anchor.diff:    @override
     Widget build(BuildContext context) {
       final Size screenSize = MediaQuery.of(context).size;
  -    _begin = new Point(screenSize.width * 0.5, screenSize.height * 0.2);
  -    _end = new Point(screenSize.width * 0.1, screenSize.height * 0.4);
  +    if (_screenSize == null || _screenSize != screenSize) {
  +      _screenSize = screenSize;
  +      _begin = new Point(screenSize.width * 0.5, screenSize.height * 0.2);
  +      _end = new Point(screenSize.width * 0.1, screenSize.height * 0.4);
  +    }
   
       final MaterialPointArcTween arc = new MaterialPointArcTween(begin: _begin, end: _end);
       return new RawGestureDetector(
  
positive.code:     final Paint paint = new Paint();
  
      if (arc.center != null)
        drawPoint(canvas, arc.center, Colors.blue[400]);
  
      paint
        ..color = Colors.green[500].withOpacity(0.25)
  
negative.code: 
    CurvedAnimation _animation;
    _DragTarget _dragTarget;
    Size _screenSize;
    Point _begin;
    Point _end;
  
  