feat: Workaround for libraries that put the macOS keyboard focus on the window rather than the content view (#266)

Author: mwcampbell

bindings/c/src/macos.rs

@@ -7,8 +7,14 @@ use crate::{
     action_handler, box_from_ptr, ref_from_ptr, tree_update, tree_update_factory, BoxCastPtr,
     CastPtr,
 };
-use accesskit_macos::{Adapter, NSPoint, QueuedEvents, SubclassingAdapter};
-use std::{os::raw::c_void, ptr};
+use accesskit_macos::{
+    add_focus_forwarder_to_window_class, Adapter, NSPoint, QueuedEvents, SubclassingAdapter,
+};
+use std::{
+    ffi::CStr,
+    os::raw::{c_char, c_void},
+    ptr,
+};
 
 pub struct macos_queued_events {
     _private: [u8; 0],
@@ -200,3 +206,25 @@ impl macos_subclassing_adapter {
         }
     }
 }
+
+/// Modifies the specified class, which must be a subclass of `NSWindow`,
+/// to include an `accessibilityFocusedUIElement` method that calls
+/// the corresponding method on the window's content view. This is needed
+/// for windowing libraries such as SDL that place the keyboard focus
+/// directly on the window rather than the content view.
+///
+/// # Safety
+///
+/// This function is declared unsafe because the caller must ensure that the
+/// code for this library is never unloaded from the application process,
+/// since it's not possible to reverse this operation. It's safest
+/// if this library is statically linked into the application's main executable.
+/// Also, this function assumes that the specified class is a subclass
+/// of `NSWindow`.
+#[no_mangle]
+pub unsafe extern "C" fn accesskit_macos_add_focus_forwarder_to_window_class(
+    class_name: *const c_char,
+) {
+    let class_name = unsafe { CStr::from_ptr(class_name).to_string_lossy() };
+    add_focus_forwarder_to_window_class(&class_name);
+}

platforms/macos/src/lib.rs

@@ -16,6 +16,9 @@ pub use adapter::Adapter;
 mod event;
 pub use event::QueuedEvents;
 
+mod patch;
+pub use patch::add_focus_forwarder_to_window_class;
+
 mod subclass;
 pub use subclass::SubclassingAdapter;
 

platforms/macos/src/patch.rs

@@ -0,0 +1,92 @@
+// Copyright 2023 The AccessKit Authors. All rights reserved.
+// Licensed under the Apache License, Version 2.0 (found in
+// the LICENSE-APACHE file) or the MIT license (found in
+// the LICENSE-MIT file), at your option.
+
+use objc2::{
+    declare::MethodImplementation,
+    encode::{Encode, EncodeArguments, Encoding},
+    ffi::class_addMethod,
+    msg_send,
+    runtime::{Bool, Class, Object, Sel},
+    sel, Message,
+};
+use std::{ffi::CString, ptr::null_mut};
+
+use crate::appkit::NSWindow;
+
+extern "C" fn focus_forwarder(this: &NSWindow, _cmd: Sel) -> *mut Object {
+    this.content_view().map_or_else(null_mut, |view| unsafe {
+        msg_send![&*view, accessibilityFocusedUIElement]
+    })
+}
+
+/// Modifies the specified class, which must be a subclass of `NSWindow`,
+/// to include an `accessibilityFocusedUIElement` method that calls
+/// the corresponding method on the window's content view. This is needed
+/// for windowing libraries such as SDL that place the keyboard focus
+/// directly on the window rather than the content view.
+///
+/// # Safety
+///
+/// This function is declared unsafe because the caller must ensure that the
+/// code for this crate is never unloaded from the application process,
+/// since it's not possible to reverse this operation. It's safest
+/// if this crate is statically linked into the application's main executable.
+/// Also, this function assumes that the specified class is a subclass
+/// of `NSWindow`.
+pub unsafe fn add_focus_forwarder_to_window_class(class_name: &str) {
+    let class = Class::get(class_name).unwrap();
+    unsafe {
+        add_method(
+            class as *const Class as *mut Class,
+            sel!(accessibilityFocusedUIElement),
+            focus_forwarder as unsafe extern "C" fn(_, _) -> _,
+        )
+    };
+}
+
+// The rest of this file is copied from objc2 with only minor adaptations,
+// to allow a method to be added to an existing class.
+
+unsafe fn add_method<T, F>(class: *mut Class, sel: Sel, func: F)
+where
+    T: Message + ?Sized,
+    F: MethodImplementation<Callee = T>,
+{
+    let encs = F::Args::ENCODINGS;
+    let sel_args = count_args(sel);
+    assert_eq!(
+        sel_args,
+        encs.len(),
+        "Selector {:?} accepts {} arguments, but function accepts {}",
+        sel,
+        sel_args,
+        encs.len(),
+    );
+
+    let types = method_type_encoding(&F::Ret::ENCODING, encs);
+    let success = Bool::from_raw(unsafe {
+        class_addMethod(
+            class as *mut _,
+            sel.as_ptr(),
+            Some(func.__imp()),
+            types.as_ptr(),
+        )
+    });
+    assert!(success.as_bool(), "Failed to add method {:?}", sel);
+}
+
+fn count_args(sel: Sel) -> usize {
+    sel.name().chars().filter(|&c| c == ':').count()
+}
+
+fn method_type_encoding(ret: &Encoding, args: &[Encoding]) -> CString {
+    // First two arguments are always self and the selector
+    let mut types = format!("{}{}{}", ret, <*mut Object>::ENCODING, Sel::ENCODING);
+    for enc in args {
+        use core::fmt::Write;
+        write!(&mut types, "{}", enc).unwrap();
+    }
+    CString::new(types).unwrap()
+}