class TimeTracer {
	private var timerInterval:Number;
	public function startTimeDisplay():Void {
		stopTimeDisplay();
		var begunAt:String = new Date().toString();
		timerInterval = setInterval(displayTime, 1000);
		function displayTime():Void {
			trace("Time now: "+new Date().toString()+". "+"Timer started at: "+begunAt);
		}
	}
	public function stopTimeDisplay():Void {
		clearInterval(timerInterval);
	}
}

/*
当stopTimeDisplay( )被调用时，间隔被停止，然后嵌套函数displayTime( )被自动删除。
而且，被保存的用于displayTime( )的局部变量begunAt也不再需要，因此被自动地垃圾回收
garbage-collected（即，删除）。
*/

/* usage: 

t = new TimeTracer();
t.startTimeDisplay();

trace
Time now: Wed Nov 30 14:18:55 GMT+0800 2005. Timer started at: Wed Nov 30 14:18:54 GMT+0800 2005
Time now: Wed Nov 30 14:18:56 GMT+0800 2005. Timer started at: Wed Nov 30 14:18:54 GMT+0800 2005
Time now: Wed Nov 30 14:18:57 GMT+0800 2005. Timer started at: Wed Nov 30 14:18:54 GMT+0800 2005
...... ......

*/