eframe: read the state of native window's input focus and request it (#2900)

* Add ability to read the native window's focus state

* Add `eframe::Frame::focus()` for requesting the native window's focus

* rename the output field `active` → `focused` for consistency

crates/eframe/src/epi.rs

@@ -814,6 +814,15 @@ impl Frame {
         self.output.minimized = Some(minimized);
     }
 
+    /// Bring the window into focus (native only). Has no effect on Wayland, or if the window is minimized or invisible.
+    ///
+    /// This method puts the window on top of other applications and takes input focus away from them,
+    /// which, if unexpected, will disturb the user.
+    #[cfg(not(target_arch = "wasm32"))]
+    pub fn focus(&mut self) {
+        self.output.focus = Some(true);
+    }
+
     /// Maximize or unmaximize window. (native only)
     #[cfg(not(target_arch = "wasm32"))]
     pub fn set_maximized(&mut self, maximized: bool) {
@@ -938,6 +947,9 @@ pub struct WindowInfo {
     /// Are we maximized?
     pub maximized: bool,
 
+    /// Is the window focused and able to receive input?
+    pub focused: bool,
+
     /// Window inner size in egui points (logical pixels).
     pub size: egui::Vec2,
 
@@ -1129,6 +1141,10 @@ pub(crate) mod backend {
         #[cfg(not(target_arch = "wasm32"))]
         pub maximized: Option<bool>,
 
+        /// Set to some bool to focus window.
+        #[cfg(not(target_arch = "wasm32"))]
+        pub focus: Option<bool>,
+
         #[cfg(not(target_arch = "wasm32"))]
         pub screenshot_requested: bool,
     }

crates/eframe/src/native/epi_integration.rs

@@ -61,6 +61,7 @@ pub fn read_window_info(
         fullscreen: window.fullscreen().is_some(),
         minimized: window_state.minimized,
         maximized: window_state.maximized,
+        focused: window.has_focus(),
         size: egui::Vec2 {
             x: size.width,
             y: size.height,
@@ -231,6 +232,7 @@ pub fn handle_app_output(
         screenshot_requested: _, // handled by the rendering backend,
         minimized,
         maximized,
+        focus,
     } = app_output;
 
     if let Some(decorated) = decorated {
@@ -284,6 +286,10 @@ pub fn handle_app_output(
         window.set_maximized(maximized);
         window_state.maximized = maximized;
     }
+
+    if focus == Some(true) {
+        window.focus_window();
+    }
 }
 
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