page 30 repeating a task over time

package{import flash.display.Sprite;import flash.events.TimerEvent;import flash.utils.getTimer;import flash.utils.Timer;/** * ... * @author */public class Main extends Sprite{private var _previousTime:Number = 0;public function Main(){var tTimer:Timer = new Timer(500, 10);tTimer.addEventListener(TimerEvent.TIMER, onTimer);tTimer.start();}private function onTimer(event:TimerEvent):void{trace(getTimer() - _previousTime);_previousTime = getTimer();}}}//输出结果很奇怪。为什么每次输出都不一样的？