Flutter进阶之实现动画效果（四）

在上一篇文章：Flutter进阶—实现动画效果（三）中，实现了一个随机高度、颜色的条形。这一篇文章我们会实现多个条形，同样是随机高度、颜色。

首先在bar.dart中创建BarChart类，并使用固定长度的Bar实例列表。我们将使用5个条形，表示一周的5个工作日。然后，我们需要将创建空白和随机实例的责任从Bar转移到BarChart。

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

class BarChart {
 static const int barCount = 5;
 final List<Bar> bars;

 BarChart(this.bars) {
 assert(bars.length == barCount);
 }

 factory BarChart.empty() {
 return new BarChart(
 /*
 List.filled(
 int length,
 E fill, {
  bool growable: false
 }
 )
 创建给定长度的固定长度列表，并用fill在每个位置初始化值
 length必须是非负整数
 */
 new List.filled(
 barCount,
 new Bar(0.0, Colors.transparent)
 )
 );
 }

 factory BarChart.random(Random random) {
 final Color color = ColorPalette.primary.random(random);
 return new BarChart(
 /*
 List.generate(
 int length,
 E generator(
  int index
 ), {
 bool growable: true
 }
 )
 创建给定长度的固定长度列表，并用generator创建的值在每个位置初始化值
 创建的列表是固定长度，除非growable为true
 */
 new List.generate(
 barCount,
 (i) => new Bar(
  random.nextDouble()*100.0,
  color
 )
 )
 );
 }

 static BarChart lerp(BarChart begin, BarChart end, double t) {
 return new BarChart(
 new List.generate(
 barCount,
 (i) => Bar.lerp(begin.bars[i], end.bars[i], t)
 )
 );
 }
}

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.height, this.color);
 final double height;
 final Color color;

 static Bar lerp(Bar begin, Bar end, double t) {
 return new Bar(
 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 {
 static const barWidthFraction = 0.75;

 BarChartPainter(Animation<BarChart> animation)
 : animation = animation,
 super(repaint: animation);

 final Animation<BarChart> animation;

 @override
 void paint(Canvas canvas, Size size) {
 void drawBar(Bar bar, double x, double width, Paint paint) {
 paint.color = bar.color;
 canvas.drawRect(
 new Rect.fromLTWH(
  x,
  size.height-bar.height,
  width,
  bar.height
 ),
 paint
 );
 }

 /*
 Paint：Canvas绘制时使用的样式说明
 style：是否绘制内部的形状、形状的边缘或两者都有，默认为PaintingStyle.fill
 */
 final paint = new Paint()..style = PaintingStyle.fill;
 final chart = animation.value;
 // 每个条形占用的空间宽度
 final barDistance = size.width/(1+chart.bars.length);
 // 每个条形占用空间75%的宽度
 final barWidth = barDistance*barWidthFraction;
 // 用于计算每个条形的x坐标点
 var x = barDistance-barWidth/2;
 for (final bar in chart.bars) {
 drawBar(bar, x, barWidth, paint);
 x += barDistance;
 }
 }

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

BarChartPainter在条形之间均匀分配可用宽度，并使每个条形占用可用宽度的75％。接下来我们要更新main.dart，用BarChart、BarChartTween替换Bar、BarTween。

// ...
class _MyHomePageState extends State<MyHomePage> with TickerProviderStateMixin {
 final random = new Random();
 AnimationController animation;
 BarChartTween tween;

 @override
 void initState() {
 super.initState();
 animation = new AnimationController(
 duration: const Duration(milliseconds: 300),
 vsync: this
 );
 tween = new BarChartTween(new BarChart.empty(), new BarChart.random(random));
 animation.forward();
 }

 @override
 void dispose() {
 animation.dispose();
 super.dispose();
 }

 void changeData() {
 setState(() {
 tween = new BarChartTween(
 tween.evaluate(animation),
 new BarChart.random(random),
 );
 animation.forward(from: 0.0);
 });
 }

 @override
 Widget build(BuildContext context) {
 return new Scaffold(
 body: new Center(
  child: new CustomPaint(
  size: new Size(200.0, 100.0),
  painter: new BarChartPainter(tween.animate(animation))
  )
 ),
 floatingActionButton: new FloatingActionButton(
 onPressed: changeData,
 child: new Icon(Icons.refresh),
 ),
 );
 }
}

现在应用程序的效果如下图：

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我们。