在上一篇文章中，我们实现了统计每个产品和地区的销售额，如果现在需要统计每个产品和地区所占市场份额的百分比，那么使用堆叠条形图是不合适的，我们可以使用分组条形图，因为它可以同时在两个类别维度上进行定量比较。分组条形图的实际效果如下图所示：

要实现上面的效果，我们需要更新bar.dart文件的代码：

import 'package:flutter/material.dart';import 'package:flutter/animation.dart';import 'dart:ui' show lerpDouble;import 'dart:math';import 'color_palette.dart';import 'tween.dart';class BarChart { BarChart(this.groups); final List<BarGroup> groups; factory BarChart.empty(Size size) { return new BarChart(<BarGroup>[]); } factory BarChart.random(Size size, Random random) { const groupWidthFraction = 0.75; const barWidthFraction = 0.9; final groupRanks = _selectRanks(random, 5); final groupCount = groupRanks.length; final groupDistance = size.width / (1+groupCount); final groupWidth = groupDistance * groupWidthFraction; final startX = groupDistance - groupWidth/2; final barRanks = _selectRanks(random, ColorPalette.primary.length ~/ 2); final barCount = barRanks.length; final barDistance = groupWidth / barCount; final barWidth = barDistance * barWidthFraction; final groups = new List.generate( groupCount, (i) { final bars = new List.generate( barCount, (j) => new Bar( barRanks[j], startX + i * groupDistance + j * barDistance, barWidth, random.nextDouble() * size.height, ColorPalette.primary[barRanks[j]], ) ); return new BarGroup( groupRanks[i], bars ); } ); return new BarChart(groups); } static List<int> _selectRanks(Random random, int cap) { final ranks = <int>[]; var rank = 0; while(true) { rank += random.nextInt(2); if(cap <= rank) break; ranks.add(rank); rank++; } return ranks; }}class BarChartTween extends Tween<BarChart> { BarChartTween(BarChart begin, BarChart end) : _groupsTween = new MergeTween<BarGroup>(begin.groups, end.groups), super(begin: begin, end: end); final MergeTween<BarGroup> _groupsTween; @override BarChart lerp(double t) => new BarChart(_groupsTween.lerp(t));}class BarGroup implements MergeTweenable<BarGroup> { BarGroup(this.rank, this.bars); final int rank; final List<Bar> bars; @override BarGroup get empty => new BarGroup(rank, <Bar>[]); @override bool operator <(BarGroup other) => rank < other.rank; @override Tween<BarGroup> tweenTo(BarGroup other) => new BarGroupTween(this, other);}class BarGroupTween extends Tween<BarGroup> { BarGroupTween(BarGroup begin, BarGroup end) : _barsTween = new MergeTween<Bar>(begin.bars, end.bars), super(begin: begin, end: end) { assert(begin.rank == end.rank); } final MergeTween<Bar> _barsTween; @override BarGroup lerp(double t) => new BarGroup( begin.rank, _barsTween.lerp(t) );}class Bar extends MergeTweenable<Bar> { Bar(this.rank, this.x, this.width, this.height, this.color); final int rank; final double x; final double width; final double height; final Color color; @override Bar get empty => new Bar(rank, x, 0.0, 0.0, color); @override bool operator <(Bar other) => rank < other.rank; @override Tween<Bar> tweenTo(Bar other) => new BarTween(this, other); static Bar lerp(Bar begin, Bar end, double t) { assert(begin.rank == end.rank); return new Bar( begin.rank, lerpDouble(begin.x, end.x, t), lerpDouble(begin.width, end.width, t), lerpDouble(begin.height, end.height, t), Color.lerp(begin.color, end.color, t) ); }}class BarTween extends Tween<Bar> { BarTween(Bar begin, Bar end) : super(begin: begin, end: end) { assert(begin.rank == end.rank); } @override Bar lerp(double t) => Bar.lerp(begin, end, t);}class BarChartPainter extends CustomPainter { BarChartPainter(Animation<BarChart> animation) : animation = animation, super(repaint: animation); final Animation<BarChart> animation; @override void paint(Canvas canvas, Size size) { final paint = new Paint()..style = PaintingStyle.fill; final chart = animation.value; for(final stack in chart.groups) { for(final bar in stack.bars) { paint.color = bar.color; canvas.drawRect( new Rect.fromLTWH( bar.x, size.height - bar.height, bar.width, bar.height ), paint, ); } } } @override bool shouldRepaint(BarChartPainter old) => false;}

接着学习下一篇：Flutter进阶之实现动画效果（十）

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