分页下标动画源码

直接引用 YJPvAnimationWidget()作为控件即可看效果。

import 'package:flutter/material.dart';

enum YJPvAnimationDirection {
  left,
  right,
}

class YJPvAnimationSharedModel {
  String action; //动画动作
  YJPvAnimationDirection direction; 
  YJPvAnimationSharedModel({this.action = 'wait', this.direction = YJPvAnimationDirection.right});
}

class YJPvAnimationSharedContext extends InheritedWidget{

  YJPvAnimationSharedModel model;

  YJPvAnimationSharedContext({
    Key key,
    @required this.model,
    @required Widget child,YJPvAnimationSharedModel
  }) : super(key: key, child: child);

  static YJPvAnimationSharedContext of(BuildContext context){
    return context.inheritFromWidgetOfExactType(YJPvAnimationSharedContext);
  }

  @override
  bool updateShouldNotify(YJPvAnimationSharedContext oldWidget) {
    // TODO: implement updateShouldNotify
    print('updateShouldNotify');
    return true;
  }

}

class YJPvAnimationWidget extends StatefulWidget {
  @override
  _YJPvAnimationWidgetState createState() => _YJPvAnimationWidgetState();
}

class _YJPvAnimationWidgetState extends State<YJPvAnimationWidget> {

  YJPvAnimationSharedModel model;

  @override
  void initState() {
    // TODO: implement initState
    model = new YJPvAnimationSharedModel();
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return new YJPvAnimationSharedContext(
      model: model, 
      child: Container(
        color: Colors.grey,
        height: 60,
        width: double.infinity,
        child: Stack(      
          children: <Widget>[
            Positioned(
              child: Container(   
                color:Colors.yellow,
                alignment: Alignment.center,
                width: 100,
                height: 10,
                child: YJPvAnimationItemWidget(),
              )
            ),
            Positioned(
              left: 100,
              top: 15,
              child: RaisedButton(
                child: Text('向右启动动画'),
                onPressed: (){
                  setState(() {
                    model.action = 'restart';
                    model.direction = YJPvAnimationDirection.right;
                  });
                },
              ),
            ),
            Positioned(
              left: 250,
              top: 15,
              child: RaisedButton(
                child: Text('向左启动动画'),
                onPressed: (){
                  setState(() {
                    model.action = 'restart';
                    model.direction = YJPvAnimationDirection.left;
                  });
                },
              ),
            ),
          ],
        ),
      )
    );
  }
}

class YJPvAnimationItemWidget extends StatefulWidget {
  @override
  _YJPvAnimationItemWidgetState createState() => _YJPvAnimationItemWidgetState();
}

class _YJPvAnimationItemWidgetState extends State<YJPvAnimationItemWidget> with TickerProviderStateMixin {

  Animation<Size> _widthUpAni;
  Animation<Size> _widthDownAni;

  AnimationController _controllerUp;
  AnimationController _controllerDown;

  bool _isWidthShrink; //是否正在收缩状态
  YJPvAnimationDirection _direction;

  static const double _minWidth = 10;
  static const double _maxWidth = 50;

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    print('initState');
    _isWidthShrink = false;
    _direction = YJPvAnimationDirection.right;
    _initController();
    _initAni();
    _start();
  }

  Future _start() async{
    await _controllerUp.forward();
  }

  void _initController(){
    _controllerUp = AnimationController(
      duration: Duration(milliseconds: 500),
      vsync: this,
    );
    _controllerDown = AnimationController(
      duration: Duration(milliseconds: 500),
      vsync: this,
    );
  }

  void _initAni(){
    _widthUpAni = SizeTween(
      begin: Size.fromWidth(_minWidth),
      end: Size.fromWidth(_maxWidth),
    ).animate(
      CurvedAnimation(
        parent: _controllerUp,
        curve: Interval(0, 1, curve: Curves.fastOutSlowIn
        ),
      ),
    )
    ..addListener((){
      setState(() {});
    })
    ..addStatusListener((AnimationStatus status){
      print('--------------1');
      print(status);
      if (status == AnimationStatus.completed) {
        _isWidthShrink = true;
        _controllerDown.forward();
      }
    });

    _widthDownAni = SizeTween(
      begin: Size.fromWidth(_maxWidth),
      end: Size.fromWidth(_minWidth),
    ).animate(
      CurvedAnimation(
        parent: _controllerDown,
        curve: Interval(0, 1, curve: Curves.fastOutSlowIn
        ),
      ),
    )
    ..addListener((){
      setState(() {});
    })
    ..addStatusListener((AnimationStatus status){
      print('--------------2');
      print(status);
      if (_widthDownAni.status == AnimationStatus.completed ) {
        _isWidthShrink = false;
        _controllerUp.reset();
        _controllerDown.reset();
      }
    });

  }

  @override
  Widget build(BuildContext context) {
    
    //宽度变化
    double _width = _isWidthShrink ? _widthDownAni.value.width : _widthUpAni.value.width; 
    //margin变化
    double _margin = _isWidthShrink ? (_maxWidth -  10 + _maxWidth - _widthDownAni.value.width) : (_widthUpAni.value.width - 10);

    return Container(
      height: 10,
      decoration: BoxDecoration(
        color: Colors.blue,
        borderRadius: BorderRadius.all(Radius.circular(5)),
      ),
      width: _width,
      margin: _direction == YJPvAnimationDirection.left ? EdgeInsets.only(right: _margin) : EdgeInsets.only(left: _margin),
    );
  }

  @override
  void didChangeDependencies() {
    print('didChangeDependencies');
    // TODO: implement didChangeDependencies
    super.didChangeDependencies();
    final ctx = YJPvAnimationSharedContext.of(context);
    if (ctx.model.action == 'restart') {
      _direction = ctx.model.direction;
      _start();
    }
  }

  @override
  void didUpdateWidget(YJPvAnimationItemWidget oldWidget) {
    // TODO: implement didUpdateWidget
    super.didUpdateWidget(oldWidget);
    print('didUpdateWidget${oldWidget.runtimeType.hashCode}');
  }

  @override
  void dispose() {
    print('dispose');
    // TODO: implement dispose
    _controllerUp.dispose();
    super.dispose();
  }

}