// ...
class BarChart {
 // ...
 factory BarChart.random(Size size, Random random) {
 const barWidthFraction = 0.75;
 final ranks = selectRanks(random, ColorPalette.primary.length);
 final barCount = ranks.length;
 final barDistance = size.width / (1+barCount);
 final barWidth = barDistance * barWidthFraction;
 final startX = barDistance - barWidth/2;
 final bars = new List.generate(
  barCount,
  (i)=> new Bar(
  ranks[i],
  startX + i * barDistance,
  barWidth,
  random.nextDouble() * size.height,
  ColorPalette.primary[ranks[i]],
  ),
 );
 return new BarChart(bars);
 }

 static List selectRanks(Random random, int cap) {
 final ranks = [];
 var rank = 0;
 while(true) {
  // 模拟产品的上架下架
  if(random.nextDouble() < 0.2) rank++;
  if(cap <= rank) break;
  ranks.add(rank);
  rank++;
 }
 return ranks;
 }

 static BarChart lerp(BarChart begin, BarChart end, double t) {
 final bars = [];
 final bMax = begin.bars.length;
 final eMax = end.bars.length;
 var b = 0;
 var e = 0;
 while (b + e < bMax + eMax) {
  /*
  这里的条件判断中包含两种情况
  b < bMax && e == eMax：
  当新图表条形数减少时
  b < bMax && begin.bars[b] < end.bars[e]：
  当新图表不包含旧图表的颜色条形时
  满足一种情况，处理旧图表中多余的条形，即向左侧方清除旧条形
  */
  if(b < bMax && (e == eMax || begin.bars[b] < end.bars[e])) {
  bars.add(Bar.lerp(begin.bars[b], begin.bars[b].collapsed, t));
  b++;
  /*
  这里的条件判断中包含两种情况
  e < eMax && b == bMax：
  当新图表条形数增加时
  e < eMax && end.bars[e] < begin.bars[b]：
  当新图表包含旧图表没有的颜色条形时
  满足一种情况，处理旧图表中没有的条形，即向右侧方绘制新条形
  */
  } else if(e < eMax && (b == bMax || end.bars[e] < begin.bars[b])) {
  bars.add(Bar.lerp(end.bars[e].collapsed, end.bars[e], t));
  e++;
  // 当新图表与旧图表有同一个颜色条形时，原地变形
  } else {
  bars.add(Bar.lerp(begin.bars[b], end.bars[e], t));
  b++;
  e++;
  }
 }
 return new BarChart(bars);
 }
}

class BarChartTween extends Tween {
 BarChartTween(BarChart begin, BarChart end) : super(begin: begin, end: end);

 @override
 BarChart lerp(double t) => BarChart.lerp(begin, end, t);
}

class 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;

 Bar get collapsed => new Bar(rank, x, 0.0, 0.0, color);

 /*
 bool operator <(
 Duration other
 )
 如果此Duration的值小于other值，则返回true
 bool operator <(Duration other) => this._duration < other._duration;
 */
 bool operator <(Bar other) => rank < other.rank;

 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 {
 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 BarChartTween extends Tween {
 final _tweens = [];

 BarChartTween(BarChart begin, BarChart end) : super(begin: begin, end: end) {
 final bMax = begin.bars.length;
 final eMax = end.bars.length;
 var b = 0;
 var e = 0;
 while (b + e < bMax + eMax) {
 if(b < bMax && (e == eMax || begin.bars[b] < end.bars[e])) {
 _tweens.add(new BarTween(begin.bars[b], begin.bars[b].collapsed));
 b++;
 } else if(e < eMax && (b == bMax || end.bars[e] < begin.bars[b])) {
 _tweens.add(new BarTween(end.bars[e].collapsed, end.bars[e]));
 e++;
 } else {
 _tweens.add(new BarTween(begin.bars[b], end.bars[e]));
 b++;
 e++;
 }
 }
 }

 @override
 BarChart lerp(double t) => new BarChart(
 new List.generate(
 _tweens.length,
 (i) => _tweens[i].lerp(t))
 );
}
// ...