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Flutter進(jìn)階之實(shí)現(xiàn)動(dòng)畫(huà)效果（四）

更新時(shí)間：2018年08月20日 08:41:00 作者：何小有

這篇文章主要為大家詳細(xì)介紹了Flutter進(jìn)階之實(shí)現(xiàn)動(dòng)畫(huà)效果，具有一定的參考價(jià)值，感興趣的小伙伴們可以參考一下

在上一篇文章：Flutter進(jìn)階—實(shí)現(xiàn)動(dòng)畫(huà)效果（三）中，實(shí)現(xiàn)了一個(gè)隨機(jī)高度、顏色的條形。這一篇文章我們會(huì)實(shí)現(xiàn)多個(gè)條形，同樣是隨機(jī)高度、顏色。

首先在bar.dart中創(chuàng)建BarChart類(lèi)，并使用固定長(zhǎng)度的Bar實(shí)例列表。我們將使用5個(gè)條形，表示一周的5個(gè)工作日。然后，我們需要將創(chuàng)建空白和隨機(jī)實(shí)例的責(zé)任從Bar轉(zhuǎn)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(chuàng)建給定長(zhǎng)度的固定長(zhǎng)度列表，并用fill在每個(gè)位置初始化值
 length必須是非負(fù)整數(shù)
 */
 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(chuàng)建給定長(zhǎng)度的固定長(zhǎng)度列表，并用generator創(chuàng)建的值在每個(gè)位置初始化值
 創(chuàng)建的列表是固定長(zhǎng)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(shí)使用的樣式說(shuō)明
 style：是否繪制內(nèi)部的形狀、形狀的邊緣或兩者都有，默認(rèn)為PaintingStyle.fill
 */
 final paint = new Paint()..style = PaintingStyle.fill;
 final chart = animation.value;
 // 每個(gè)條形占用的空間寬度
 final barDistance = size.width/(1+chart.bars.length);
 // 每個(gè)條形占用空間75%的寬度
 final barWidth = barDistance*barWidthFraction;
 // 用于計(jì)算每個(gè)條形的x坐標(biāo)點(diǎn)
 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(gè)條形占用可用寬度的75％。接下來(lái)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(xiàn)在應(yīng)用程序的效果如下圖：