feat: Add window-based constructor to macOS subclassing adapter

bindings/c/src/macos.rs

@@ -128,6 +128,24 @@ impl macos_subclassing_adapter {
         BoxCastPtr::to_mut_ptr(adapter)
     }
 
+    /// This function takes ownership of `handler`.
+    #[no_mangle]
+    pub unsafe extern "C" fn accesskit_macos_subclassing_adapter_for_window(
+        window: *mut c_void,
+        source: tree_update_factory,
+        source_userdata: *mut c_void,
+        handler: *mut action_handler,
+    ) -> *mut macos_subclassing_adapter {
+        let source = source.unwrap();
+        let handler = box_from_ptr(handler);
+        let adapter = SubclassingAdapter::for_window(
+            window,
+            move || box_from_ptr(source(source_userdata)).into(),
+            handler,
+        );
+        BoxCastPtr::to_mut_ptr(adapter)
+    }
+
     #[no_mangle]
     pub extern "C" fn accesskit_macos_subclassing_adapter_free(
         adapter: *mut macos_subclassing_adapter,

platforms/macos/src/appkit/window.rs

@@ -6,10 +6,12 @@
 use objc2::{
     extern_class, extern_methods,
     foundation::{NSObject, NSPoint, NSRect},
+    msg_send_id,
+    rc::{Id, Shared},
     ClassType,
 };
 
-use super::NSResponder;
+use super::{NSResponder, NSView};
 
 extern_class!(
     #[derive(Debug)]
@@ -28,5 +30,9 @@ extern_methods!(
 
         #[sel(convertPointFromScreen:)]
         pub(crate) fn convert_point_from_screen(&self, point: NSPoint) -> NSPoint;
+
+        pub(crate) fn content_view(&self) -> Option<Id<NSView, Shared>> {
+            unsafe { msg_send_id![self, contentView] }
+        }
     }
 );

platforms/macos/src/subclass.rs

@@ -20,7 +20,11 @@ use objc2::{
 use once_cell::{sync::Lazy as SyncLazy, unsync::Lazy};
 use std::{collections::HashMap, ffi::c_void, sync::Mutex};
 
-use crate::{appkit::NSView, event::QueuedEvents, Adapter};
+use crate::{
+    appkit::{NSView, NSWindow},
+    event::QueuedEvents,
+    Adapter,
+};
 
 static SUBCLASSES: SyncLazy<Mutex<HashMap<&'static Class, &'static Class>>> =
     SyncLazy::new(|| Mutex::new(HashMap::new()));
@@ -116,13 +120,22 @@ impl SubclassingAdapter {
     ) -> Self {
         let view = view as *mut NSView;
         let retained_view = unsafe { Id::retain(view) }.unwrap();
+        Self::new_internal(retained_view, source, action_handler)
+    }
+
+    fn new_internal(
+        retained_view: Id<NSView, Shared>,
+        source: impl 'static + FnOnce() -> TreeUpdate,
+        action_handler: Box<dyn ActionHandler>,
+    ) -> Self {
         let adapter: LazyAdapter = {
             let retained_view = retained_view.clone();
             Lazy::new(Box::new(move || {
                 let view = Id::as_ptr(&retained_view) as *mut c_void;
                 unsafe { Adapter::new(view, source(), action_handler) }
             }))
         };
+        let view = Id::as_ptr(&retained_view) as *mut NSView;
         // Cast to a pointer and back to force the lifetime to 'static
         // SAFETY: We know the class will live as long as the instance,
         // and we only use this reference while the instance is alive.
@@ -171,6 +184,29 @@ impl SubclassingAdapter {
         }
     }
 
+    /// Create an adapter that dynamically subclasses the content view
+    /// of the specified window.
+    ///
+    /// The action handler will always be called on the main thread.
+    ///
+    /// # Safety
+    ///
+    /// `window` must be a valid, unreleased pointer to an `NSWindow`.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if the specified window doesn't currently have
+    /// a content view.
+    pub unsafe fn for_window(
+        window: *mut c_void,
+        source: impl 'static + FnOnce() -> TreeUpdate,
+        action_handler: Box<dyn ActionHandler>,
+    ) -> Self {
+        let window = unsafe { &*(window as *const NSWindow) };
+        let retained_view = window.content_view().unwrap();
+        Self::new_internal(retained_view, source, action_handler)
+    }
+
     /// Initialize the tree if it hasn't been initialized already, then apply
     /// the provided update.
     ///