flutter以中心旋转控件

1.gif

其实就实现这么一个旋转动画，一开始想到的是用AnimatedContainer的transform属性来实现这个动画，结果发现不行，三角形的旋转是以左上角为中心的。
效果就不演示了，随便试试就知道了
看下为什么以中心旋转，有什么办法使中心旋转

transform 通过矩阵变换实现，矩阵的变换是以canvas的（0,0）点作为起始点的，所以需要图像中心变换的话，需要canvas先移动到（-width*0.5,-height*0.5），将图像中心点移动到(0,0)位置，进行变换，然后再移动回来
显然我们在AnimatedContainer里获取不到canvas，所以这方案实现不了
分析源码

@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，显然是通过Animation改变Container的属性，来实现动画效果的
继续追踪Container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中有一个alignment，这个可以移动到中心点，但是Container未设置，所以默认使用左上角为起始点

既然AnimatedContainer实现不了，那就自定义一个
我们使用Transform控件，使用其Transform.rotate构造函数

Transform.rotate({
    Key key,
    @required double angle,
    this.origin,
    this.alignment = Alignment.center, //已经设置为中心了
    this.transformHitTests = true,
    Widget child,
  })

但是这个没动画效果，所以我们需要加上动画

import 'dart:math';
import 'package:flutter/material.dart';
/*
* 中心处旋转动画，AnimatedContainer针对的是左上角的旋转，注意角度传递的是π
* */
class RotateContainer extends StatefulWidget {
  final double endAngle; // 注意这个角度，π为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,
    );
  }
}

附上其他代码：

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(  //自己画个三角形，当然也可以使用Icon
                painter: TrianglePainter(Colors.grey),
                child: SizedBox(
                  width: 12,
                  height: 12,
                ),
              ),
            ),
          ],
        ),
      ),
      onTap: onPressed,
    );
  }
}

/*
* 画个三角形
* */
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: "筛选",
          rotated: tagsMenuOpen,
          onPressed: () {
            tagsMenuOpen = !tagsMenuOpen;
            setState(() {});
          },
        ),
      ),
    );
  }