countdown timer

AnimationController controller;

  String get timerString {
    Duration duration = controller.duration * controller.value;
    return '${(duration.inMinutes % 60).toString().padLeft(2, '0')}:${(duration.inSeconds % 60).toString().padLeft(2, '0')}';
  }

  @override
  void initState() {
    super.initState();
    controller = AnimationController(
      vsync: this,
      duration: Duration(seconds: 3),

    );