api: show example for platform specific application trait

src/platform/wayland.rs

@@ -13,6 +13,7 @@
 //! * `wayland-csd-adwaita` (default).
 //! * `wayland-csd-adwaita-crossfont`.
 //! * `wayland-csd-adwaita-notitle`.
+use crate::application::ApplicationHandler;
 use crate::event_loop::{ActiveEventLoop, EventLoop, EventLoopBuilder};
 use crate::monitor::MonitorHandle;
 pub use crate::window::Theme;
@@ -24,6 +25,10 @@ pub trait ActiveEventLoopExtWayland {
     fn is_wayland(&self) -> bool;
 }
 
+pub trait WaylandApplicationHandler: ApplicationHandler + 'static {
+    fn wayland_callback(&mut self);
+}
+
 impl ActiveEventLoopExtWayland for &dyn ActiveEventLoop {
     #[inline]
     fn is_wayland(&self) -> bool {
@@ -35,6 +40,8 @@ impl ActiveEventLoopExtWayland for &dyn ActiveEventLoop {
 pub trait EventLoopExtWayland {
     /// True if the [`EventLoop`] uses Wayland.
     fn is_wayland(&self) -> bool;
+
+    fn register_wayland_callback<T: WaylandApplicationHandler>(&mut self);
 }
 
 impl EventLoopExtWayland for EventLoop {
@@ -42,6 +49,22 @@ impl EventLoopExtWayland for EventLoop {
     fn is_wayland(&self) -> bool {
         self.event_loop.is_wayland()
     }
+
+    fn register_wayland_callback<T: WaylandApplicationHandler>(&mut self) {
+        let event_loop = match &self.event_loop {
+            crate::platform_impl::EventLoop::Wayland(event_loop) => &event_loop.active_event_loop,
+            #[cfg(x11_platform)]
+            crate::platform_impl::EventLoop::X(_) => return,
+        };
+
+        event_loop.wayland_callback.set(Some(|app: &mut dyn ApplicationHandler| {
+            app.as_any()
+                .expect("as_any_mut is not implemented")
+                .downcast_mut::<T>()
+                .unwrap()
+                .wayland_callback()
+        }));
+    }
 }
 
 /// Additional methods on [`EventLoopBuilder`] that are specific to Wayland.