Drop application handler on run loop exit (#4149)

Calling the `Drop` impl of the user's `ApplicationHandler` is important on
iOS and Web, since they don't return from `EventLoop::run_app`.

And now that we reliably call `Drop`, the `ApplicationHandler::exited`
event/callback is unnecessary; using `Drop` composes much better (open files
etc. stored in the app state will be automatically flushed), and prevents
weirdness like attempting to create a new window while exiting.

examples/application.rs

@@ -592,18 +592,18 @@ impl ApplicationHandler for Application {
         }
     }
 
-    #[cfg(not(android_platform))]
-    fn exiting(&mut self, _event_loop: &dyn ActiveEventLoop) {
-        // We must drop the context here.
-        self.context = None;
-    }
-
     #[cfg(target_os = "macos")]
     fn macos_handler(&mut self) -> Option<&mut dyn ApplicationHandlerExtMacOS> {
         Some(self)
     }
 }
 
+impl Drop for Application {
+    fn drop(&mut self) {
+        info!("Application exited");
+    }
+}
+
 #[cfg(target_os = "macos")]
 impl ApplicationHandlerExtMacOS for Application {
     fn standard_key_binding(

examples/window.rs

@@ -11,6 +11,8 @@ use winit::window::{Window, WindowAttributes, WindowId};
 
 #[path = "util/fill.rs"]
 mod fill;
+#[path = "util/tracing.rs"]
+mod tracing;
 
 #[derive(Default, Debug)]
 struct App {
@@ -70,9 +72,12 @@ fn main() -> Result<(), Box<dyn Error>> {
     #[cfg(web_platform)]
     console_error_panic_hook::set_once();
 
+    tracing::init();
+
     let event_loop = EventLoop::new()?;
-    let mut app = App::default();
 
     // For alternative loop run options see `pump_events` and `run_on_demand` examples.
-    event_loop.run_app(&mut app).map_err(Into::into)
+    event_loop.run_app(App::default())?;
+
+    Ok(())
 }