feat(Windows): add with_msg_hook (#2213)

CHANGELOG.md

@@ -8,6 +8,7 @@ And please only add new entries to the top of this list, right below the `# Unre
 
 # Unreleased
 
+- On Windows, Added `EventLoopBuilderExtWindows::with_msg_hook`
 - On Windows, remove internally unique DC per window.
 - macOS: Remove the need to call `set_ime_position` after moving the window.
 - Added `Window::is_visible`.

src/event_loop.rs

@@ -50,7 +50,7 @@ pub struct EventLoopWindowTarget<T: 'static> {
 ///
 /// This is used to make specifying options that affect the whole application
 /// easier. But note that constructing multiple event loops is not supported.
-#[derive(Debug, Clone, Default)]
+#[derive(Default)]
 pub struct EventLoopBuilder<T: 'static> {
     pub(crate) platform_specific: platform_impl::PlatformSpecificEventLoopAttributes,
     _p: PhantomData<T>,
@@ -95,7 +95,7 @@ impl<T> EventLoopBuilder<T> {
     #[inline]
     pub fn build(&mut self) -> EventLoop<T> {
         EventLoop {
-            event_loop: platform_impl::EventLoop::new(&self.platform_specific),
+            event_loop: platform_impl::EventLoop::new(&mut self.platform_specific),
             _marker: PhantomData,
         }
     }

src/platform/windows.rs

@@ -1,6 +1,6 @@
 #![cfg(target_os = "windows")]
 
-use std::path::Path;
+use std::{ffi::c_void, path::Path};
 
 use crate::{
     dpi::PhysicalSize,
@@ -56,6 +56,36 @@ pub trait EventLoopBuilderExtWindows {
     /// # }
     /// ```
     fn with_dpi_aware(&mut self, dpi_aware: bool) -> &mut Self;
+
+    /// A callback to be executed before dispatching a win32 message to the window procedure.
+    /// Return true to disable winit's internal message dispatching.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # use windows_sys::Win32::UI::WindowsAndMessaging::{ACCEL, CreateAcceleratorTableW, TranslateAcceleratorW, DispatchMessageW, TranslateMessage, MSG};
+    /// use winit::event_loop::EventLoopBuilder;
+    /// #[cfg(target_os = "windows")]
+    /// use winit::platform::windows::EventLoopBuilderExtWindows;
+    ///
+    /// let mut builder = EventLoopBuilder::new();
+    /// #[cfg(target_os = "windows")]
+    /// builder.with_msg_hook(|msg|{
+    ///     let msg = msg as *const MSG;
+    /// #   let accels: Vec<ACCEL> = Vec::new();
+    ///     let translated = unsafe {
+    ///         TranslateAcceleratorW(
+    ///             (*msg).hwnd,
+    ///             CreateAcceleratorTableW(accels.as_ptr() as _, 1),
+    ///             msg,
+    ///         ) == 1
+    ///     };
+    ///     translated
+    /// });
+    /// ```
+    fn with_msg_hook<F>(&mut self, callback: F) -> &mut Self
+    where
+        F: FnMut(*const c_void) -> bool + 'static;
 }
 
 impl<T> EventLoopBuilderExtWindows for EventLoopBuilder<T> {
@@ -70,6 +100,15 @@ impl<T> EventLoopBuilderExtWindows for EventLoopBuilder<T> {
         self.platform_specific.dpi_aware = dpi_aware;
         self
     }
+
+    #[inline]
+    fn with_msg_hook<F>(&mut self, callback: F) -> &mut Self
+    where
+        F: FnMut(*const c_void) -> bool + 'static,
+    {
+        self.platform_specific.msg_hook = Some(Box::new(callback));
+        self
+    }
 }
 
 /// Additional methods on `Window` that are specific to Windows.

src/platform_impl/windows/event_loop.rs

@@ -6,6 +6,7 @@ use parking_lot::Mutex;
 use std::{
     cell::Cell,
     collections::VecDeque,
+    ffi::c_void,
     marker::PhantomData,
     mem, panic, ptr,
     rc::Rc,
@@ -150,12 +151,13 @@ impl<T> ThreadMsgTargetData<T> {
 pub struct EventLoop<T: 'static> {
     thread_msg_sender: Sender<T>,
     window_target: RootELW<T>,
+    msg_hook: Option<Box<dyn FnMut(*const c_void) -> bool + 'static>>,
 }
 
-#[derive(Debug, Copy, Clone, PartialEq, Hash)]
 pub(crate) struct PlatformSpecificEventLoopAttributes {
     pub(crate) any_thread: bool,
     pub(crate) dpi_aware: bool,
+    pub(crate) msg_hook: Option<Box<dyn FnMut(*const c_void) -> bool + 'static>>,
 }
 
 impl Default for PlatformSpecificEventLoopAttributes {
@@ -163,6 +165,7 @@ impl Default for PlatformSpecificEventLoopAttributes {
         Self {
             any_thread: false,
             dpi_aware: true,
+            msg_hook: None,
         }
     }
 }
@@ -174,7 +177,7 @@ pub struct EventLoopWindowTarget<T: 'static> {
 }
 
 impl<T: 'static> EventLoop<T> {
-    pub(crate) fn new(attributes: &PlatformSpecificEventLoopAttributes) -> Self {
+    pub(crate) fn new(attributes: &mut PlatformSpecificEventLoopAttributes) -> Self {
         let thread_id = unsafe { GetCurrentThreadId() };
 
         if !attributes.any_thread && thread_id != main_thread_id() {
@@ -211,6 +214,7 @@ impl<T: 'static> EventLoop<T> {
                 },
                 _marker: PhantomData,
             },
+            msg_hook: attributes.msg_hook.take(),
         }
     }
 
@@ -251,8 +255,16 @@ impl<T: 'static> EventLoop<T> {
                 if GetMessageW(&mut msg, 0, 0, 0) == false.into() {
                     break 'main 0;
                 }
-                TranslateMessage(&msg);
-                DispatchMessageW(&msg);
+
+                let handled = if let Some(callback) = self.msg_hook.as_deref_mut() {
+                    callback(&mut msg as *mut _ as *mut _)
+                } else {
+                    false
+                };
+                if !handled {
+                    TranslateMessage(&msg);
+                    DispatchMessageW(&msg);
+                }
 
                 if let Err(payload) = runner.take_panic_error() {
                     runner.reset_runner();