fixes GLFW Plugin thread safety

Using HandleFuncSync instead of HandleFunc to ensure the plugin method
handler is called from the go-flutter main thread.

fixes go-flutter-desktop/go-flutter#214

Author: pchampio

draggable_borderless/go/cmd/options.go

@@ -16,6 +16,8 @@ var options = []flutter.Option{
 type AppBarDraggable struct {
 	window           *glfw.Window
 	windowDragActive chan bool
+	cursorPosY       int
+	cursorPosX       int
 }
 
 var _ flutter.Plugin = &AppBarDraggable{}     // compile-time type check
@@ -27,7 +29,8 @@ func (p *AppBarDraggable) InitPlugin(messenger plugin.BinaryMessenger) error {
 	p.windowDragActive = make(chan bool)
 	channel := plugin.NewMethodChannel(messenger, "samples.go-flutter.dev/draggable", plugin.StandardMethodCodec{})
 	channel.HandleFunc("onPanStart", p.onPanStart)
-	channel.HandleFunc("onPanEnd", p.onPanEnd)
+	channel.HandleFuncSync("onPanUpdate", p.onPanUpdate) // MUST RUN ON THE MAIN THREAD (use of HandleFuncSync)
+	channel.HandleFunc("onClose", p.onClose)
 	return nil
 }
 
@@ -37,31 +40,31 @@ func (p *AppBarDraggable) InitPluginGLFW(window *glfw.Window) error {
 	return nil
 }
 
-// onPanStart a golang / flutter implemantation of:
+// onPanStart/onPanUpdate a golang / flutter implemantation of:
 // "GLFW how to drag undecorated window without lag"
 // https://stackoverflow.com/a/46205940
 func (p *AppBarDraggable) onPanStart(arguments interface{}) (reply interface{}, err error) {
 	argumentsMap := arguments.(map[interface{}]interface{})
-	cursorPosX := int(argumentsMap["dx"].(float64))
-	cursorPosY := int(argumentsMap["dy"].(float64))
-	for {
-		select {
-		case <-p.windowDragActive:
-			return
-		default:
-			xpos, ypos := p.window.GetCursorPos()
-			deltaX := int(xpos) - cursorPosX
-			deltaY := int(ypos) - cursorPosY
+	p.cursorPosX = int(argumentsMap["dx"].(float64))
+	p.cursorPosY = int(argumentsMap["dy"].(float64))
+	return nil, nil
+}
+
+// onPanUpdate calls GLFW functions that aren't thread safe.
+// to run function on the main go-flutter thread, use HandleFuncSync instead of HandleFunc!
+func (p *AppBarDraggable) onPanUpdate(arguments interface{}) (reply interface{}, err error) {
+	xpos, ypos := p.window.GetCursorPos() // This function must only be called from the main thread.
+	deltaX := int(xpos) - p.cursorPosX
+	deltaY := int(ypos) - p.cursorPosY
 
-			x, y := p.window.GetPos()
-			p.window.SetPos(x+deltaX, y+deltaY)
-		}
+	x, y := p.window.GetPos()           // This function must only be called from the main thread.
+	p.window.SetPos(x+deltaX, y+deltaY) // This function must only be called from the main thread.
 
-	}
 	return nil, nil
 }
 
-func (p *AppBarDraggable) onPanEnd(arguments interface{}) (reply interface{}, err error) {
-	p.windowDragActive <- false
+func (p *AppBarDraggable) onClose(arguments interface{}) (reply interface{}, err error) {
+	// This function may be called from any thread. Access is not synchronized.
+	p.window.SetShouldClose(true)
 	return nil, nil
 }

draggable_borderless/lib/main_desktop.dart

@@ -14,6 +14,7 @@ class MyApp extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return MaterialApp(
+      debugShowCheckedModeBanner: false,
       title: 'Flutter Code Sample for widgets.Listener',
       theme: ThemeData(
         // If the host is missing some fonts, it can cause the
@@ -22,11 +23,7 @@ class MyApp extends StatelessWidget {
         primarySwatch: Colors.blue,
       ),
       home: Scaffold(
-        appBar: DraggebleAppBar(
-          appBar: AppBar(
-            title: Text("Draggable borderless"),
-          ),
-        ),
+        appBar: DraggebleAppBar(title: "Draggable borderless"),
         body: Center(
           child: Column(
             mainAxisAlignment: MainAxisAlignment.center,
@@ -47,25 +44,37 @@ class DraggebleAppBar extends StatelessWidget implements PreferredSizeWidget {
   static const platform_channel_draggable =
       MethodChannel('samples.go-flutter.dev/draggable');
 
-  final AppBar appBar;
+  AppBar appBar;
 
-  const DraggebleAppBar({Key key, this.appBar}) : super(key: key);
+  DraggebleAppBar({@required String title}) {
+    this.appBar = AppBar(
+      title: Text(title),
+      actions: <Widget>[
+        new IconButton(
+          icon: new Icon(Icons.close),
+          onPressed: () async =>
+              await platform_channel_draggable.invokeMethod("onClose"),
+        ),
+      ],
+      leading: new Container(),
+    );
+  }
 
   @override
   Widget build(BuildContext context) {
     return GestureDetector(
-        child: appBar, onPanStart: onPanStart, onPanEnd: onPanEnd);
+        child: appBar, onPanStart: onPanStart, onPanUpdate: onPanUpdate);
   }
 
   @override
   Size get preferredSize => new Size.fromHeight(kToolbarHeight);
 
+  void onPanUpdate(DragUpdateDetails details) async {
+    await platform_channel_draggable.invokeMethod('onPanUpdate');
+  }
+
   void onPanStart(DragStartDetails details) async {
     await platform_channel_draggable.invokeMethod('onPanStart',
         {"dx": details.globalPosition.dx, "dy": details.globalPosition.dy});
   }
-
-  void onPanEnd(DragEndDetails details) async {
-    await platform_channel_draggable.invokeMethod('onPanEnd');
-  }
 }