Android CountDownTimer的使用

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) {            @Override            public void onTick(long millisUntilFinished) {                // do something            }            @Override            public 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 down    private Handler mHandler = new Handler() {        @Override        public 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 execute                    long lastTickDuration = SystemClock.elapsedRealtime() - lastTickStart;                    long delay;                    if (millisLeft < mCountdownInterval) {                        // just delay until done                        delay = millisLeft - lastTickDuration;                        // special case: user's onTick took more than interval to                        // complete, trigger onFinish without delay                        if (delay < 0) delay = 0;                    } else {                        delay = mCountdownInterval - lastTickDuration;                        // special case: user's onTick took more than interval to                        // complete, skip to next interval                        while (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

看过CountDownTimer的源码后，有几个细节我们需要稍微注意一下：

在源码第38行中，CountDownTimer会判断是否倒计时已结束，如果是则调用onFinish方法，否则调用onTick方法。因此，在倒计时的最后一秒时，我们并不会收到onTick的回调，取而代之的是onFinish的回调。
从源码可以看出，CountDownTimer其实与Timer完全没有任何关系，它的倒计时实现是使用Handler机制实现的，因此当我们在非UI线程使用该工具时，需要先初始化Looper
同上，由于CountDownTimer是基于Handler实现的，其处理以及发送message以及回调onTick处于同一线程，因此当我们在回调方法onTick耗时过多时，可能会影响CountDownTimer预估的回调次数（见源码144行）

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