Flutter - AnimationController

class _WelcomeScreenState extends State<WelcomeScreen>
    with SingleTickerProviderStateMixin {
  AnimationController controller;
  Animation animation;

  @override
  void initState() {
    super.initState();

    controller = AnimationController(
      duration: Duration(seconds: 1), //0 to 1 in 60 steps tickers.
      vsync: this, 
      //SingleTickerProviderStateMixin class is going to provide animation Ticker
      // upperBound: 100.0,有curve class必須小於1，以避免衝突
    );
    animation = CurvedAnimation(
        parent: controller, curve: Curves.easeIn); //upperBound 需小於1

    controller.forward();
    //Get the animation to start. proceed the animation forward(small to big)
    controller.addStatusListener((status) {
      if (status == AnimationStatus.completed) {

        controller.reverse(from: 1.0);//值由小到大
      } else if (status == AnimationStatus.dismissed) {
        controller.forward();
      }
      print(animation.status);
    });

    //Calls the listener every time the value of the animation changes.
    controller.addListener(() {
      setState(() {}); //call build()again and again.
   
      print(controller.value);
    });
  }

  @override 
  void dispose() { //在頁面毀之前，釋放記憶體，別再暫用資料。
    super.dispose();
    controller.dispose();

}