flutter_circular_chart/lib/src/painter.dart

import 'dart:math' as Math;

import 'package:flutter/material.dart';
import 'package:flutter_circular_chart/src/animated_circular_chart.dart';
import 'package:flutter_circular_chart/src/circular_chart.dart';
import 'package:flutter_circular_chart/src/stack.dart';

class AnimatedCircularChartPainter extends CustomPainter {
  AnimatedCircularChartPainter(this.animation) : super(repaint: animation);

  final Animation<CircularChart> animation;

  @override
  void paint(Canvas canvas, Size size) {
    _paintChart(canvas, size, animation.value);
  }

  @override
  bool shouldRepaint(AnimatedCircularChartPainter old) => false;
}

class CircularChartPainter extends CustomPainter {
  CircularChartPainter(this.chart);

  final CircularChart chart;

  @override
  void paint(Canvas canvas, Size size) {
    _paintChart(canvas, size, chart);
  }

  @override
  bool shouldRepaint(CircularChartPainter old) => false;
}

const double _kRadiansPerDegree = Math.PI / 180;

void _paintChart(Canvas canvas, Size size, CircularChart chart) {
  final Paint segmentPaint = new Paint()
    ..style = chart.chartType == CircularChartType.Radial
        ? PaintingStyle.stroke
        : PaintingStyle.fill;

  for (final CircularChartStack stack in chart.stacks) {
    for (final segment in stack.segments) {
      segmentPaint.color = segment.color;
      segmentPaint.strokeWidth = stack.width;
      canvas.drawArc(
        new Rect.fromCircle(
          center: new Offset(size.width / 2, size.height / 2),
          radius: stack.radius,
        ),
        stack.startAngle * _kRadiansPerDegree,
        segment.sweepAngle * _kRadiansPerDegree,
        chart.chartType == CircularChartType.Pie,
        segmentPaint,
      );
    }
  }
}