颤振负荷指示器

Question

class Loader extends StatefulWidget {
  @override
  State createState() => LoaderState();
}

class LoaderState extends State<Loader> with SingleTickerProviderStateMixin {
  AnimationController controller;
  Animation<double> animation;

  @override
  void initState() {
    super.initState();
    controller = AnimationController(
        duration: Duration(milliseconds: 1200), vsync: this);
    animation = CurvedAnimation(parent: controller, curve: Curves.elasticOut);
    animation.addListener(() {
      this.setState(() {});
    });
    animation.addStatusListener((AnimationStatus status) {});
    controller.repeat();
  }

  @override
  void dispose() {
    controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        Container(
          color: Colors.blue,
          height: 3.0,
          width: animation.value * 100.0,
        ),
        Padding(
          padding: EdgeInsets.only(bottom: 5.0),
        ),
        Container(
          color: Colors.blue[300],
          height: 3.0,
          width: animation.value * 75.0,
        ),
        Padding(
          padding: EdgeInsets.only(bottom: 5.0),
        ),
        Container(
          color: Colors.blue,
          height: 3.0,
          width: animation.value * 50.0,
        )
      ],
    );
  }
}    

Expanded(
            child: Padding(
                      padding:
                          EdgeInsets.only(left: 20.0, right: 5.0, top: 20.0),
                      child: GestureDetector(
                        onTap: () {
                          Navigator.push(
                              context,
                              MaterialPageRoute(
                                  builder: (context) => FirstScreen()));
                        },
                        child: Container(
                            alignment: Alignment.center,
                            height: 45.0,
                            decoration: BoxDecoration(
                                color: Color(0xFF1976D2),
                                borderRadius: BorderRadius.circular(9.0)),
                            child: Text('Login',
                                style: TextStyle(
                                    fontSize: 20.0, color: Colors.white))),
                      ),
                    ),
                  ),