Add Context::request_repaint_after

eframe/src/native/run.rs

@@ -107,7 +107,7 @@ pub fn run_glow(
 
             let egui::FullOutput {
                 platform_output,
-                needs_repaint,
+                repaint_after,
                 textures_delta,
                 shapes,
             } = integration.update(app.as_mut(), window);
@@ -133,9 +133,18 @@ pub fn run_glow(
 
             *control_flow = if integration.should_quit() {
                 winit::event_loop::ControlFlow::Exit
-            } else if needs_repaint {
+            } else if repaint_after.is_zero() {
                 window.request_redraw();
                 winit::event_loop::ControlFlow::Poll
+            } else if let Some(repaint_after_instant) =
+                std::time::Instant::now().checked_add(repaint_after)
+            {
+                // if repaint_after is something huge and can't be added to Instant,
+                // we will use `ControlFlow::Wait` instead.
+                // technically, this might lead to some weird corner cases where the user *WANTS*
+                // winit to use `WaitUntil(MAX_INSTANT)` explicitly. they can roll their own
+                // egui backend impl i guess.
+                winit::event_loop::ControlFlow::WaitUntil(repaint_after_instant)
             } else {
                 winit::event_loop::ControlFlow::Wait
             };
@@ -152,7 +161,6 @@ pub fn run_glow(
                 std::thread::sleep(std::time::Duration::from_millis(10));
             }
         };
-
         match event {
             // Platform-dependent event handlers to workaround a winit bug
             // See: https://github.com/rust-windowing/winit/issues/987
@@ -194,7 +202,12 @@ pub fn run_glow(
                 painter.destroy();
             }
             winit::event::Event::UserEvent(RequestRepaintEvent) => window.request_redraw(),
-            _ => (),
+            winit::event::Event::NewEvents(winit::event::StartCause::ResumeTimeReached {
+                ..
+            }) => {
+                window.request_redraw();
+            }
+            _ => {}
         }
     });
 }
@@ -279,7 +292,7 @@ pub fn run_wgpu(
 
             let egui::FullOutput {
                 platform_output,
-                needs_repaint,
+                repaint_after,
                 textures_delta,
                 shapes,
             } = integration.update(app.as_mut(), window);
@@ -300,9 +313,18 @@ pub fn run_wgpu(
 
             *control_flow = if integration.should_quit() {
                 winit::event_loop::ControlFlow::Exit
-            } else if needs_repaint {
+            } else if repaint_after.is_zero() {
                 window.request_redraw();
                 winit::event_loop::ControlFlow::Poll
+            } else if let Some(repaint_after_instant) =
+                std::time::Instant::now().checked_add(repaint_after)
+            {
+                // if repaint_after is something huge and can't be added to Instant,
+                // we will use `ControlFlow::Wait` instead.
+                // technically, this might lead to some weird corner cases where the user *WANTS*
+                // winit to use `WaitUntil(MAX_INSTANT)` explicitly. they can roll their own
+                // egui backend impl i guess.
+                winit::event_loop::ControlFlow::WaitUntil(repaint_after_instant)
             } else {
                 winit::event_loop::ControlFlow::Wait
             };

eframe/src/web/backend.rs

@@ -248,7 +248,7 @@ impl AppRunner {
     /// Returns `true` if egui requests a repaint.
     ///
     /// Call [`Self::paint`] later to paint
-    pub fn logic(&mut self) -> Result<(bool, Vec<egui::ClippedPrimitive>), JsValue> {
+    pub fn logic(&mut self) -> Result<(std::time::Duration, Vec<egui::ClippedPrimitive>), JsValue> {
         let frame_start = now_sec();
 
         resize_canvas_to_screen_size(self.canvas_id(), self.app.max_size_points());
@@ -260,7 +260,7 @@ impl AppRunner {
         });
         let egui::FullOutput {
             platform_output,
-            needs_repaint,
+            repaint_after,
             textures_delta,
             shapes,
         } = full_output;
@@ -282,7 +282,7 @@ impl AppRunner {
         }
 
         self.frame.info.cpu_usage = Some((now_sec() - frame_start) as f32);
-        Ok((needs_repaint, clipped_primitives))
+        Ok((repaint_after, clipped_primitives))
     }
 
     pub fn clear_color_buffer(&self) {

eframe/src/web/events.rs

@@ -9,9 +9,9 @@ pub fn paint_and_schedule(
         let mut runner_lock = runner_ref.lock();
         if runner_lock.needs_repaint.fetch_and_clear() {
             runner_lock.clear_color_buffer();
-            let (needs_repaint, clipped_primitives) = runner_lock.logic()?;
+            let (repaint_after, clipped_primitives) = runner_lock.logic()?;
             runner_lock.paint(&clipped_primitives)?;
-            if needs_repaint {
+            if repaint_after.is_zero() {
                 runner_lock.needs_repaint.set_true();
             }
             runner_lock.auto_save();

egui-winit/src/lib.rs

@@ -475,7 +475,6 @@ impl State {
             mutable_text_under_cursor: _, // only used in eframe web
             text_cursor_pos,
         } = platform_output;
-
         self.current_pixels_per_point = egui_ctx.pixels_per_point(); // someone can have changed it to scale the UI
 
         self.set_cursor_icon(window, cursor_icon);

egui/src/context.rs

@@ -28,7 +28,6 @@ impl Default for WrappedTextureManager {
 }
 
 // ----------------------------------------------------------------------------
-
 #[derive(Default)]
 struct ContextImpl {
     /// `None` until the start of the first frame.
@@ -47,7 +46,9 @@ struct ContextImpl {
     output: PlatformOutput,
 
     paint_stats: PaintStats,
-
+    /// the duration backend will poll for new events, before forcing another egui update
+    /// even if there's no new events.
+    repaint_after: std::time::Duration,
     /// While positive, keep requesting repaints. Decrement at the end of each frame.
     repaint_requests: u32,
     request_repaint_callbacks: Option<Box<dyn Fn() + Send + Sync>>,
@@ -574,6 +575,39 @@ impl Context {
         }
     }
 
+    /// request repaint after the specified duration elapses in the case of no new input
+    /// events being received.
+    ///
+    /// function can be multiple times, but only the *smallest* duration will be considered.
+    /// so, if the function is called two times with `1 second` and `2 seconds`, egui will repaint
+    /// after `1 second`
+    ///
+    /// this is primarily useful for applications who would like to save battery by avoiding wasted
+    /// redraws when the app is not in focus. but sometimes the GUI of the app might become stale
+    /// and outdated if it is not updated for too long.
+    ///
+    /// lets say, something like a stop watch widget that displays the time in seconds. you would waste
+    /// resources repainting multiple times within the same second (when you have no input),
+    /// just calculate the difference of duration between current time and next second change,
+    /// and call this function, to make sure that you are displaying the latest updated time, but
+    /// not wasting resources on needless repaints within the same second.
+    ///
+    /// NOTE: only works if called before `Context::end_frame()`. to force egui to update,
+    /// use `Context::request_repaint()` instead.
+    ///
+    /// Quirk:
+    ///
+    ///     duration begins at the next frame. lets say for example that its a very inefficient app
+    ///     and takes 500 milliseconds per frame at 2 fps. the widget / user might want a repaint in
+    ///     next 500 milliseconds. now, app takes 1000 ms per frame (1 fps) because the backend event
+    ///     timeout takes 500 milli seconds AFTER the vsync swap buffer.
+    ///     so, its not that we are requesting repaint within X duration. we are rather timing out
+    ///     during app idle time where we are not receiving any new input events.
+    pub fn request_repaint_after(&self, duration: std::time::Duration) {
+        // maybe we can check if duration is ZERO, and call self.request_repaint()?
+        let mut ctx = self.write();
+        ctx.repaint_after = ctx.repaint_after.min(duration);
+    }
     /// For integrations: this callback will be called when an egui user calls [`Self::request_repaint`].
     ///
     /// This lets you wake up a sleeping UI thread.
@@ -805,19 +839,18 @@ impl Context {
 
         let platform_output: PlatformOutput = std::mem::take(&mut self.output());
 
-        let needs_repaint = if self.read().repaint_requests > 0 {
-            self.write().repaint_requests -= 1;
-            true
-        } else {
-            false
-        };
-        self.write().requested_repaint_last_frame = needs_repaint;
-
+        let repaint_after = self.read().repaint_after;
+        self.write().requested_repaint_last_frame = repaint_after.is_zero();
+        // make sure we reset the repaint_after duration.
+        // otherwise, if repaint_after is low, then any widget setting repaint_after next frame,
+        // will fail to overwrite the previous lower value. and thus, repaints will never
+        // go back to higher values.
+        self.write().repaint_after = std::time::Duration::MAX;
         let shapes = self.drain_paint_lists();
 
         FullOutput {
             platform_output,
-            needs_repaint,
+            repaint_after,
             textures_delta,
             shapes,
         }

egui/src/data/output.rs

@@ -10,10 +10,15 @@ pub struct FullOutput {
     /// Non-rendering related output.
     pub platform_output: PlatformOutput,
 
-    /// If `true`, egui is requesting immediate repaint (i.e. on the next frame).
+    /// If `Duration::is_zero()`, egui is requesting immediate repaint (i.e. on the next frame).
     ///
     /// This happens for instance when there is an animation, or if a user has called `Context::request_repaint()`.
-    pub needs_repaint: bool,
+    ///
+    /// If `Duration` is greater than zero, egui wants to be repainted at or before the specified
+    /// duration elapses. when in reactive mode, egui spends forever waiting for input and only then,
+    /// will it repaint itself. this can be used to make sure that backend will only wait for a
+    /// specified amount of time, and repaint egui without any new input.
+    pub repaint_after: std::time::Duration,
 
     /// Texture changes since last frame (including the font texture).
     ///
@@ -32,13 +37,13 @@ impl FullOutput {
     pub fn append(&mut self, newer: Self) {
         let Self {
             platform_output,
-            needs_repaint,
+            repaint_after,
             textures_delta,
             shapes,
         } = newer;
 
         self.platform_output.append(platform_output);
-        self.needs_repaint = needs_repaint; // if the last frame doesn't need a repaint, then we don't need to repaint
+        self.repaint_after = repaint_after; // if the last frame doesn't need a repaint, then we don't need to repaint
         self.textures_delta.append(textures_delta);
         self.shapes = shapes; // Only paint the latest
     }
@@ -49,7 +54,7 @@ impl FullOutput {
 /// You can access (and modify) this with [`crate::Context::output`].
 ///
 /// The backend should use this.
-#[derive(Clone, Default, PartialEq)]
+#[derive(Default, Clone, PartialEq)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct PlatformOutput {
     /// Set the cursor to this icon.

egui_demo_app/src/backend_panel.rs

@@ -1,3 +1,5 @@
+use egui::Widget;
+
 /// How often we repaint the demo app by default
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 enum RunMode {
@@ -41,7 +43,6 @@ impl Default for RunMode {
 
 // ----------------------------------------------------------------------------
 
-#[derive(Default)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 #[cfg_attr(feature = "serde", serde(default))]
 pub struct BackendPanel {
@@ -51,6 +52,10 @@ pub struct BackendPanel {
     // go back to [`Reactive`] mode each time we start
     run_mode: RunMode,
 
+    #[cfg_attr(feature = "serde", serde(skip))]
+    // reset to 1 second as default repaint_after idle timeout.
+    repaint_after_timeout: std::time::Duration,
+
     /// current slider value for current gui scale
     #[cfg_attr(feature = "serde", serde(skip))]
     pixels_per_point: Option<f32>,
@@ -61,14 +66,32 @@ pub struct BackendPanel {
     egui_windows: EguiWindows,
 }
 
+impl Default for BackendPanel {
+    fn default() -> Self {
+        Self {
+            open: false,
+            run_mode: Default::default(),
+            repaint_after_timeout: std::time::Duration::from_secs(1),
+            pixels_per_point: None,
+            frame_history: Default::default(),
+            egui_windows: Default::default(),
+        }
+    }
+}
+
 impl BackendPanel {
     pub fn update(&mut self, ctx: &egui::Context, frame: &mut eframe::Frame) {
         self.frame_history
             .on_new_frame(ctx.input().time, frame.info().cpu_usage);
 
-        if self.run_mode == RunMode::Continuous {
-            // Tell the backend to repaint as soon as possible
-            ctx.request_repaint();
+        match self.run_mode {
+            RunMode::Reactive => {
+                ctx.request_repaint_after(self.repaint_after_timeout);
+            }
+            RunMode::Continuous => {
+                // Tell the backend to repaint as soon as possible
+                ctx.request_repaint();
+            }
         }
     }
 
@@ -220,6 +243,16 @@ impl BackendPanel {
             ));
         } else {
             ui.label("Only running UI code when there are animations or input.");
+            ui.label("but if there's no input for the repaint_after duration, we force an update");
+            ui.label("repaint_after (in seconds)");
+            let mut milli_seconds = self.repaint_after_timeout.as_secs_f32();
+            if egui::DragValue::new(&mut milli_seconds)
+                .clamp_range(0.1..=10.0)
+                .ui(ui)
+                .changed()
+            {
+                self.repaint_after_timeout = std::time::Duration::from_secs_f32(milli_seconds);
+            }
         }
     }
 }

egui_glium/src/lib.rs

@@ -56,13 +56,17 @@ impl EguiGlium {
     /// Returns `true` if egui requests a repaint.
     ///
     /// Call [`Self::paint`] later to paint.
-    pub fn run(&mut self, display: &glium::Display, run_ui: impl FnMut(&egui::Context)) -> bool {
+    pub fn run(
+        &mut self,
+        display: &glium::Display,
+        run_ui: impl FnMut(&egui::Context),
+    ) -> std::time::Duration {
         let raw_input = self
             .egui_winit
             .take_egui_input(display.gl_window().window());
         let egui::FullOutput {
             platform_output,
-            needs_repaint,
+            repaint_after,
             textures_delta,
             shapes,
         } = self.egui_ctx.run(raw_input, run_ui);
@@ -76,7 +80,7 @@ impl EguiGlium {
         self.shapes = shapes;
         self.textures_delta.append(textures_delta);
 
-        needs_repaint
+        repaint_after
     }
 
     /// Paint the results of the last call to [`Self::run`].