Web: set control flow strategies on EventLoop (#3740)

src/changelog/unreleased.md

@@ -40,6 +40,11 @@ changelog entry.
 
 ## Unreleased
 
+### Added
+
+- On Web, add `EventLoopExtWebSys::(set_)poll_strategy()` to allow setting
+  control flow strategies before starting the event loop.
+
 ### Changed
 
 - On Web, let events wake up event loop immediately when using

src/platform/web.rs

@@ -183,6 +183,20 @@ pub trait EventLoopExtWebSys {
     )]
     /// [^1]: `run_app()` is _not_ available on WASM when the target supports `exception-handling`.
     fn spawn_app<A: ApplicationHandler<Self::UserEvent> + 'static>(self, app: A);
+
+    /// Sets the strategy for [`ControlFlow::Poll`].
+    ///
+    /// See [`PollStrategy`].
+    ///
+    /// [`ControlFlow::Poll`]: crate::event_loop::ControlFlow::Poll
+    fn set_poll_strategy(&self, strategy: PollStrategy);
+
+    /// Gets the strategy for [`ControlFlow::Poll`].
+    ///
+    /// See [`PollStrategy`].
+    ///
+    /// [`ControlFlow::Poll`]: crate::event_loop::ControlFlow::Poll
+    fn poll_strategy(&self) -> PollStrategy;
 }
 
 impl<T> EventLoopExtWebSys for EventLoop<T> {
@@ -191,6 +205,14 @@ impl<T> EventLoopExtWebSys for EventLoop<T> {
     fn spawn_app<A: ApplicationHandler<Self::UserEvent> + 'static>(self, app: A) {
         self.event_loop.spawn_app(app);
     }
+
+    fn set_poll_strategy(&self, strategy: PollStrategy) {
+        self.event_loop.set_poll_strategy(strategy);
+    }
+
+    fn poll_strategy(&self) -> PollStrategy {
+        self.event_loop.poll_strategy()
+    }
 }
 
 pub trait ActiveEventLoopExtWebSys {

src/platform_impl/web/event_loop/mod.rs

@@ -5,6 +5,7 @@ use crate::application::ApplicationHandler;
 use crate::error::EventLoopError;
 use crate::event::Event;
 use crate::event_loop::ActiveEventLoop as RootActiveEventLoop;
+use crate::platform::web::{ActiveEventLoopExtWebSys, PollStrategy};
 
 use super::{backend, device, window};
 
@@ -76,6 +77,14 @@ impl<T> EventLoop<T> {
     pub fn window_target(&self) -> &RootActiveEventLoop {
         &self.elw
     }
+
+    pub fn set_poll_strategy(&self, strategy: PollStrategy) {
+        self.elw.set_poll_strategy(strategy);
+    }
+
+    pub fn poll_strategy(&self) -> PollStrategy {
+        self.elw.poll_strategy()
+    }
 }
 
 fn handle_event<T: 'static, A: ApplicationHandler<T>>(