custom_switch.dart

Animation _circleAnimation;
AnimationController _animationController;

  @override
  void initState() {
    super.initState();
    _animationController =
        AnimationController(vsync: this, duration: Duration(milliseconds: 60));
    _circleAnimation = AlignmentTween(
            begin: widget.value ? Alignment.centerRight : Alignment.centerLeft,
            end: widget.value ? Alignment.centerLeft : Alignment.centerRight)
        .animate(CurvedAnimation(
            parent: _animationController, curve: Curves.linear));
  }