web: Add EventLoop::spawn (#2208)

* web: Add `EventLoop::spawn`

This is the same as `EventLoop::run`, but doesn't throw an exception in order to return `!`.

I decided to name it `spawn` rather than `run_web` because I think that's more descriptive, but I'm happy to change it to `run_web`.

Resolves #1714

* Update src/platform/web.rs

Co-authored-by: Markus Røyset <maroider@protonmail.com>

* Fix outdated names

Co-authored-by: Markus Røyset <maroider@protonmail.com>

src/platform/web.rs

@@ -5,6 +5,10 @@
 //! to retrieve the canvas from the Window. Alternatively, use the [`WindowBuilderExtWebSys`] trait
 //! to provide your own canvas.
 
+use crate::event::Event;
+use crate::event_loop::ControlFlow;
+use crate::event_loop::EventLoop;
+use crate::event_loop::EventLoopWindowTarget;
 use crate::window::WindowBuilder;
 
 use web_sys::HtmlCanvasElement;
@@ -27,3 +31,38 @@ impl WindowBuilderExtWebSys for WindowBuilder {
         self
     }
 }
+
+/// Additional methods on `EventLoop` that are specific to the web.
+pub trait EventLoopExtWebSys {
+    /// A type provided by the user that can be passed through `Event::UserEvent`.
+    type UserEvent;
+
+    /// Initializes the winit event loop.
+    ///
+    /// Unlike `run`, this returns immediately, and doesn't throw an exception in order to
+    /// satisfy its `!` return type.
+    fn spawn<F>(self, event_handler: F)
+    where
+        F: 'static
+            + FnMut(
+                Event<'_, Self::UserEvent>,
+                &EventLoopWindowTarget<Self::UserEvent>,
+                &mut ControlFlow,
+            );
+}
+
+impl<T> EventLoopExtWebSys for EventLoop<T> {
+    type UserEvent = T;
+
+    fn spawn<F>(self, event_handler: F)
+    where
+        F: 'static
+            + FnMut(
+                Event<'_, Self::UserEvent>,
+                &EventLoopWindowTarget<Self::UserEvent>,
+                &mut ControlFlow,
+            ),
+    {
+        self.event_loop.spawn(event_handler)
+    }
+}