Nothing wrong with the code I just need an explanation

programming flash
1

[COLOR=“DarkRed”]Hi,

This is a simple (simple?) particle system that came in the Learning ActionScript 3.0 book.

I was trying to create a particle system and than I thought could use this as a reference but I am lost.

I am confused, why is that without having numbers assigned to the variables _xpos, _ypos etc. it is working fine? Could someone explain this code, please?

Any help would be appreciated.

thanks,
fs_tigre[/COLOR]

package {

import flash.display.Sprite;
import flash.events.Event;

public class ParticleDemo extends Sprite {

	public var emitterX:Number = stage.stageWidth/2;
	public var emitterY:Number = stage.stageHeight/2;

	public function ParticleDemo() {
		stage.addEventListener(Event.ENTER_FRAME,onLoop, false, 0, true);
	}
	
	private function onLoop(evt:Event):void {
		var p:Particle = new Particle(emitterX,
									  emitterY,
									  Math.random()*11 - 6,
									  Math.random()*-20,
									  1,
									  Math.random()*0xFFFFFF);
		addChild(p);
	}
	
}

}

[SIZE=“4”][COLOR=“Black”][COLOR=“DarkRed”]This is the particle classs[/COLOR][/COLOR][/SIZE]

package{

import flash.display.*;
import flash.geom.*
import flash.events.Event;

public class Particle extends Sprite {
	
	private var _xpos:Number;
	private var _ypos:Number;
	private var _xvel:Number;
	private var _yvel:Number;
	private var _grav:Number;

	public function Particle(xp:Number, yp:Number, xvel:Number, yvel:Number, grav:Number, col:uint) {
		_xpos = xp;
		_ypos = yp;
		_xvel = xvel
		_yvel = yvel
		_grav = grav;
		
		var ball:Sprite = new Ball();
		addChild(ball);
		
		x = _xpos;
		y = _ypos;
		alpha = .8;
		scaleX = scaleY = Math.random() * 1.9 + .1;
		
		var colorInfo:ColorTransform = ball.transform.colorTransform;
		colorInfo.color = col;
		ball.transform.colorTransform = colorInfo;
		
		addEventListener(Event.ENTER_FRAME, onRun, false, 0, true);
	}
	
	private function onRun(evt:Event):void {
		_yvel += _grav;
		_xpos += _xvel;
		_ypos += _yvel;
		x = _xpos;
		y = _ypos;
		if (_xpos < 0 || _ypos < 0 || _xpos > stage.stageWidth || _ypos > stage.stageHeight) {
		   removeEventListener(Event.ENTER_FRAME, onRun);
		   parent.removeChild(this);
		}
	}
}

}