Android CountDownTimer的使用

本文主要是介绍Android CountDownTimer的使用，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

最近博主需要实现一个倒计时相关的功能，被推荐了Android的CountDownTimer工具类，在此说一下CountDownTimer的使用以及源码的解读

以下是一个总计10秒倒计时，每间隔1秒进行回调的例子：

        CountDownTimer timer = new CountDownTimer(10000, 1000) {@Overridepublic void onTick(long millisUntilFinished) {// do something}@Overridepublic void onFinish() {// do something}};timer.start();

每间隔1秒，CountDownTimer便会调用onTick回调方法执行相应操作
当倒计时结束后，CountDownTimer会调用onFinish回调方法执行相应的操作

看完CountDownTimer的例子后，我们可以看一下CountDownTimer的源码以加深对该工具类的理解，源码如下：

public abstract class CountDownTimer {/*** Millis since epoch when alarm should stop.*/private final long mMillisInFuture;/*** The interval in millis that the user receives callbacks*/private final long mCountdownInterval;private long mStopTimeInFuture;/*** boolean representing if the timer was cancelled*/private boolean mCancelled = false;/*** @param millisInFuture The number of millis in the future from the call*   to {@link #start()} until the countdown is done and {@link #onFinish()}*   is called.* @param countDownInterval The interval along the way to receive*   {@link #onTick(long)} callbacks.*/public CountDownTimer(long millisInFuture, long countDownInterval) {mMillisInFuture = millisInFuture;mCountdownInterval = countDownInterval;}/*** Cancel the countdown.*/public synchronized final void cancel() {mCancelled = true;mHandler.removeMessages(MSG);}/*** Start the countdown.*/public synchronized final CountDownTimer start() {mCancelled = false;if (mMillisInFuture <= 0) {onFinish();return this;}mStopTimeInFuture = SystemClock.elapsedRealtime() + mMillisInFuture;mHandler.sendMessage(mHandler.obtainMessage(MSG));return this;}/*** Callback fired on regular interval.* @param millisUntilFinished The amount of time until finished.*/public abstract void onTick(long millisUntilFinished);/*** Callback fired when the time is up.*/public abstract void onFinish();private static final int MSG = 1;// handles counting downprivate Handler mHandler = new Handler() {@Overridepublic void handleMessage(Message msg) {synchronized (CountDownTimer.this) {if (mCancelled) {return;}final long millisLeft = mStopTimeInFuture - SystemClock.elapsedRealtime();if (millisLeft <= 0) {onFinish();} else {long lastTickStart = SystemClock.elapsedRealtime();onTick(millisLeft);// take into account user's onTick taking time to executelong lastTickDuration = SystemClock.elapsedRealtime() - lastTickStart;long delay;if (millisLeft < mCountdownInterval) {// just delay until donedelay = millisLeft - lastTickDuration;// special case: user's onTick took more than interval to// complete, trigger onFinish without delayif (delay < 0) delay = 0;} else {delay = mCountdownInterval - lastTickDuration;// special case: user's onTick took more than interval to// complete, skip to next intervalwhile (delay < 0) delay += mCountdownInterval;}sendMessageDelayed(obtainMessage(MSG), delay);}}}};
}

源码并不算长，CountDownTimer作为一个抽象类，其主要方法有如下几个：

• start：开始进行倒计时
• cancel：取消倒计时
• onTick：抽象方法，用于倒计时间隔回调
• onFinish：抽象方法，用于倒计时结束时回调

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