通过学习前面的文章，我们现在终于能为更复杂的图表制作动画效果了。接着上一篇文章讲，如果公司的产品销往全国各地，那么我们的图表要展示的内容就需要加上地区。我们可以使用堆叠条形图来试试效果，实际效果如下图所示：

堆叠条用于数据集，其中类别是二维的，并且将由bar高度表示的数值加起来是合理的。其中一个颜色表示一个地区，每个条形表示一个产品，产品条形图使公司可以对比全国市场上各个产品的销售额，同时按区域堆叠显示各个地区的销售额。

动画效果用于可视化数据集更改，从而引入附加维度（通常为时间），且不会使图表混乱。为了使动画变得漂亮而实用，我们需要确保我们仅在语义对应的组件之间运行。因此，用于表示2016年特定产品/区域/渠道的收入的条形细分应变为一个代表2017年同一产品/地区/渠道（如果存在）的收入。

我们可以使用合并算法来确保这一点，合并将在多个层次上进行工作，反映类别的维度。为了实现这一点，并且没有大量的代码重复，我们将把合并算法抽象成一个通用的算法，并把它放在一个新建的tween.dart文件中：

import 'package:flutter/animation.dart';import 'package:flutter/material.dart';abstract class MergeTweenable<T> { T get empty; Tween<T> tweenTo(T other); bool operator <(T other);}class MergeTween<T extends MergeTweenable<T>> extends Tween<List<T>> { final _tweens = <Tween<T>>[]; MergeTween(List<T> begin, List<T> end) : super(begin: begin, end: end) { final bMax = begin.length; final eMax = end.length; var b = 0; var e = 0; while (b + e < bMax + eMax) { if(b < bMax && (e == eMax || begin[b] < end[e])) { _tweens.add(begin[b].tweenTo(begin[b].empty)); b++; } else if(e < eMax && (b == bMax || end[e] < begin[b])) { _tweens.add(end[e].empty.tweenTo(end[e])); e++; } else { _tweens.add(begin[b].tweenTo(end[e])); b++; e++; } } } @override List<T> lerp(double t) => new List.generate( _tweens.length, (i) => _tweens[i].lerp(t) );}

接口MergeTweenable精确地捕获了通过合并创建两个排序列表T的补间所需要的内容，我们将使用Bar、BarStack和BarGroup实例化类型参数T，并使所有这些类型实现MergeTweenable。

最后我们需要更新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.stacks); final List<BarStack> stacks; factory BarChart.empty(Size size) { return new BarChart(<BarStack>[]); } factory BarChart.random(Size size, Random random) { const stackWidthFraction = 0.75; final stackRanks = _selectRanks(random, 10); final stackCount = stackRanks.length; final stackDistance = size.width / (1+stackCount); final stackWidth = stackDistance * stackWidthFraction; final startX = stackDistance - stackWidth/2; final stacks = new List.generate( stackCount, (i) { final barRanks = _selectRanks(random, ColorPalette.primary.length ~/ 2); final bars = new List.generate( barRanks.length, (j) => new Bar( barRanks[j], random.nextDouble() * size.height / 2, ColorPalette.primary[barRanks[j]], ) ); return new BarStack( stackRanks[i], startX + i * stackDistance, stackWidth, bars ); } ); return new BarChart(stacks); } 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) : _stacksTween = new MergeTween<BarStack>(begin.stacks, end.stacks), super(begin: begin, end: end); final MergeTween<BarStack> _stacksTween; @override BarChart lerp(double t) => new BarChart(_stacksTween.lerp(t));}class BarStack implements MergeTweenable<BarStack> { BarStack(this.rank, this.x, this.width, this.bars); final int rank; final double x; final double width; final List<Bar> bars; @override BarStack get empty => new BarStack(rank, x, 0.0, <Bar>[]); @override bool operator <(BarStack other) => rank < other.rank; @override Tween<BarStack> tweenTo(BarStack other) => new BarStackTween(this, other);}class BarStackTween extends Tween<BarStack> { BarStackTween(BarStack begin, BarStack end) : _barsTween = new MergeTween<Bar>(begin.bars, end.bars), super(begin: begin, end: end) { assert(begin.rank == end.rank); } final MergeTween<Bar> _barsTween; @override BarStack lerp(double t) => new BarStack( begin.rank, lerpDouble(begin.x, end.x, t), lerpDouble(begin.width, end.width, t), _barsTween.lerp(t) );}class Bar extends MergeTweenable<Bar> { Bar(this.rank, this.height, this.color); final int rank; final double height; final Color color; @override Bar get empty => new Bar(rank, 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.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 barPaint = new Paint()..style = PaintingStyle.fill; final linePaint = new Paint() ..style = PaintingStyle.stroke ..color = Colors.white /** *当style设置为PaintingStyle.stroke时绘制边缘有多宽 宽度以与路径方向正交的方向测量的逻辑像素给出 */ ..strokeWidth = 1.0; final linePath = new Path(); final chart = animation.value; for(final stack in chart.stacks) { var y = size.height; for(final bar in stack.bars) { barPaint.color = bar.color; canvas.drawRect( new Rect.fromLTWH( stack.x, y - bar.height, stack.width, bar.height ), barPaint, ); if(y < size.height) { linePath.moveTo(stack.x, y); linePath.lineTo(stack.x + stack.width, y); } y -= bar.height; } canvas.drawPath(linePath, linePaint); linePath.reset(); } } @override bool shouldRepaint(BarChartPainter old) => false;}

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。