Box::new(app);
-
-        // SAFETY: The `transmute` is necessary because `run()` requires `'static`. This is safe
-        // because this function will never return and all resources not cleaned up by the point we
-        // `throw` will leak, making this actually `'static`.
-        let app = unsafe {
-            std::mem::transmute::<
-                Box<dyn ApplicationHandler + '_>,
-                Box<dyn ApplicationHandler + 'static>,
-            >(app)
-        };
-
-        self.elw.run(app, false);
-
-        // Throw an exception to break out of Rust execution and use unreachable to tell the
-        // compiler this function won't return, giving it a return type of '!'
-        backend::throw(
-            "Using exceptions for control flow, don't mind me. This isn't actually an error!",
-        );
-
-        unreachable!();
-    }
-
-    pub fn spawn_app<A: ApplicationHandler + 'static>(self, app: A) {
-        self.elw.run(Box::new(app), true);
+    pub fn run_app<A: ApplicationHandler + 'static>(self, app: A) -> Result<(), EventLoopError> {
+        self.elw.run(Box::new(app));
+        Ok(())
     }
 
     pub fn window_target(&self) -> &dyn RootActiveEventLoop {

winit-web/src/event_loop/runner.rs

@@ -48,7 +48,6 @@ struct Execution {
     exit: Cell<bool>,
     runner: RefCell<RunnerEnum>,
     suspended: Cell<bool>,
-    event_loop_recreation: Cell<bool>,
     events: RefCell<VecDeque<Event>>,
     id: Cell<usize>,
     window: web_sys::Window,
@@ -195,7 +194,6 @@ impl Shared {
                 exit: Cell::new(false),
                 runner: RefCell::new(RunnerEnum::Pending),
                 suspended: Cell::new(false),
-                event_loop_recreation: Cell::new(false),
                 events: RefCell::new(VecDeque::new()),
                 window,
                 navigator,
@@ -772,9 +770,7 @@ impl Shared {
         // * For each undropped `Window`:
         //     * The `register_redraw_request` closure.
         //     * The `destroy_fn` closure.
-        if self.0.event_loop_recreation.get() {
-            EventLoop::allow_event_loop_recreation();
-        }
+        EventLoop::allow_event_loop_recreation();
     }
 
     // Check if the event loop is currently closed
@@ -809,10 +805,6 @@ impl Shared {
         }
     }
 
-    pub fn event_loop_recreation(&self, allow: bool) {
-        self.0.event_loop_recreation.set(allow)
-    }
-
     pub(crate) fn control_flow(&self) -> ControlFlow {
         self.0.control_flow.get()
     }

winit-web/src/event_loop/window_target.rs

@@ -56,8 +56,7 @@ impl ActiveEventLoop {
         Self { runner: runner::Shared::new(), modifiers: ModifiersShared::default() }
     }
 
-    pub(crate) fn run(&self, app: Box<dyn ApplicationHandler>, event_loop_recreation: bool) {
-        self.runner.event_loop_recreation(event_loop_recreation);
+    pub(crate) fn run(&self, app: Box<dyn ApplicationHandler>) {
         self.runner.start(app, self.clone());
     }
 

winit-web/src/lib.rs

@@ -85,7 +85,6 @@ use std::task::{Context, Poll};
 use ::web_sys::HtmlCanvasElement;
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Serialize};
-use winit_core::application::ApplicationHandler;
 use winit_core::cursor::{CustomCursor, CustomCursorSource};
 use winit_core::error::NotSupportedError;
 use winit_core::event_loop::ActiveEventLoop;
@@ -237,30 +236,6 @@ impl Default for WindowAttributesWeb {
 
 /// Additional methods on `EventLoop` that are specific to the Web.
 pub trait EventLoopExtWeb {
-    /// Initializes the winit event loop.
-    ///
-    /// Unlike
-    #[cfg_attr(target_feature = "exception-handling", doc = "`run_app()`")]
-    #[cfg_attr(
-        not(target_feature = "exception-handling"),
-        doc = "[`run_app()`]"
-    )]
-    /// [^1], this returns immediately, and doesn't throw an exception in order to
-    /// satisfy its [`!`] return type.
-    ///
-    /// Once the event loop has been destroyed, it's possible to reinitialize another event loop
-    /// by calling this function again. This can be useful if you want to recreate the event loop
-    /// while the WebAssembly module is still loaded. For example, this can be used to recreate the
-    /// event loop when switching between tabs on a single page application.
-    #[rustfmt::skip]
-    ///
-    #[cfg_attr(
-        not(target_feature = "exception-handling"),
-        doc = "[`run_app()`]: EventLoop::run_app()"
-    )]
-    /// [^1]: `run_app()` is _not_ available on Wasm when the target supports `exception-handling`.
-    fn spawn_app<A: ApplicationHandler + 'static>(self, app: A);
-
     /// Sets the strategy for [`ControlFlow::Poll`].
     ///
     /// See [`PollStrategy`].

winit-web/src/web_sys/mod.rs

@@ -25,10 +25,6 @@ pub use self::resize_scaling::ResizeScaleHandle;
 pub use self::safe_area::SafeAreaHandle;
 pub use self::schedule::Schedule;
 
-pub fn throw(msg: &str) {
-    wasm_bindgen::throw_str(msg);
-}
-
 pub struct PageTransitionEventHandle {
     _show_listener: event_handle::EventListenerHandle<dyn FnMut(PageTransitionEvent)>,
     _hide_listener: event_handle::EventListenerHandle<dyn FnMut(PageTransitionEvent)>,