Flutter进阶—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,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：

未完待续～～～