Flutter进阶—实现动画效果（八）

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【手动斜眼笑】。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) {
          /*
          void moveTo(
            double x,
            double y
          )
          在给定的坐标开始一个新的子路径。
           */
          linePath.moveTo(stack.x, y);
          /*
          void lineTo(
            double x,
            double y
          )
          将直线段从当前点添加到给定点。
           */
          linePath.lineTo(stack.x + stack.width, y);
        }
        y -= bar.height;
      }
      /*
      void drawPath(
        Path path,
        Paint paint
      )
      用给定的paint绘制给定的path
       */
      canvas.drawPath(linePath, linePaint);
      linePath.reset();
    }
  }

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