淡出粒子
为了实现粒子在视觉上消失的效果，通过设置Particle的fade属性为0~1之间的小数，使它的alpha逐渐降低，从而达到淡出粒子的目的。[code lang="as3"]package{
import flash.display.Sprite;
import flash.events.Event;
[SWF(width="800",height="600",backgroundColor="0xffffff",frameRate="31")]
public class ParticleFade extends Sprite{
private var numbers:Number = 20;
private var particles:Array;
public function ParticleFade(){
particles=new Array();
stage.addEventListener(Event.ENTER_FRAME, onEnterFrame);
}
private function onEnterFrame(e:Event):void{
if(particles.length < numbers){
var ball:Ball=new Ball(12, 0x00ff00, 1, 0x000000, 1);
addChild(ball);
ball.xVelocity=Math.random() * 20 - 10;
ball.yVelocity=Math.random() * 20 - 10;
ball.gravity=0.9;
ball.growX = 1.05;
ball.growY = 1.05;
ball.fade = 0.98;
ball.x=stage.stageWidth/2;
ball.y=stage.stageHeight/4;
particles.push(ball);
}
for (var i:int=0; i < particles.length; i++){
var particle:Ball=particles[i];
particle.update();
if (particle.x - particle.radius > stage.stageWidth || particle.x + particle.radius < 0 || particle.y - particle.radius > stage.stageHeight || particle.y + particle.radius < 0){
particle.xVelocity=Math.random() * 20 - 10;
particle.yVelocity=Math.random() * 20 - 10;
particle.x=stage.stageWidth / 2;
particle.y=stage.stageHeight / 4;
particle.scaleX = particle.scaleY = 1;
particle.alpha = 1;
}
}
}
}
}[/code]
在初始化粒子的循环中将粒子的fade属性设置为0.98，在遍历粒子数组并使其运动的循环中调用update函数使粒子运动并改变它的alpha属性。超过环境边界后，再重置它的速度和位置，并将它的alpha设置回1，从而在下一次的遍历循环中演示它的fade属性。
<img class="alignnone size-medium wp-image-116" title="ParticleFade" src="http://www.everyinch.net/wp-content/uploads/2011/09/ParticleFade-300x273.jpg" alt="" width="300" height="273" />