controller =

19
37

AnimationController(duration: const Duration(seconds: 2), vsync: this);

20

animation = Tween<double>(begin: 0, end: 300).animate(controller)

21

..addListener(() {

22

setState(() {

23

// The state that has changed here is the animation object’s value.

24

});

25

});

38

+
                animation = Tween<double>(begin: 0, end: 300).animate(controller);

26
39

controller.forward();

27
40

28
41