Allow recreating wasm event loop with spawn (#2897)

CHANGELOG.md

@@ -8,6 +8,7 @@ And please only add new entries to the top of this list, right below the `# Unre
 
 # Unreleased
 
+- On Web, allow event loops to be recreated with `spawn`.
 - **Breaking:** Rename `Window::set_ime_position` to `Window::set_ime_cursor_area` adding a way to set exclusive zone.
 - On Android, changed default behavior of Android to ignore volume keys letting the operating system handle them.
 - On Android, added `EventLoopBuilderExtAndroid::handle_volume_keys` to indicate that the application will handle the volume keys manually.

src/event_loop.rs

@@ -9,9 +9,9 @@
 //! handle events.
 use std::marker::PhantomData;
 use std::ops::Deref;
+use std::sync::atomic::{AtomicBool, Ordering};
 use std::{error, fmt};
 
-use once_cell::sync::OnceCell;
 use raw_window_handle::{HasRawDisplayHandle, RawDisplayHandle};
 #[cfg(not(wasm_platform))]
 use std::time::{Duration, Instant};
@@ -69,6 +69,8 @@ impl EventLoopBuilder<()> {
     }
 }
 
+static EVENT_LOOP_CREATED: AtomicBool = AtomicBool::new(false);
+
 impl<T> EventLoopBuilder<T> {
     /// Start building a new event loop, with the given type as the user event
     /// type.
@@ -114,10 +116,10 @@ impl<T> EventLoopBuilder<T> {
     )]
     #[inline]
     pub fn build(&mut self) -> EventLoop<T> {
-        static EVENT_LOOP_CREATED: OnceCell<()> = OnceCell::new();
+        if EVENT_LOOP_CREATED.swap(true, Ordering::Relaxed) {
-        if EVENT_LOOP_CREATED.set(()).is_err() {
             panic!("Creating EventLoop multiple times is not supported.");
         }
+
         // Certain platforms accept a mutable reference in their API.
         #[allow(clippy::unnecessary_mut_passed)]
         EventLoop {
@@ -125,6 +127,11 @@ impl<T> EventLoopBuilder<T> {
             _marker: PhantomData,
         }
     }
+
+    #[cfg(wasm_platform)]
+    pub(crate) fn allow_event_loop_recreation() {
+        EVENT_LOOP_CREATED.store(false, Ordering::Relaxed);
+    }
 }
 
 impl<T> fmt::Debug for EventLoop<T> {

src/platform/web.rs

@@ -66,6 +66,11 @@ pub trait EventLoopExtWebSys {
     ///
     /// Unlike `run`, 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.
     fn spawn<F>(self, event_handler: F)
     where
         F: 'static

src/platform_impl/web/event_loop/mod.rs

@@ -8,7 +8,9 @@ pub use self::window_target::EventLoopWindowTarget;
 
 use super::{backend, device, window};
 use crate::event::Event;
-use crate::event_loop::{ControlFlow, EventLoopWindowTarget as RootEventLoopWindowTarget};
+use crate::event_loop::{
+    ControlFlow, EventLoopBuilder, EventLoopWindowTarget as RootEventLoopWindowTarget,
+};
 
 use std::marker::PhantomData;
 
@@ -33,7 +35,7 @@ impl<T> EventLoop<T> {
     where
         F: 'static + FnMut(Event<'_, T>, &RootEventLoopWindowTarget<T>, &mut ControlFlow),
     {
-        self.spawn(event_handler);
+        self.spawn_inner(event_handler);
 
         // 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 '!'
@@ -44,7 +46,15 @@ impl<T> EventLoop<T> {
         unreachable!();
     }
 
-    pub fn spawn<F>(self, mut event_handler: F)
+    pub fn spawn<F>(self, event_handler: F)
+    where
+        F: 'static + FnMut(Event<'_, T>, &RootEventLoopWindowTarget<T>, &mut ControlFlow),
+    {
+        EventLoopBuilder::<T>::allow_event_loop_recreation();
+        self.spawn_inner(event_handler);
+    }
+
+    fn spawn_inner<F>(self, mut event_handler: F)
     where
         F: 'static + FnMut(Event<'_, T>, &RootEventLoopWindowTarget<T>, &mut ControlFlow),
     {