switch scale mode if 3 fingers tap

flutter_hbb/lib/drag.dart

@@ -0,0 +1,104 @@
+import 'package:flutter/gestures.dart';
+
+class MultiTouchGestureRecognizer extends MultiTapGestureRecognizer {
+  MultiTouchGestureRecognizerCallback onMultiTap;
+  var numberOfTouches = 0;
+
+  MultiTouchGestureRecognizer() {
+    this
+      ..onTapDown = addTouch
+      ..onTapUp = removeTouch
+      ..onTapCancel = cancelTouch
+      ..onTap = captureDefaultTap;
+  }
+
+  void addTouch(int pointer, TapDownDetails details) {
+    numberOfTouches++;
+    onMultiTap(numberOfTouches, true);
+  }
+
+  void removeTouch(int pointer, TapUpDetails details) {
+    numberOfTouches--;
+    onMultiTap(numberOfTouches, false);
+  }
+
+  void cancelTouch(int pointer) {
+    numberOfTouches = 0;
+  }
+
+  void captureDefaultTap(int pointer) {}
+}
+
+typedef MultiTouchGestureRecognizerCallback = void Function(
+    int touchCount, bool addOrRemove);
+
+typedef OnUpdate(DragUpdateDetails details);
+
+class CustomMultiDrag extends Drag {
+  CustomMultiDrag({this.events, this.offset});
+
+  List<PointerDownEvent> events;
+  Offset offset;
+
+  @override
+  void update(DragUpdateDetails details) {
+    var n = events.length;
+    print('$n $details');
+  }
+
+  @override
+  void end(DragEndDetails details) {
+    super.end(details);
+  }
+}
+
+typedef OnDisposeState();
+
+// clone _ImmediatePointerState
+class CustomPointerState extends MultiDragPointerState {
+  final OnDisposeState onDisposeState;
+  CustomPointerState(Offset initialPosition, PointerDeviceKind kind,
+      {this.onDisposeState})
+      : super(initialPosition, kind);
+
+  @override
+  void checkForResolutionAfterMove() {
+    assert(pendingDelta != null);
+    if (pendingDelta.distance > computeHitSlop(kind))
+      resolve(GestureDisposition.accepted);
+  }
+
+  @override
+  void accepted(GestureMultiDragStartCallback starter) {
+    starter(initialPosition);
+  }
+
+  @override
+  void dispose() {
+    onDisposeState.call();
+    super.dispose();
+  }
+}
+
+// clone ImmediateMultiDragGestureRecognizer
+class CustomMultiDragGestureRecognizer
+    extends MultiDragGestureRecognizer<CustomPointerState> {
+  var events = List<PointerDownEvent>();
+
+  /// Create a gesture recognizer for tracking multiple pointers at once.
+  CustomMultiDragGestureRecognizer({
+    Object debugOwner,
+    PointerDeviceKind kind,
+  }) : super(debugOwner: debugOwner, kind: kind);
+
+  @override
+  CustomPointerState createNewPointerState(PointerDownEvent event) {
+    events.add(event);
+    return CustomPointerState(event.position, event.kind, onDisposeState: () {
+      events.remove(event);
+    });
+  }
+
+  @override
+  String get debugDescription => 'custom_multidrag';
+}