用到CountDownTimer这个类，非常简单，只有四个方法，

public final void cancel () //取消后，再次启动会重新开始倒计时

public abstract void onFinish ()

public abstract void onTick (long millisUntilFinished) //onTick的调用是同步的，保证这次调用不会在之前调用完成前发生

public synchronized final CountDownTimer start ()

实例

1         new CountDownTimer(60 * 1000, 1000) {  2             public void onTick(long millisUntilFinished) {  3                 testView.setText("seconds remaining: " + millisUntilFinished / 1000);  4             }  5             public void onFinish() {  6                 testView.setText("done!");  7             }  8          }.start();