今日无事，闲来看了看别人的一些demo，然后自己琢磨着写了一个CircleProgressBar组件。CircleProgressBar组件是我们经常使用的一个控件，一般包括这几个重要的属性：min, max, progress, 构成则是由Arc路径成像。但是在flutter中使用Arc绘制图形会出现一些显示问题而根本达不到你想要的效果。为此下面的代码中我们使用了一点数学小知识。

CircleProgressBar最终效果图

首先，编写CircleProgressBar

class CircleProgressBar {
  CircleProgressBar(this.progress, {this.min = 0.0, this.max = 100.0});

  double progress;

  double min;

  double max;

  static CircleProgressBar lerp(
      CircleProgressBar begin, CircleProgressBar end, double t) {
    return new CircleProgressBar(lerpDouble(begin.progress, end.progress, t));
  }
  
}

接着，编写CircleProgressBar的Tween动画

因为是补间动画，所以会有begin, end两个参数，而lerp函数则是相应动画的线性插件函数，相当于Android原生中的Interpolator。

class CircleProgressBarTween extends Tween<CircleProgressBar> {
  CircleProgressBarTween(CircleProgressBar begin, CircleProgressBar end)
      : super(begin: begin, end: end);

  @override
  CircleProgressBar lerp(double t) => CircleProgressBar.lerp(begin, end, t);
}

最后，编写CircleProgressBar自定义UI组件。

构建自定义UI需要继承CustomPainter方法，并重写paint方法和shouldRepaint方法。重写paint方法，使用笔刷和画布绘制控件UI。

class CircleProgressBarPainter extends CustomPainter {
  CircleProgressBarPainter(Animation<CircleProgressBar> animation)
      : animation = animation,
        super(repaint: animation);

  Animation<CircleProgressBar> animation;

  @override
  void paint(Canvas canvas, Size size) {
    final progressBar = animation.value;
    final paint = new Paint()
      ..color = Colors.grey[400]
      ..style = PaintingStyle.stroke
      ..strokeWidth = 8.0;
    final double wh = max(size.width, size.height);
    final centerX = (size.width - wh) / 2.0,
        centerY = (size.height - wh) / 2.0;
    final radius = wh / 2.0;
    canvas.drawPath(getArcPath(centerX, centerY, radius, true), paint);
    paint.color = Colors.red;
    canvas.drawPath(getArcPath(centerX, centerY, radius, false, (progressBar.progress / progressBar.max) * 360), paint);
  }

  ///
  /// 获取Arc的Path
  /// <pre>
  /// centerX 圆心坐标x
  /// centerY 圆心坐标Y
  /// isClosePath 是否闭合路径
  /// arcAngle 角度，默认为360°
  ///
  /// 说明：下面代码中使用了一个数学公式
  /// 已知圆心坐标(x, y)，半径(r)，和角度(a)，求圆上某个点的坐标，则有：
  /// x1 = x + r * cos(a * π / 180.0)
  /// y1 = y + r * sin(a * π / 180.0)
  /// </pre>
  ///
  Path getArcPath(
      double centerX, double centerY, double radius, bool isClosePath, [double arcAngle = 360.0]) {
    var path = Path();
    for (double i = 0.0; i < arcAngle; i += 0.1) {
      double x = centerX + radius * cos(i * 3.141592653 / 180);
      double y = centerY / 2.0 + radius * sin(i * 3.141592653 / 180);
      if (i != 0)
        path.lineTo(x, y);
      else
        path.moveTo(x, y);
    }
    if (isClosePath) path.close();
    return path;
  }

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

最后上测试代码

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> with TickerProviderStateMixin {
  final random = new Random();
  var progress = 0.0;
  AnimationController animationController;
  CircleProgressBarTween progressBarTween;

  @override
  void initState() {
    super.initState();
    animationController = new AnimationController(
        vsync: this, duration: const Duration(milliseconds: 300));
    progressBarTween = new CircleProgressBarTween(
        new CircleProgressBar(0.0, max: 360.0), new CircleProgressBar(0.0, max: 360.0));
    animationController.forward();
  }

  void _incrementCounter() {
    setState(() {
      progress = random.nextDouble() * 360.0;
      animationController = new AnimationController(
          vsync: this, duration: const Duration(milliseconds: 300));
      progressBarTween = new CircleProgressBarTween(
          progressBarTween.evaluate(animationController),
          new CircleProgressBar(progress, max: 360.0));
      animationController.forward();
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new CustomPaint(
              size: const Size(100.0, 100.0),
              painter: new CircleProgressBarPainter(
                  progressBarTween.animate(animationController)),
            ),
            new Text(
              '进度条值为: $progress',
            ),
          ],
        ),
      ),
      floatingActionButton: new AnimatedContainer(
        duration: const Duration(seconds: 2),
        transform: Matrix4.translationValues(0.5, 0.5, 0.5),
        child: new FloatingActionButton(
          onPressed: _incrementCounter,
          tooltip: 'Increment',
          child: new Icon(Icons.add),
        ),
      ),
    );
  }
}