flutter 做一个带线指示的饼状图

看到一个文章https://www.jianshu.com/p/df33f4595ce6

是一个饼状图，但是不适用，因为数据比较多的时候显示就很密集，所以要用线，添加了一些，不过目前文字大小还没怎么计算，所以是写固定的

整个代码如下

import 'package:flutter/material.dart';
import 'piechart.dart';
import 'PieData.dart';
import 'dart:math';
class View extends CustomPainter{

  //中间文字
  var text='111';
  bool isChange=false;
  //当前选中的扇形
  var currentSelect=0;
  //画笔
  Paint _mPaint;
  Paint TextPaint;
  // 扇形大小
  int mWidth, mHeight;
  // 圆半径
  num mRadius, mInnerRadius,mBigRadius;
  // 扇形起始弧度（Andorid中是角度）
  num mStartAngle = 0;
  // 矩形（扇形绘制的区域）
  Rect mOval,mBigOval;
// 扇形 数据
  List mData;
  PieData pieData;
  // 构造函数，接受需要的参数值
  View(this.mData,this.pieData,this.currentSelect,this.isChange);

  /**
   * 重写 paint方法，在其中写绘制饼状图的逻辑
   */
  @override
  void paint(Canvas canvas, Size size) {

    // 初始化各类工具等
    _mPaint = new Paint();
    TextPaint = new Paint();
    mHeight=100;mWidth=100;

    /// 生成纵轴文字的TextPainter
    TextPainter textPainter = TextPainter(
      textDirection: TextDirection.ltr,
      maxLines: 1,
    );

    // 文字画笔 风格定义
    TextPainter _newVerticalAxisTextPainter(String text) {
      return textPainter
        ..text = TextSpan(
          text: text,
          style: new TextStyle(
            color: Colors.black,
            fontSize: 10.0,
          ),
        );
    }

    // 正常半径
    mRadius = 50.0;
    //加大半径  用来绘制被选中的扇形区域
    mBigRadius=55.0;
    //內园半径
    mInnerRadius = mRadius * 0.50;
    // 未选中的扇形绘制的矩形区域
    mOval = Rect.fromLTRB(-mRadius, -mRadius, mRadius, mRadius);
    // 选中的扇形绘制的矩形区域
    mBigOval = Rect.fromLTRB(-mBigRadius, -mBigRadius, mBigRadius,
        mBigRadius);
    //当没有数据时 直接返回
    if (mData.length == null || mData.length <= 0) {
      return;
    }

    ///绘制逻辑与Android差不多
    canvas.save();
    // 将坐标点移动到View的中心
    canvas.translate(50.0, 50.0);
    // 1. 画扇形
    num startAngle = 0.0;
    bool draw=false;
    for (int i = 0; i < mData.length; i++) {
      PieData p = mData[i];
      double hudu=p.percentage;
      //计算当前偏移量（单位为弧度）
      double sweepAngle = 2*pi*hudu;
     // print(sweepAngle);
      //画笔的颜色
      _mPaint..color = p.color;
      if(currentSelect>=0 && i==currentSelect){
        //如果当前为所选中的扇形 则将其半径加大  突出显示
        canvas.drawArc(mBigOval, startAngle, sweepAngle, true, _mPaint);
        Paint _paint=new Paint();
        _paint.color=Color(0xff91c954);
        _paint.strokeWidth=1.0;
        Offset offset=Offset(mRadius*0.75*cos(startAngle+sweepAngle/2.0),mRadius*0.75*sin(startAngle+sweepAngle/2.0));
       // print(mRadius*0.75*sin(2*pi*270.0/360.0));
       // print(mRadius*0.75*cos(2*pi*270.0/360.0));
      //  if(!draw){print(offset);
//print(mRadius*0.75*sin(2*pi*270.0/360.0));
        //  print(mRadius*0.75*cos(2*pi*270.0/360.0));
        TextPainter t=TextPainter(text: TextSpan(text: "11%",style: TextStyle(color: Colors.black),),textDirection: TextDirection.ltr,
          maxLines: 1,);
        canvas.drawLine(offset, Offset(mRadius*1.25*cos(startAngle+sweepAngle/2.0),mRadius*1.25*sin(startAngle+sweepAngle/2.0)), _paint);
        if(mRadius*0.75*cos(startAngle+sweepAngle/2.0)<0){
          Offset offset1=new Offset(mRadius*1.25*cos(startAngle+sweepAngle/2.0)-mRadius*0.2, mRadius*1.25*sin(startAngle+sweepAngle/2.0));
          canvas.drawLine(Offset(mRadius*1.25*cos(startAngle+sweepAngle/2.0),mRadius*1.25*sin(startAngle+sweepAngle/2.0)), offset1, _paint);
        //  canvas.restore();
          t.layout();
          t.paint(canvas, Offset(mRadius*1.25*cos(startAngle+sweepAngle/2.0)-mRadius*0.2-20.0, mRadius*1.25*sin(startAngle+sweepAngle/2.0)-10.0));
        }else{
          Offset offset1=new Offset(mRadius*1.25*cos(startAngle+sweepAngle/2.0)+mRadius*0.2, mRadius*1.25*sin(startAngle+sweepAngle/2.0));
          canvas.drawLine(Offset(mRadius*1.25*cos(startAngle+sweepAngle/2.0),mRadius*1.25*sin(startAngle+sweepAngle/2.0)), offset1, _paint);
         // canvas.restore();
          t.layout();
          t.paint(canvas, Offset(mRadius*1.25*cos(startAngle+sweepAngle/2.0)-mRadius*0.2+20.0, mRadius*1.25*sin(startAngle+sweepAngle/2.0)-10.0));
        }


      //  draw=true;
       // }

      }else{
        // 绘制没被选中的扇形  正常半径
        canvas.drawArc(mOval, startAngle, sweepAngle, true, _mPaint);
        Offset origin=Offset((-mRadius+mRadius)/2.0, (-mRadius+mRadius)/2.0);
        Paint _paint=new Paint();
        _paint.color=Color(0xff91c954);
        _paint.strokeWidth=1.0;
     
Offset offset=Offset( mRadius*0.75*cos(startAngle+sweepAngle/2.0),mRadius*0.75*sin(startAngle+sweepAngle/2.0));

        canvas.drawLine(offset, Offset(mRadius*1.25*cos(startAngle+sweepAngle/2.0),mRadius*1.25*sin(startAngle+sweepAngle/2.0)), _paint);
        TextPainter t=TextPainter(text: TextSpan(text: "11%",style: TextStyle(color: Colors.black),),textDirection: TextDirection.ltr,
          maxLines: 1,);
        if(mRadius*0.75*cos(startAngle+sweepAngle/2.0)<0){
          Offset offset1=new Offset(mRadius*1.25*cos(startAngle+sweepAngle/2.0)-mRadius*0.2, mRadius*1.25*sin(startAngle+sweepAngle/2.0));
          canvas.drawLine(Offset(mRadius*1.25*cos(startAngle+sweepAngle/2.0),mRadius*1.25*sin(startAngle+sweepAngle/2.0)), offset1, _paint);
         // canvas.restore();
          t.layout();
          t.paint(canvas, Offset(mRadius*1.25*cos(startAngle+sweepAngle/2.0)-mRadius*0.2-25.0, mRadius*1.25*sin(startAngle+sweepAngle/2.0)-10.0));
        }else{
          Offset offset1=new Offset(mRadius*1.25*cos(startAngle+sweepAngle/2.0)+mRadius*0.2, mRadius*1.25*sin(startAngle+sweepAngle/2.0));
          canvas.drawLine(Offset(mRadius*1.25*cos(startAngle+sweepAngle/2.0),mRadius*1.25*sin(startAngle+sweepAngle/2.0)), offset1, _paint);
         // canvas.restore();
          t.layout();
          t.paint(canvas, Offset(mRadius*1.25*cos(startAngle+sweepAngle/2.0)-mRadius*0.2+20.0, mRadius*1.25*sin(startAngle+sweepAngle/2.0)-10.0));
        }
      //  draw=true;
      //  }
      }
      //计算每次开始绘制的弧度
      startAngle += sweepAngle ;
     // print(startAngle);
    }

//    canvas.drawRect(mOval, _mPaint);  // 矩形区域

    // 2.画内圆
    _mPaint..color = Colors.white;

    canvas.drawCircle(Offset.zero, mInnerRadius, _mPaint);

    canvas.restore();

    //当前百分比值
    double percentage = pieData.percentage*100;
    // 绘制文字内容
    var texts ='${percentage}%';
    var tp = _newVerticalAxisTextPainter(texts)..layout();

    // Text的绘制起始点 = 可用宽度 - 文字宽度 - 左边距
    var textLeft = 35.0;
    tp.paint(canvas, Offset(textLeft, 50.0 - tp.height / 2));

  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) {
    return true;
  }
}

绘制text之前，一定要调用layout这个函数，这个跟安卓很像