Flutter進階之實現動畫效果（四）

2019-12-12 00:34:27

字體：大中小

來源：轉載

供稿：網友

在上一篇文章：Flutter進階―實現動畫效果（三）中，實現了一個隨機高度、顏色的條形。這一篇文章我們會實現多個條形，同樣是隨機高度、顏色。

首先在bar.dart中創建BarChart類，并使用固定長度的Bar實例列表。我們將使用5個條形，表示一周的5個工作日。然后，我們需要將創建空白和隨機實例的責任從Bar轉移到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 } ) 創建給定長度的固定長度列表，并用fill在每個位置初始化值 length必須是非負整數 */ 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 } ) 創建給定長度的固定長度列表，并用generator創建的值在每個位置初始化值 創建的列表是固定長度，除非growable為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繪制時使用的樣式說明 style：是否繪制內部的形狀、形狀的邊緣或兩者都有，默認為PaintingStyle.fill */ final paint = new Paint()..style = PaintingStyle.fill; final chart = animation.value; // 每個條形占用的空間寬度 final barDistance = size.width/(1+chart.bars.length); // 每個條形占用空間75%的寬度 final barWidth = barDistance*barWidthFraction; // 用于計算每個條形的x坐標點 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在條形之間均勻分配可用寬度，并使每個條形占用可用寬度的75％。接下來我們要更新main.dart，用BarChart、BarChartTween替換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), ), ); }}

現在應用程序的效果如下圖：