Add EventLoopBuilder

This commit adds an `EventLoopBuilder` struct to simplify event loop
customization and providing options to it upon creation. It also
deprecates the use of `EventLoop::with_user_event` in favor of the same
method on new builder, and replaces old platforms specific extension
traits with the new ones on the `EventLoopBuilder`.

src/event_loop.rs

@@ -10,6 +10,7 @@
 //! [event_loop_proxy]: crate::event_loop::EventLoopProxy
 //! [send_event]: crate::event_loop::EventLoopProxy::send_event
 use instant::Instant;
+use std::marker::PhantomData;
 use std::ops::Deref;
 use std::{error, fmt};
 
@@ -31,7 +32,7 @@ use crate::{event::Event, monitor::MonitorHandle, platform_impl};
 ///
 pub struct EventLoop<T: 'static> {
     pub(crate) event_loop: platform_impl::EventLoop<T>,
-    pub(crate) _marker: ::std::marker::PhantomData<*mut ()>, // Not Send nor Sync
+    pub(crate) _marker: PhantomData<*mut ()>, // Not Send nor Sync
 }
 
 /// Target that associates windows with an `EventLoop`.
@@ -42,7 +43,62 @@ pub struct EventLoop<T: 'static> {
 /// `&EventLoop`.
 pub struct EventLoopWindowTarget<T: 'static> {
     pub(crate) p: platform_impl::EventLoopWindowTarget<T>,
-    pub(crate) _marker: ::std::marker::PhantomData<*mut ()>, // Not Send nor Sync
+    pub(crate) _marker: PhantomData<*mut ()>, // Not Send nor Sync
+}
+
+/// Object that allows building the event loop.
+///
+/// This is used to make specifying options that affect the whole application
+/// easier. But note that constructing multiple event loops is not supported.
+#[derive(Debug, Clone, Default)]
+pub struct EventLoopBuilder<T: 'static> {
+    pub(crate) platform_specific: platform_impl::PlatformSpecificEventLoopAttributes,
+    _p: PhantomData<T>,
+}
+
+impl EventLoopBuilder<()> {
+    /// Start building a new event loop.
+    #[inline]
+    pub fn new() -> Self {
+        Self::with_user_event()
+    }
+}
+
+impl<T> EventLoopBuilder<T> {
+    /// Start building a new event loop, with the given type as the user event
+    /// type.
+    #[inline]
+    pub fn with_user_event() -> Self {
+        Self {
+            platform_specific: Default::default(),
+            _p: PhantomData,
+        }
+    }
+
+    /// Builds a new event loop.
+    ///
+    /// ***For cross-platform compatibility, the `EventLoop` must be created on the main thread.***
+    /// Attempting to create the event loop on a different thread will panic. This restriction isn't
+    /// strictly necessary on all platforms, but is imposed to eliminate any nasty surprises when
+    /// porting to platforms that require it. `EventLoopBuilderExt::any_thread` functions are exposed
+    /// in the relevant `platform` module if the target platform supports creating an event loop on
+    /// any thread.
+    ///
+    /// Usage will result in display backend initialisation, this can be controlled on linux
+    /// using an environment variable `WINIT_UNIX_BACKEND`. Legal values are `x11` and `wayland`.
+    /// If it is not set, winit will try to connect to a wayland connection, and if it fails will
+    /// fallback on x11. If this variable is set with any other value, winit will panic.
+    ///
+    /// ## Platform-specific
+    ///
+    /// - **iOS:** Can only be called on the main thread.
+    #[inline]
+    pub fn build(&mut self) -> EventLoop<T> {
+        EventLoop {
+            event_loop: platform_impl::EventLoop::new(&self.platform_specific),
+            _marker: PhantomData,
+        }
+    }
 }
 
 impl<T> fmt::Debug for EventLoop<T> {
@@ -119,41 +175,17 @@ impl Default for ControlFlow {
 }
 
 impl EventLoop<()> {
-    /// Builds a new event loop with a `()` as the user event type.
-    ///
-    /// ***For cross-platform compatibility, the `EventLoop` must be created on the main thread.***
-    /// Attempting to create the event loop on a different thread will panic. This restriction isn't
-    /// strictly necessary on all platforms, but is imposed to eliminate any nasty surprises when
-    /// porting to platforms that require it. `EventLoopExt::new_any_thread` functions are exposed
-    /// in the relevant `platform` module if the target platform supports creating an event loop on
-    /// any thread.
-    ///
-    /// Usage will result in display backend initialisation, this can be controlled on linux
-    /// using an environment variable `WINIT_UNIX_BACKEND`. Legal values are `x11` and `wayland`.
-    /// If it is not set, winit will try to connect to a wayland connection, and if it fails will
-    /// fallback on x11. If this variable is set with any other value, winit will panic.
-    ///
-    /// ## Platform-specific
-    ///
-    /// - **iOS:** Can only be called on the main thread.
+    /// Alias for `EventLoopBuilder::new().build()`.
+    #[inline]
     pub fn new() -> EventLoop<()> {
-        EventLoop::<()>::with_user_event()
+        EventLoopBuilder::new().build()
     }
 }
 
 impl<T> EventLoop<T> {
-    /// Builds a new event loop.
-    ///
-    /// All caveats documented in [`EventLoop::new`] apply to this function.
-    ///
-    /// ## Platform-specific
-    ///
-    /// - **iOS:** Can only be called on the main thread.
+    #[deprecated = "Use `EventLoopBuiler::<T>::with_user_event().build()` instead."]
     pub fn with_user_event() -> EventLoop<T> {
-        EventLoop {
-            event_loop: platform_impl::EventLoop::new(),
-            _marker: ::std::marker::PhantomData,
-        }
+        EventLoopBuilder::<T>::with_user_event().build()
     }
 
     /// Hijacks the calling thread and initializes the winit event loop with the provided