epi: drag native window (#728)

* feat: drag window

* Update epi/src/lib.rs

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

* Update egui_demo_lib/src/backend_panel.rs

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

* cargo fmt

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

egui_demo_lib/src/backend_panel.rs

@@ -208,6 +208,13 @@ impl BackendPanel {
             if ui.button("Quit").clicked() {
                 frame.quit();
             }
+
+            if ui
+                .button("Drag me to drag window")
+                .is_pointer_button_down_on()
+            {
+                frame.drag_window();
+            }
         }
     }
 

egui_glium/src/backend.rs

@@ -328,6 +328,7 @@ pub fn run(mut app: Box<dyn epi::App>, native_options: &epi::NativeOptions) {
                     quit,
                     window_size,
                     decorated,
+                    drag_window,
                 } = app_output;
 
                 if let Some(decorated) = decorated {
@@ -344,6 +345,10 @@ pub fn run(mut app: Box<dyn epi::App>, native_options: &epi::NativeOptions) {
                     );
                 }
 
+                if drag_window {
+                    let _ = display.gl_window().window().drag_window();
+                }
+
                 if quit {
                     running = false;
                 } else if needs_repaint {

egui_web/src/backend.rs

@@ -259,6 +259,7 @@ impl AppRunner {
                 quit: _,        // Can't quit a web page
                 window_size: _, // Can't resize a web page
                 decorated: _,   // Can't show decorations
+                drag_window: _, // Can't be dragged
             } = app_output;
         }
 

epi/src/lib.rs

@@ -252,6 +252,14 @@ impl<'a> Frame<'a> {
         self.0.output.decorated = Some(decorated);
     }
 
+    /// When called, the native window will follow the
+    /// movement of the cursor while the primary mouse button is down.
+    ///
+    /// Does not work on the web, and works badly on Mac.
+    pub fn drag_window(&mut self) {
+        self.0.output.drag_window = true;
+    }
+
     /// If you need to request a repaint from another thread, clone this and send it to that other thread.
     pub fn repaint_signal(&self) -> std::sync::Arc<dyn RepaintSignal> {
         self.0.repaint_signal.clone()
@@ -410,5 +418,8 @@ pub mod backend {
 
         /// Set to some bool to change window decorations
         pub decorated: Option<bool>,
+
+        /// Set to true to drap window
+        pub drag_window: bool,
     }
 }