Remove symlinking between winit-appkit and winit-uikit (#4530)

winit-common/src/foundation/mod.rs

@@ -0,0 +1,3 @@
+mod notification_center;
+
+pub use self::notification_center::*;

winit-common/src/foundation/notification_center.rs

@@ -0,0 +1,30 @@
+use std::ptr::NonNull;
+
+use block2::RcBlock;
+use objc2::rc::Retained;
+use objc2::runtime::ProtocolObject;
+use objc2_foundation::{
+    NSNotification, NSNotificationCenter, NSNotificationName, NSObjectProtocol,
+};
+
+/// Observe the given notification.
+///
+/// This is used in Winit as an alternative to declaring an application delegate, as we want to
+/// give the user full control over those.
+pub fn create_observer(
+    center: &NSNotificationCenter,
+    name: &NSNotificationName,
+    handler: impl Fn(&NSNotification) + 'static,
+) -> Retained<ProtocolObject<dyn NSObjectProtocol>> {
+    let block = RcBlock::new(move |notification: NonNull<NSNotification>| {
+        handler(unsafe { notification.as_ref() });
+    });
+    unsafe {
+        center.addObserverForName_object_queue_usingBlock(
+            Some(name),
+            None, // No sender filter
+            None, // No queue, run on posting thread (i.e. main thread)
+            &block,
+        )
+    }
+}