Implement accessibility APIs via AccessKit (#2294)

* squash before rebase

* Update AccessKit, introducing support for editable spinners on Windows and an important fix for navigation order on macOS

* Restore support for increment and decrement actions in DragValue

* Avoid VoiceOver race condition bug

* fix clippy lint

* Tell AccessKit that the default action for a text edit (equivalent to a click) is to set the focus. This matters to some platform adapters.

* Refactor InputState functions for AccessKit actions

* Support the AccessKit SetValue for DragValue; this is the only way for a Windows AT to programmatically adjust the value

* Same for Slider

* Properly associate the slider label with both the slider and the drag value

* Lazily activate egui's AccessKit support

* fix clippy lint

* Update AccessKit

* More documentation, particularly around lazy activation

* Tweak one of the doc comments

* See if I can get AccessKit exempted from the 'missing backticks' lint

* Make PlatformOutput::accesskit_update an Option

* Refactor lazy activation

* Refactor node mutation (again)

* Eliminate the need for an explicit is_accesskit_active method, at least for now

* Fix doc comment

* More refactoring of tree construction; don't depend on Arc::get_mut

* Override a clippy lint; I seem to have no other choice

* Final planned refactor: a more flexible approach to hierarchy

* Last AccessKit update for this PR; includes an important macOS DPI fix

* Move and document the optional accesskit dependency

* Fix comment typo

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

* reformat

* More elegant code for conditionally creating a node

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

* Set step to 1.0 for all integer sliders

* Add doc example for Response::labelled_by

* Clarify a TODO comment I left for myself

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui-winit/src/lib.rs

@@ -11,7 +11,11 @@
 
 use std::os::raw::c_void;
 
+#[cfg(feature = "accesskit")]
+pub use accesskit_winit;
 pub use egui;
+#[cfg(feature = "accesskit")]
+use egui::accesskit;
 pub use winit;
 
 pub mod clipboard;
@@ -86,6 +90,9 @@ pub struct State {
 
     /// track ime state
     input_method_editor_started: bool,
+
+    #[cfg(feature = "accesskit")]
+    accesskit: Option<accesskit_winit::Adapter>,
 }
 
 impl State {
@@ -114,9 +121,26 @@ impl State {
             pointer_touch_id: None,
 
             input_method_editor_started: false,
+
+            #[cfg(feature = "accesskit")]
+            accesskit: None,
         }
     }
 
+    #[cfg(feature = "accesskit")]
+    pub fn init_accesskit<T: From<accesskit_winit::ActionRequestEvent> + Send>(
+        &mut self,
+        window: &winit::window::Window,
+        event_loop_proxy: winit::event_loop::EventLoopProxy<T>,
+        initial_tree_update_factory: impl 'static + FnOnce() -> accesskit::TreeUpdate + Send,
+    ) {
+        self.accesskit = Some(accesskit_winit::Adapter::new(
+            window,
+            initial_tree_update_factory,
+            event_loop_proxy,
+        ));
+    }
+
     /// Call this once a graphics context has been created to update the maximum texture dimensions
     /// that egui will use.
     pub fn set_max_texture_side(&mut self, max_texture_side: usize) {
@@ -374,6 +398,16 @@ impl State {
         }
     }
 
+    /// Call this when there is a new [`accesskit::ActionRequest`].
+    ///
+    /// The result can be found in [`Self::egui_input`] and be extracted with [`Self::take_egui_input`].
+    #[cfg(feature = "accesskit")]
+    pub fn on_accesskit_action_request(&mut self, request: accesskit::ActionRequest) {
+        self.egui_input
+            .events
+            .push(egui::Event::AccessKitActionRequest(request));
+    }
+
     fn on_mouse_button_input(
         &mut self,
         state: winit::event::ElementState,
@@ -592,6 +626,8 @@ impl State {
             events: _,                    // handled above
             mutable_text_under_cursor: _, // only used in eframe web
             text_cursor_pos,
+            #[cfg(feature = "accesskit")]
+            accesskit_update,
         } = platform_output;
         self.current_pixels_per_point = egui_ctx.pixels_per_point(); // someone can have changed it to scale the UI
 
@@ -608,6 +644,13 @@ impl State {
         if let Some(egui::Pos2 { x, y }) = text_cursor_pos {
             window.set_ime_position(winit::dpi::LogicalPosition { x, y });
         }
+
+        #[cfg(feature = "accesskit")]
+        if let Some(accesskit) = self.accesskit.as_ref() {
+            if let Some(update) = accesskit_update {
+                accesskit.update_if_active(|| update);
+            }
+        }
     }
 
     fn set_cursor_icon(&mut self, window: &winit::window::Window, cursor_icon: egui::CursorIcon) {