Reduce iOS cpu usage (#16548)

# Objective

- Avoid recreating the monitor every loop (temp fix until it's done
properly on winit side)
- Add a new `WinitSettings` preset for mobile that makes the winit loop
wait more and recommend its usage

crates/bevy_winit/src/system.rs

@@ -158,6 +158,16 @@ pub fn create_monitors(
                 seen_monitors[idx] = true;
                 continue 'outer;
             }
+            // on iOS, equality doesn't work, so we need to compare the names
+            // otherwise the monitor entity is recreated every time
+            // TODO: remove after https://github.com/rust-windowing/winit/pull/4013 has been released
+            #[cfg(target_os = "ios")]
+            {
+                if monitor.name() == m.name() {
+                    seen_monitors[idx] = true;
+                    continue 'outer;
+                }
+            }
         }
 
         let size = monitor.size();

crates/bevy_winit/src/winit_config.rs

@@ -35,6 +35,19 @@ impl WinitSettings {
         }
     }
 
+    /// Default settings for mobile.
+    ///
+    /// [`Reactive`](UpdateMode::Reactive) if windows have focus,
+    /// [`reactive_low_power`](UpdateMode::reactive_low_power) otherwise.
+    ///
+    /// Use the [`EventLoopProxy`](crate::EventLoopProxy) to request a redraw from outside bevy.
+    pub fn mobile() -> Self {
+        WinitSettings {
+            focused_mode: UpdateMode::reactive(Duration::from_secs_f32(1.0 / 60.0)),
+            unfocused_mode: UpdateMode::reactive_low_power(Duration::from_secs(1)),
+        }
+    }
+
     /// Returns the current [`UpdateMode`].
     ///
     /// **Note:** The output depends on whether the window has focus or not.

examples/mobile/src/lib.rs

@@ -6,6 +6,7 @@ use bevy::{
     log::{Level, LogPlugin},
     prelude::*,
     window::{AppLifecycle, WindowMode},
+    winit::WinitSettings,
 };
 
 // the `bevy_main` proc_macro generates the required boilerplate for iOS and Android
@@ -32,6 +33,9 @@ fn main() {
                 ..default()
             }),
     )
+    // Make the winit loop wait more aggressively when no user input is received
+    // This can help reduce cpu usage on mobile devices
+    .insert_resource(WinitSettings::mobile())
     .add_systems(Startup, (setup_scene, setup_music))
     .add_systems(Update, (touch_camera, button_handler, handle_lifetime))
     .run();