1.gif

其實(shí)就實(shí)現(xiàn)這么一個(gè)旋轉(zhuǎn)動(dòng)畫，一開始想到的是用AnimatedContainer的transform屬性來實(shí)現(xiàn)這個(gè)動(dòng)畫，結(jié)果發(fā)現(xiàn)不行，三角形的旋轉(zhuǎn)是以左上角為中心的。
效果就不演示了，隨便試試就知道了
看下為什么以中心旋轉(zhuǎn)，有什么辦法使中心旋轉(zhuǎn)

1. transform 通過矩陣變換實(shí)現(xiàn)，矩陣的變換是以canvas的（0,0）點(diǎn)作為起始點(diǎn)的，所以需要圖像中心變換的話，需要canvas先移動(dòng)到（-width*0.5,-height*0.5），將圖像中心點(diǎn)移動(dòng)到(0,0)位置，進(jìn)行變換，然后再移動(dòng)回來
   顯然我們在AnimatedContainer里獲取不到canvas，所以這方案實(shí)現(xiàn)不了
2. 分析源碼

@override
  Widget build(BuildContext context) {
    return Container(
      child: widget.child,
      alignment: _alignment?.evaluate(animation),
      padding: _padding?.evaluate(animation),
      decoration: _decoration?.evaluate(animation),
      foregroundDecoration: _foregroundDecoration?.evaluate(animation),
      constraints: _constraints?.evaluate(animation),
      margin: _margin?.evaluate(animation),
      transform: _transform?.evaluate(animation),
    );
  }

在AnimatedContainer的build中找到了Container，顯然是通過Animation改變Container的屬性，來實(shí)現(xiàn)動(dòng)畫效果的
繼續(xù)追蹤C(jī)ontainer的transform

@override
  Widget build(BuildContext context) {
    ...
    if (transform != null)
      current = Transform(transform: transform, child: current);

    return current;
  }

build中找到了使用了Transform

const Transform({
    Key key,
    @required this.transform,
    this.origin,
    this.alignment,
    this.transformHitTests = true,
    Widget child,
  }) : assert(transform != null),
       super(key: key, child: child);

Transform中有一個(gè)alignment，這個(gè)可以移動(dòng)到中心點(diǎn)，但是Container未設(shè)置，所以默認(rèn)使用左上角為起始點(diǎn)

既然AnimatedContainer實(shí)現(xiàn)不了，那就自定義一個(gè)
我們使用Transform控件，使用其Transform.rotate構(gòu)造函數(shù)

Transform.rotate({
    Key key,
    @required double angle,
    this.origin,
    this.alignment = Alignment.center, //已經(jīng)設(shè)置為中心了
    this.transformHitTests = true,
    Widget child,
  })

但是這個(gè)沒動(dòng)畫效果，所以我們需要加上動(dòng)畫

import 'dart:math';
import 'package:flutter/material.dart';
/*
* 中心處旋轉(zhuǎn)動(dòng)畫，AnimatedContainer針對的是左上角的旋轉(zhuǎn)，注意角度傳遞的是π
* */
class RotateContainer extends StatefulWidget {
  final double endAngle; // 注意這個(gè)角度，π為180°
  final bool rotated;
  final Widget child;

  @override
  _RotateContainerState createState() => _RotateContainerState();

  RotateContainer({this.endAngle, this.child, this.rotated = false});
}

class _RotateContainerState extends State<RotateContainer>
    with SingleTickerProviderStateMixin {
  AnimationController _controller;
  Animation<double> _animation;
  double angle = 0;

  @override
  void initState() {
    _controller =
        AnimationController(vsync: this, duration: Duration(milliseconds: duration));
    _animation =
        Tween(begin: 0.0, end: widget.endAngle).animate(_controller)
          ..addListener(() {
            setState(() {
              angle = _animation.value;
            });
          });
    super.initState();
  }

  @override
  void didUpdateWidget(RotateContainer oldWidget) {
    if (oldWidget.rotated == widget.rotated) return; //防止多余刷新
    if (!widget.rotated) {
      _controller.reverse();
    } else {
      _controller.forward();
    }
    super.didUpdateWidget(oldWidget);
  }

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

  @override
  Widget build(BuildContext context) {
    return Transform.rotate(
      angle: angle,
      child: widget.child,
    );
  }
}

附上其他代碼：

const int duration = 400;

class TagsButton extends StatelessWidget {
  final String text;
  final bool rotated;
  final VoidCallback onPressed;

  TagsButton({this.text, this.onPressed, this.rotated = false});

  @override
  Widget build(BuildContext context) {
    return InkWell(
      child: Container(
        padding: EdgeInsets.all(10.0),
        child: Row(
          children: <Widget>[
            Text(text),
            RotateContainer(
              endAngle: pi,
              rotated: rotated,
              child: CustomPaint(  //自己畫個(gè)三角形，當(dāng)然也可以使用Icon
                painter: TrianglePainter(Colors.grey),
                child: SizedBox(
                  width: 12,
                  height: 12,
                ),
              ),
            ),
          ],
        ),
      ),
      onTap: onPressed,
    );
  }
}

/*
* 畫個(gè)三角形
* */
class TrianglePainter extends CustomPainter {
  Color color;
  Paint _paint;
  Path _path;
  double angle;

  TrianglePainter(this.color) {
    _paint = Paint()
      ..strokeWidth = 1.0
      ..color = color
      ..isAntiAlias = true;

    _path = Path();
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) => false;

  @override
  void paint(Canvas canvas, Size size) {
    final baseX = size.width * 0.5;
    final baseY = size.height * 0.5;
    //三角形
    _path.moveTo(baseX - 0.86 * baseX, 0.5 * baseY);
    _path.lineTo(baseX, 1.5 * baseY);
    _path.lineTo(baseX + 0.86 * baseX, 0.5 * baseY);
    canvas.drawPath(_path, _paint);
  }
}

使用

  bool tagsMenuOpen = false;

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Container(
        width: 80,
        child: TagsButton(
          text: "篩選",
          rotated: tagsMenuOpen,
          onPressed: () {
            tagsMenuOpen = !tagsMenuOpen;
            setState(() {});
          },
        ),
      ),
    );
  }