Implement keyboard event hook on Windows (#212)

This PR implements **keyboard event hooking** on Windows, a technique also used by JUCE to get around restrictions on keyboard input imposed by various hosts. By intercepting Win32 messages using `SetWindowsHookEx`, we can bypass the host and forward keyboard messages to the `baseview` window directly.

This resolves the longstanding issue with keyboard input in Ableton, and likely renders many existing hacks for this problem obsolete (though I haven't tested that part yet.)

The meat of the implementation is in the new `src/win/hook.rs` module, which registers a simple event hook that intercepts keyboard-related `WM_` events addressed to `baseview` windows, and forwards them into the relevant window's event handler directly, consuming the message in the process.

---------

Co-authored-by: ash taylor?! <estrobiologist@outlook.com>
Co-authored-by: Micah Johnston <micahrjohnston@gmail.com>

Author: estroBiologist

src/win/hook.rs

@@ -0,0 +1,142 @@
+use std::{
+    collections::HashSet,
+    ffi::c_int,
+    ptr,
+    sync::{LazyLock, RwLock},
+};
+
+use winapi::{
+    shared::{
+        minwindef::{LPARAM, WPARAM},
+        windef::{HHOOK, HWND, POINT},
+    },
+    um::{
+        libloaderapi::GetModuleHandleW,
+        processthreadsapi::GetCurrentThreadId,
+        winuser::{
+            CallNextHookEx, SetWindowsHookExW, UnhookWindowsHookEx, HC_ACTION, MSG, PM_REMOVE,
+            WH_GETMESSAGE, WM_CHAR, WM_KEYDOWN, WM_KEYUP, WM_SYSCHAR, WM_SYSKEYDOWN, WM_SYSKEYUP,
+            WM_USER,
+        },
+    },
+};
+
+use crate::win::wnd_proc;
+
+// track all windows opened by this instance of baseview
+// we use an RwLock here since the vast majority of uses (event interceptions)
+// will only need to read from the HashSet
+static HOOK_STATE: LazyLock<RwLock<KeyboardHookState>> = LazyLock::new(|| RwLock::default());
+
+pub(crate) struct KeyboardHookHandle(HWNDWrapper);
+
+#[derive(Default)]
+struct KeyboardHookState {
+    hook: Option<HHOOK>,
+    open_windows: HashSet<HWNDWrapper>,
+}
+
+#[derive(Hash, PartialEq, Eq, Clone, Copy)]
+struct HWNDWrapper(HWND);
+
+// SAFETY: it's a pointer behind an RwLock. we'll live
+unsafe impl Send for KeyboardHookState {}
+unsafe impl Sync for KeyboardHookState {}
+
+// SAFETY: we never access the underlying HWND ourselves, just use it as a HashSet entry
+unsafe impl Send for HWNDWrapper {}
+unsafe impl Sync for HWNDWrapper {}
+
+impl Drop for KeyboardHookHandle {
+    fn drop(&mut self) {
+        deinit_keyboard_hook(self.0);
+    }
+}
+
+// initialize keyboard hook
+// some DAWs (particularly Ableton) intercept incoming keyboard messages,
+// but we're naughty so we intercept them right back
+pub(crate) fn init_keyboard_hook(hwnd: HWND) -> KeyboardHookHandle {
+    let state = &mut *HOOK_STATE.write().unwrap();
+
+    // register hwnd to global window set
+    state.open_windows.insert(HWNDWrapper(hwnd));
+
+    if state.hook.is_some() {
+        // keyboard hook already exists, just return handle
+        KeyboardHookHandle(HWNDWrapper(hwnd))
+    } else {
+        // keyboard hook doesn't exist (no windows open before this), create it
+        let new_hook = unsafe {
+            SetWindowsHookExW(
+                WH_GETMESSAGE,
+                Some(keyboard_hook_callback),
+                GetModuleHandleW(ptr::null()),
+                GetCurrentThreadId(),
+            )
+        };
+
+        state.hook = Some(new_hook);
+
+        KeyboardHookHandle(HWNDWrapper(hwnd))
+    }
+}
+
+fn deinit_keyboard_hook(hwnd: HWNDWrapper) {
+    let state = &mut *HOOK_STATE.write().unwrap();
+
+    state.open_windows.remove(&hwnd);
+
+    if state.open_windows.is_empty() {
+        if let Some(hhook) = state.hook {
+            unsafe {
+                UnhookWindowsHookEx(hhook);
+            }
+
+            state.hook = None;
+        }
+    }
+}
+
+unsafe extern "system" fn keyboard_hook_callback(
+    n_code: c_int, wparam: WPARAM, lparam: LPARAM,
+) -> isize {
+    let msg = lparam as *mut MSG;
+
+    if n_code == HC_ACTION && wparam == PM_REMOVE as usize && offer_message_to_baseview(msg) {
+        *msg = MSG {
+            hwnd: ptr::null_mut(),
+            message: WM_USER,
+            wParam: 0,
+            lParam: 0,
+            time: 0,
+            pt: POINT { x: 0, y: 0 },
+        };
+
+        0
+    } else {
+        CallNextHookEx(ptr::null_mut(), n_code, wparam, lparam)
+    }
+}
+
+// check if `msg` is a keyboard message addressed to a window
+// in KeyboardHookState::open_windows, and intercept it if so
+unsafe fn offer_message_to_baseview(msg: *mut MSG) -> bool {
+    let msg = &*msg;
+
+    // if this isn't a keyboard message, ignore it
+    match msg.message {
+        WM_KEYDOWN | WM_SYSKEYDOWN | WM_KEYUP | WM_SYSKEYUP | WM_CHAR | WM_SYSCHAR => {}
+
+        _ => return false,
+    }
+
+    // check if this is one of our windows. if so, intercept it
+    if HOOK_STATE.read().unwrap().open_windows.contains(&HWNDWrapper(msg.hwnd)) {
+        let _ = wnd_proc(msg.hwnd, msg.message, msg.wParam, msg.lParam);
+
+        return true;
+    }
+
+    false
+}

src/win/mod.rs

@@ -1,5 +1,6 @@
 mod cursor;
 mod drop_target;
+mod hook;
 mod keyboard;
 mod window;
 

src/win/window.rs

@@ -33,6 +33,7 @@ use raw_window_handle::{
 
 const BV_WINDOW_MUST_CLOSE: UINT = WM_USER + 1;
 
+use crate::win::hook::{self, KeyboardHookHandle};
 use crate::{
     Event, MouseButton, MouseCursor, MouseEvent, PhyPoint, PhySize, ScrollDelta, Size, WindowEvent,
     WindowHandler, WindowInfo, WindowOpenOptions, WindowScalePolicy,
@@ -118,7 +119,7 @@ impl Drop for ParentHandle {
     }
 }
 
-unsafe extern "system" fn wnd_proc(
+pub(crate) unsafe extern "system" fn wnd_proc(
     hwnd: HWND, msg: UINT, wparam: WPARAM, lparam: LPARAM,
 ) -> LRESULT {
     if msg == WM_CREATE {
@@ -507,6 +508,11 @@ pub(super) struct WindowState {
     scale_policy: WindowScalePolicy,
     dw_style: u32,
 
+    // handle to the win32 keyboard hook
+    // we don't need to read from this, just carry it around so the Drop impl can run
+    #[allow(dead_code)]
+    kb_hook: KeyboardHookHandle,
+
     /// Tasks that should be executed at the end of `wnd_proc`. This is needed to avoid mutably
     /// borrowing the fields from `WindowState` more than once. For instance, when the window
     /// handler requests a resize in response to a keyboard event, the window state will already be
@@ -686,6 +692,8 @@ impl Window<'_> {
             );
             // todo: manage error ^
 
+            let kb_hook = hook::init_keyboard_hook(hwnd);
+
             #[cfg(feature = "opengl")]
             let gl_context: Option<GlContext> = options.gl_config.map(|gl_config| {
                 let mut handle = Win32WindowHandle::empty();
@@ -716,6 +724,8 @@ impl Window<'_> {
 
                 deferred_tasks: RefCell::new(VecDeque::with_capacity(4)),
 
+                kb_hook,
+
                 #[cfg(feature = "opengl")]
                 gl_context,
             });