Add X11 opt-in function for device events

Previously on X11, by default all global events were broadcasted to
every winit application. This unnecessarily drains battery due to
excessive CPU usage when moving the mouse.

To resolve this, device events are now ignored by default and users must
manually opt into it using
`EventLoopWindowTarget::set_filter_device_events`.

Fixes (#1634) on Linux.

CHANGELOG.md

@@ -50,6 +50,7 @@ And please only add new entries to the top of this list, right below the `# Unre
 - Added `Window::set_ime_allowed` supported on desktop platforms.
 - **Breaking:** IME input on desktop platforms won't be received unless it's explicitly allowed via `Window::set_ime_allowed` and new `WindowEvent::Ime` events are handled.
 - On macOS, `WindowEvent::Resized` is now emitted in `frameDidChange` instead of `windowDidResize`.
+- **Breaking:** On X11, device events are now ignored for unfocused windows by default, use `EventLoopWindowTarget::set_device_event_filter` to set the filter level.
 
 # 0.26.1 (2022-01-05)
 

src/event_loop.rs

@@ -286,6 +286,28 @@ impl<T> EventLoopWindowTarget<T> {
     pub fn primary_monitor(&self) -> Option<MonitorHandle> {
         self.p.primary_monitor()
     }
+
+    /// Change [`DeviceEvent`] filter mode.
+    ///
+    /// Since the [`DeviceEvent`] capture can lead to high CPU usage for unfocused windows, winit
+    /// will ignore them by default for unfocused windows on Linux/BSD. This method allows changing
+    /// this filter at runtime to explicitly capture them again.
+    ///
+    /// ## Platform-specific
+    ///
+    /// - **Wayland / Windows / macOS / iOS / Android / Web**: Unsupported.
+    ///
+    /// [`DeviceEvent`]: crate::event::DeviceEvent
+    pub fn set_device_event_filter(&mut self, _filter: DeviceEventFilter) {
+        #[cfg(any(
+            target_os = "linux",
+            target_os = "dragonfly",
+            target_os = "freebsd",
+            target_os = "netbsd",
+            target_os = "openbsd"
+        ))]
+        self.p.set_device_event_filter(_filter);
+    }
 }
 
 /// Used to send custom events to `EventLoop`.
@@ -330,3 +352,20 @@ impl<T> fmt::Display for EventLoopClosed<T> {
 }
 
 impl<T: fmt::Debug> error::Error for EventLoopClosed<T> {}
+
+/// Fiter controlling the propagation of device events.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
+pub enum DeviceEventFilter {
+    /// Always filter out device events.
+    Always,
+    /// Filter out device events while the window is not focused.
+    Unfocused,
+    /// Report all device events regardless of window focus.
+    Never,
+}
+
+impl Default for DeviceEventFilter {
+    fn default() -> Self {
+        Self::Unfocused
+    }
+}

src/platform_impl/linux/mod.rs

@@ -30,7 +30,9 @@ use crate::{
     dpi::{PhysicalPosition, PhysicalSize, Position, Size},
     error::{ExternalError, NotSupportedError, OsError as RootOsError},
     event::Event,
-    event_loop::{ControlFlow, EventLoopClosed, EventLoopWindowTarget as RootELW},
+    event_loop::{
+        ControlFlow, DeviceEventFilter, EventLoopClosed, EventLoopWindowTarget as RootELW,
+    },
     icon::Icon,
     monitor::{MonitorHandle as RootMonitorHandle, VideoMode as RootVideoMode},
     window::{CursorIcon, Fullscreen, UserAttentionType, WindowAttributes},
@@ -732,7 +734,7 @@ impl<T: 'static> EventLoop<T> {
     }
 
     pub fn window_target(&self) -> &crate::event_loop::EventLoopWindowTarget<T> {
-        x11_or_wayland!(match self; EventLoop(evl) => evl.window_target())
+        x11_or_wayland!(match self; EventLoop(evlp) => evlp.window_target())
     }
 }
 
@@ -793,6 +795,16 @@ impl<T> EventLoopWindowTarget<T> {
             }
         }
     }
+
+    #[inline]
+    pub fn set_device_event_filter(&mut self, _filter: DeviceEventFilter) {
+        match *self {
+            #[cfg(feature = "wayland")]
+            EventLoopWindowTarget::Wayland(_) => (),
+            #[cfg(feature = "x11")]
+            EventLoopWindowTarget::X(ref mut evlp) => evlp.set_device_event_filter(_filter),
+        }
+    }
 }
 
 fn sticky_exit_callback<T, F>(

src/platform_impl/linux/x11/event_processor.rs

@@ -49,7 +49,7 @@ impl<T: 'static> EventProcessor<T> {
         let mut devices = self.devices.borrow_mut();
         if let Some(info) = DeviceInfo::get(&wt.xconn, device) {
             for info in info.iter() {
-                devices.insert(DeviceId(info.deviceid), Device::new(self, info));
+                devices.insert(DeviceId(info.deviceid), Device::new(info));
             }
         }
     }
@@ -907,6 +907,8 @@ impl<T: 'static> EventProcessor<T> {
                         if self.active_window != Some(xev.event) {
                             self.active_window = Some(xev.event);
 
+                            wt.update_device_event_filter(true);
+
                             let window_id = mkwid(xev.event);
                             let position = PhysicalPosition::new(xev.event_x, xev.event_y);
 
@@ -956,6 +958,7 @@ impl<T: 'static> EventProcessor<T> {
                         if !self.window_exists(xev.event) {
                             return;
                         }
+
                         wt.ime
                             .borrow_mut()
                             .unfocus(xev.event)
@@ -964,6 +967,8 @@ impl<T: 'static> EventProcessor<T> {
                         if self.active_window.take() == Some(xev.event) {
                             let window_id = mkwid(xev.event);
 
+                            wt.update_device_event_filter(false);
+
                             // Issue key release events for all pressed keys
                             Self::handle_pressed_keys(
                                 wt,

src/platform_impl/linux/x11/mod.rs

@@ -50,7 +50,9 @@ use self::{
 use crate::{
     error::OsError as RootOsError,
     event::{Event, StartCause},
-    event_loop::{ControlFlow, EventLoopClosed, EventLoopWindowTarget as RootELW},
+    event_loop::{
+        ControlFlow, DeviceEventFilter, EventLoopClosed, EventLoopWindowTarget as RootELW,
+    },
     platform_impl::{platform::sticky_exit_callback, PlatformSpecificWindowBuilderAttributes},
     window::WindowAttributes,
 };
@@ -105,6 +107,7 @@ pub struct EventLoopWindowTarget<T> {
     ime: RefCell<Ime>,
     windows: RefCell<HashMap<WindowId, Weak<UnownedWindow>>>,
     redraw_sender: WakeSender<WindowId>,
+    device_event_filter: DeviceEventFilter,
     _marker: ::std::marker::PhantomData<T>,
 }
 
@@ -229,8 +232,7 @@ impl<T: 'static> EventLoop<T> {
         let (user_sender, user_channel) = std::sync::mpsc::channel();
         let (redraw_sender, redraw_channel) = std::sync::mpsc::channel();
 
-        let target = Rc::new(RootELW {
+        let window_target = EventLoopWindowTarget {
-            p: super::EventLoopWindowTarget::X(EventLoopWindowTarget {
             ime,
             root,
             windows: Default::default(),
@@ -243,7 +245,14 @@ impl<T: 'static> EventLoop<T> {
                 sender: redraw_sender, // not used again so no clone
                 waker: waker.clone(),
             },
-            }),
+            device_event_filter: Default::default(),
+        };
+
+        // Set initial device event filter.
+        window_target.update_device_event_filter(true);
+
+        let target = Rc::new(RootELW {
+            p: super::EventLoopWindowTarget::X(window_target),
             _marker: ::std::marker::PhantomData,
         });
 
@@ -524,6 +533,29 @@ impl<T> EventLoopWindowTarget<T> {
     pub fn x_connection(&self) -> &Arc<XConnection> {
         &self.xconn
     }
+
+    pub fn set_device_event_filter(&mut self, filter: DeviceEventFilter) {
+        self.device_event_filter = filter;
+    }
+
+    /// Update the device event filter based on window focus.
+    pub fn update_device_event_filter(&self, focus: bool) {
+        let filter_events = self.device_event_filter == DeviceEventFilter::Never
+            || (self.device_event_filter == DeviceEventFilter::Unfocused && !focus);
+
+        let mut mask = 0;
+        if !filter_events {
+            mask = ffi::XI_RawMotionMask
+                | ffi::XI_RawButtonPressMask
+                | ffi::XI_RawButtonReleaseMask
+                | ffi::XI_RawKeyPressMask
+                | ffi::XI_RawKeyReleaseMask;
+        }
+
+        self.xconn
+            .select_xinput_events(self.root, ffi::XIAllDevices, mask)
+            .queue();
+    }
 }
 
 impl<T: 'static> EventLoopProxy<T> {
@@ -698,24 +730,11 @@ enum ScrollOrientation {
 }
 
 impl Device {
-    fn new<T: 'static>(el: &EventProcessor<T>, info: &ffi::XIDeviceInfo) -> Self {
+    fn new(info: &ffi::XIDeviceInfo) -> Self {
         let name = unsafe { CStr::from_ptr(info.name).to_string_lossy() };
         let mut scroll_axes = Vec::new();
 
-        let wt = get_xtarget(&el.target);
-
         if Device::physical_device(info) {
-            // Register for global raw events
-            let mask = ffi::XI_RawMotionMask
-                | ffi::XI_RawButtonPressMask
-                | ffi::XI_RawButtonReleaseMask
-                | ffi::XI_RawKeyPressMask
-                | ffi::XI_RawKeyReleaseMask;
-            // The request buffer is flushed when we poll for events
-            wt.xconn
-                .select_xinput_events(wt.root, info.deviceid, mask)
-                .queue();
-
             // Identify scroll axes
             for class_ptr in Device::classes(info) {
                 let class = unsafe { &**class_ptr };