Implement windows focus key press/release on Windows (#1307)

* X11: Sync key press/release with window focus

* When a window loses focus, key release events are issued for all pressed keys
* When a window gains focus, key press events are issued for all pressed keys
* Adds `is_synthetic` field to `WindowEvent` variant `KeyboardInput`
  to indicate that these events are synthetic.
* Adds `is_synthetic: false` to `WindowEvent::KeyboardInput` events issued
  on all other platforms

* Implement windows focus key press/release on Windows

* Docs

Co-authored-by: Murarth <murarth@gmail.com>

CHANGELOG.md

@@ -32,6 +32,9 @@
 - On X11, generate synthetic key events for keys held when a window gains or loses focus.
 - On X11, issue a `CursorMoved` event when a `Touch` event occurs,
   as X11 implicitly moves the cursor for such events.
+- Add `is_synthetic` field to `WindowEvent` variant `KeyboardInput`,
+  indicating that the event is generated by winit.
+- On X11 and Windows, generate synthetic key events for keys held when a window gains or loses focus.
 
 # 0.20.0 Alpha 4 (2019-10-18)
 

src/event.rs

@@ -150,9 +150,10 @@ pub enum WindowEvent {
         /// If `true`, the event was generated synthetically by winit
         /// in one of the following circumstances:
         ///
-        /// * **X11**: Synthetic key press events are generated for all keys pressed
+        /// * Synthetic key press events are generated for all keys pressed
         ///   when a window gains focus. Likewise, synthetic key release events
         ///   are generated for all keys pressed when a window goes out of focus.
+        ///   ***Currently, this is only functional on X11 and Windows***
         ///
         /// Otherwise, this value is always `false`.
         is_synthetic: bool,

src/platform_impl/windows/event.rs

@@ -27,6 +27,16 @@ pub fn get_key_mods() -> ModifiersState {
     mods
 }
 
+pub fn get_pressed_keys() -> impl Iterator<Item = c_int> {
+    let mut keyboard_state = vec![0u8; 256];
+    unsafe { winuser::GetKeyboardState(keyboard_state.as_mut_ptr()) };
+    keyboard_state
+        .into_iter()
+        .enumerate()
+        .filter(|(_, p)| (*p & (1 << 7)) != 0) // whether or not a key is pressed is communicated via the high-order bit
+        .map(|(i, _)| i as c_int)
+}
+
 unsafe fn get_char(keyboard_state: &[u8; 256], v_key: u32, hkl: HKL) -> Option<char> {
     let mut unicode_bytes = [0u16; 5];
     let len = winuser::ToUnicodeEx(

src/platform_impl/windows/event_loop.rs

@@ -1421,7 +1421,27 @@ unsafe extern "system" fn public_window_callback<T>(
         }
 
         winuser::WM_SETFOCUS => {
-            use crate::event::WindowEvent::Focused;
+            use crate::event::{ElementState::Released, WindowEvent::Focused};
+            for windows_keycode in event::get_pressed_keys() {
+                let scancode =
+                    winuser::MapVirtualKeyA(windows_keycode as _, winuser::MAPVK_VK_TO_VSC);
+                let virtual_keycode = event::vkey_to_winit_vkey(windows_keycode);
+
+                subclass_input.send_event(Event::WindowEvent {
+                    window_id: RootWindowId(WindowId(window)),
+                    event: WindowEvent::KeyboardInput {
+                        device_id: DEVICE_ID,
+                        input: KeyboardInput {
+                            scancode,
+                            virtual_keycode,
+                            state: Released,
+                            modifiers: event::get_key_mods(),
+                        },
+                        is_synthetic: true,
+                    },
+                })
+            }
+
             subclass_input.send_event(Event::WindowEvent {
                 window_id: RootWindowId(WindowId(window)),
                 event: Focused(true),
@@ -1431,7 +1451,27 @@ unsafe extern "system" fn public_window_callback<T>(
         }
 
         winuser::WM_KILLFOCUS => {
-            use crate::event::WindowEvent::Focused;
+            use crate::event::{ElementState::Released, WindowEvent::Focused};
+            for windows_keycode in event::get_pressed_keys() {
+                let scancode =
+                    winuser::MapVirtualKeyA(windows_keycode as _, winuser::MAPVK_VK_TO_VSC);
+                let virtual_keycode = event::vkey_to_winit_vkey(windows_keycode);
+
+                subclass_input.send_event(Event::WindowEvent {
+                    window_id: RootWindowId(WindowId(window)),
+                    event: WindowEvent::KeyboardInput {
+                        device_id: DEVICE_ID,
+                        input: KeyboardInput {
+                            scancode,
+                            virtual_keycode,
+                            state: Released,
+                            modifiers: event::get_key_mods(),
+                        },
+                        is_synthetic: true,
+                    },
+                })
+            }
+
             subclass_input.send_event(Event::WindowEvent {
                 window_id: RootWindowId(WindowId(window)),
                 event: Focused(false),