• 仪表盘绘制，效果图如下：

  
  
  仪表盘效果图

• 仪表盘基本参数
  min - 最小值
  max - 最大值
  progress - 进度值
  仪表盘总共300度，那么每个点的刻度为：300 / (max-min)

• 效果图主要掌握如下，
  Canvas: 1. ARC绘制，2.Paragraph文本绘制，3.图形旋转；

• 绘制方法：
  - _drawProgressArc 绘制扇形⭕️
  - _drawArcPointLine 绘制仪表盘刻度线
  - _drawArcProgressPoint 绘制仪表盘刻度文字

• 用到的基本公式：
  1.角度转弧度：num _toRadius(num degree) => degree * Math.pi / 180;
  2.圆上的点坐标(x, y)求值:
  已知圆心坐标(ox, oy)，半径为r，则圆上点坐标可如下求解,
  x = ox + r * cos(rad);
  y = oy + r * sin(rad);

• 注意点：
  绘制文本，需要PragraphBuilder 执行 layout 方法。

• 基本代码如下：

import 'dart:math' as Math;
import 'dart:ui' as UI;
import 'package:flutter/material.dart';

class ArcProgressBar extends StatelessWidget {
  final double width;
  final double height;
  final double min;
  final double max;
  final double progress;

  ArcProgressBar({
    Key key,
    this.width = 180,
    this.height = 180,
    this.min = 0,
    this.max = 100,
    this.progress = 0,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) => AspectRatio(
      aspectRatio: 1,
      child: CustomPaint(
        size: Size(width, height),
        painter: _ArcProgressBarPainter(8, progress, min: min, max: max),
      ));
}

class _ArcProgressBarPainter extends CustomPainter {
  Paint _paint = Paint();

  double _strokeSize;

  double get _margin => _strokeSize / 2;

  num progress = 0;

  num min;

  num max;

  _ArcProgressBarPainter(double strokeSize, this.progress,
      {this.min = 0, this.max = 100}) {
    this._strokeSize = strokeSize;
    if (progress == null || progress < min) progress = 0;
    if (progress > max) progress = max;
    if (min == null || min <= 0) min = 0;
    if (max == null || max <= min) max = 100;
  }

  @override
  void paint(Canvas canvas, Size size) {
    num radius = size.width / 2;
    num cx = radius;
    num cy = radius;
    _drawProgressArc(canvas, size);
    _drawArcProgressPoint(canvas, cx, cy, radius);
    _drawArcPointLine(canvas, cx, cy, radius);
  }

  void _drawProgressArc(Canvas canvas, Size size) {
    _paint
      ..isAntiAlias = true
      ..color = Colors.grey[400]
      ..style = PaintingStyle.stroke
      ..strokeCap = StrokeCap.round
      ..strokeWidth = _strokeSize;
    canvas.drawArc(
        Rect.fromLTWH(_margin, _margin, size.width - _strokeSize,
            size.height - _strokeSize),
        _toRadius(120),
        _toRadius(300),
        false,
        _paint);

    _paint
      ..color = Colors.cyan
      ..strokeWidth = _strokeSize - 1;
    canvas.drawArc(
        Rect.fromLTWH(_margin + 1, _margin + 1, size.width - _strokeSize - 2,
            size.height - _strokeSize - 2),
        _toRadius(120),
        progress * _toRadius(300 / (max - min)),
        false,
        _paint);
  }

  void _drawArcProgressPoint(Canvas canvas, num cx, num cy, num radius) {
    _paint.strokeWidth = 1;
    canvas.save();
    canvas.translate(cx, cy);
    canvas.rotate(_toRadius(120));
    canvas.translate(-cx, -cy);
    for (int i = 0; i <= (max - min); i++) {
      num evaDegree = i * _toRadius(300 / (max - min));
      num b = i % 10 == 0 ? -5 : 0;
      num x = cx + (radius - 20 + b) * Math.cos(evaDegree);
      num y = cy + (radius - 20 + b) * Math.sin(evaDegree);
      num x1 = cx + (radius - 12) * Math.cos(evaDegree);
      num y1 = cx + (radius - 12) * Math.sin(evaDegree);
      canvas.drawLine(Offset(x, y), Offset(x1, y1), _paint);
    }
    canvas.translate(cx, cy);
    canvas.rotate(_toRadius(-120));
    canvas.translate(-cx, -cy);
    for (int i = min; i <= max; i += 10) {
      var pb = UI.ParagraphBuilder(
          UI.ParagraphStyle(fontSize: 15, textAlign: TextAlign.start))
        ..pushStyle(UI.TextStyle(color: Colors.black))
        ..addText(i.toString());
      UI.Paragraph p = pb.build()..layout(UI.ParagraphConstraints(width: 30));
      num evaDegree = _toRadius(120) + i * _toRadius(300 / (max - min));
      num x = cx + (radius - 40) * Math.cos(evaDegree);
      num y = cy + (radius - 40) * Math.sin(evaDegree);
      canvas.drawParagraph(p, Offset(x - 8, y - 10));
    }
    canvas.restore();
  }

  void _drawArcPointLine(UI.Canvas canvas, num cx, num cy, num radius) {
    canvas.save();
    canvas.translate(cx, cy);
    canvas.rotate(_toRadius(120));
    canvas.translate(-cx, -cy);
    _paint
      ..color = Colors.amber[800]
      ..style = PaintingStyle.fill
      ..strokeWidth = 3;
    num degree = _toRadius(300 / (max - min)) * progress;
    num x = cx + radius * 3 / 5 * Math.cos(degree);
    num y = cy + radius * 3 / 5 * Math.sin(degree);
    canvas.drawLine(Offset(cx, cy), Offset(x, y), _paint);
    _paint.color = Colors.amber[900];
    canvas.drawCircle(Offset(cx, cy), 12, _paint);
    canvas.restore();
  }

  num _toRadius(num degree) => degree * Math.pi / 180;

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