fix(macos): ensure app delegate is defined (#775)

.github/fix-macos-delegate-usage.md

@@ -0,0 +1,5 @@
+---
+"tao": patch
+---
+
+Ensure the macOS app delegate is defined before accessing it.

src/platform_impl/macos/event_loop.rs

@@ -139,8 +139,28 @@ impl<T> EventLoop<T> {
   pub fn new() -> Self {
     let panic_info: Rc<PanicInfo> = Default::default();
     setup_control_flow_observers(Rc::downgrade(&panic_info));
+
+    let delegate = unsafe {
+      let is_main_thread: BOOL = msg_send!(class!(NSThread), isMainThread);
+      if is_main_thread == NO {
+        panic!("On macOS, `EventLoop` must be created on the main thread!");
+      }
+
+      // This must be done before `NSApp()` (equivalent to sending
+      // `sharedApplication`) is called anywhere else, or we'll end up
+      // with the wrong `NSApplication` class and the wrong thread could
+      // be marked as main.
+      let app: id = msg_send![APP_CLASS.0, sharedApplication];
+
+      let delegate = IdRef::new(msg_send![APP_DELEGATE_CLASS.0, new]);
+      let pool = NSAutoreleasePool::new(nil);
+      let _: () = msg_send![app, setDelegate:*delegate];
+      let _: () = msg_send![pool, drain];
+      delegate
+    };
+
     EventLoop {
-      delegate: IdRef::new(nil),
+      delegate,
       window_target: Rc::new(RootWindowTarget {
         p: Default::default(),
         _marker: PhantomData,
@@ -166,29 +186,6 @@ impl<T> EventLoop<T> {
   where
     F: FnMut(Event<'_, T>, &RootWindowTarget<T>, &mut ControlFlow),
   {
-    // initialize app delegate if needed
-    if self.delegate.is_null() {
-      let delegate = unsafe {
-        let is_main_thread: BOOL = msg_send!(class!(NSThread), isMainThread);
-        if is_main_thread == NO {
-          panic!("On macOS, `EventLoop` must be created on the main thread!");
-        }
-
-        // This must be done before `NSApp()` (equivalent to sending
-        // `sharedApplication`) is called anywhere else, or we'll end up
-        // with the wrong `NSApplication` class and the wrong thread could
-        // be marked as main.
-        let app: id = msg_send![APP_CLASS.0, sharedApplication];
-
-        let delegate = IdRef::new(msg_send![APP_DELEGATE_CLASS.0, new]);
-        let pool = NSAutoreleasePool::new(nil);
-        let _: () = msg_send![app, setDelegate:*delegate];
-        let _: () = msg_send![pool, drain];
-        delegate
-      };
-      self.delegate = delegate;
-    }
-
     // This transmute is always safe, in case it was reached through `run`, since our
     // lifetime will be already 'static. In other cases caller should ensure that all data
     // they passed to callback will actually outlive it, some apps just can't move