Flutter進(jìn)階之實(shí)現(xiàn)動(dòng)畫效果（五）

import 'package:flutter/material.dart';
import 'package:flutter/animation.dart';
import 'dart:ui' show lerpDouble;
import 'dart:math';
import 'color_palette.dart';

class BarChart {
 final List<Bar> bars;

 BarChart(this.bars);

 factory BarChart.empty(Size size) {
 return new BarChart(<Bar>[]);
 }

 factory BarChart.random(Size size, Random random) {
 const barWidthFraction = 0.75;
 const minBarDistance = 20.0;
 // floor()：返回不大于此的最大整數(shù)
 final barCount = random.nextInt((size.width/minBarDistance).floor()) + 1;
 final barDistance = size.width / (1+barCount);
 final barWidth = barDistance * barWidthFraction;
 final startX = barDistance - barWidth/2;
 final color = ColorPalette.primary.random(random);
 final bars = new List.generate(
  barCount,
  (i)=> new Bar(
  startX + i * barDistance,
  barWidth,
  random.nextDouble() * size.height,
  color,
  ),
 );
 return new BarChart(bars);
 }

 static BarChart lerp(BarChart begin, BarChart end, double t) {
 // max：返回兩個(gè)數(shù)字中較大的一個(gè)
 final barCount = max(begin.bars.length, end.bars.length);
 final bars = new List.generate(
  barCount,
  (i) => Bar.lerp(
  // ??：如果為空時(shí)取的默認(rèn)值
  begin._barOrNull(i) ?? end.bars[i].collapsed,
  end._barOrNull(i) ?? begin.bars[i].collapsed,
  t,
  )
 );
 return new BarChart(bars);
 }

 Bar _barOrNull(int index) => (index<bars.length ? bars[index] : null);
}

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

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

class Bar {
 Bar(this.x, this.width, this.height, this.color);
 final double x;
 final double width;
 final double height;
 final Color color;

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

 static Bar lerp(Bar begin, Bar end, double t) {
 return new Bar(
  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);

 @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 bar in chart.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;
}

class _MyHomePageState extends State<MyHomePage> with TickerProviderStateMixin {
 static const size = const Size(200.0, 100.0);
 // ...
 @override
 void initState() {
 // ...
 tween = new BarChartTween(
  new BarChart.empty(size),
  new BarChart.random(size, random));
 animation.forward();
 }
 // ...
 void changeData() {
 setState(() {
  tween = new BarChartTween(
  tween.evaluate(animation),
  new BarChart.random(size, random),
  );
  animation.forward(from: 0.0);
 });
 }
 // ...
 }
}