Allow Bevy to start from non-main threads on supported platforms (#10020

)

# Objective

Allow Bevy apps to run without requiring to start from the main thread.
This allows other projects and applications to do things like spawning a
normal or scoped
thread and run Bevy applications there.

The current behaviour if you try this is a panic.

## Solution

Allow this by default on platforms winit supports this behaviour on
(x11, Wayland, Windows).

---

## Changelog

### Added

- Added the ability to start Bevy apps outside of the main thread on
x11, Wayland, Windows

---------

Signed-off-by: Torstein Grindvik <torstein.grindvik@nordicsemi.no>
Signed-off-by: Torstein Grindvik <torstein.grindvik@muybridge.com>
Co-authored-by: Torstein Grindvik <torstein.grindvik@muybridge.com>
Co-authored-by: James Liu <contact@jamessliu.com>

crates/bevy_internal/src/default_plugins.rs

@@ -64,7 +64,7 @@ impl PluginGroup for DefaultPlugins {
 
         #[cfg(feature = "bevy_winit")]
         {
-            group = group.add(bevy_winit::WinitPlugin);
+            group = group.add(bevy_winit::WinitPlugin::default());
         }
 
         #[cfg(feature = "bevy_render")]

crates/bevy_winit/src/lib.rs

@@ -72,11 +72,51 @@ pub static ANDROID_APP: std::sync::OnceLock<AndroidApp> = std::sync::OnceLock::n
 /// replace the existing [`App`] runner with one that constructs an [event loop](EventLoop) to
 /// receive window and input events from the OS.
 #[derive(Default)]
-pub struct WinitPlugin;
+pub struct WinitPlugin {
+    /// Allows the window (and the event loop) to be created on any thread
+    /// instead of only the main thread.
+    ///
+    /// See [`EventLoopBuilder::build`] for more information on this.
+    ///
+    /// # Supported platforms
+    ///
+    /// Only works on Linux (X11/Wayland) and Windows.
+    /// This field is ignored on other platforms.
+    pub run_on_any_thread: bool,
+}
 
 impl Plugin for WinitPlugin {
     fn build(&self, app: &mut App) {
         let mut event_loop_builder = EventLoopBuilder::<()>::with_user_event();
+
+        // This is needed because the features checked in the inner
+        // block might be enabled on other platforms than linux.
+        #[cfg(target_os = "linux")]
+        {
+            #[cfg(feature = "x11")]
+            {
+                use winit::platform::x11::EventLoopBuilderExtX11;
+
+                // This allows a Bevy app to be started and ran outside of the main thread.
+                // A use case for this is to allow external applications to spawn a thread
+                // which runs a Bevy app without requiring the Bevy app to need to reside on
+                // the main thread, which can be problematic.
+                event_loop_builder.with_any_thread(self.run_on_any_thread);
+            }
+
+            #[cfg(feature = "wayland")]
+            {
+                use winit::platform::wayland::EventLoopBuilderExtWayland;
+                event_loop_builder.with_any_thread(self.run_on_any_thread);
+            }
+        }
+
+        #[cfg(target_os = "windows")]
+        {
+            use winit::platform::windows::EventLoopBuilderExtWindows;
+            event_loop_builder.with_any_thread(self.run_on_any_thread);
+        }
+
         #[cfg(target_os = "android")]
         {
             use winit::platform::android::EventLoopBuilderExtAndroid;