Android－源码剖析CountDownTimer（倒计时类）

简介

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CounterDownTImer是Android系统自带的一个倒计时器，特别是在做app登录时会比较有用。

用法

非常简单，比如做个倒计时60s且每隔1s会刷新一下，可以这样写

new CountDownTimer(60000, 1000) {     public void onTick(long millisLeft) {         mTextField.setText("seconds millisLeft: " + millisLeft / 1000);     }     public void onFinish() {         mTextField.setText("done!");     }  }.start();

CountDownTimer类的构造器接受2个参数，第一个为倒计时总时间，第二个为间隔时间单位。注意2个参数单位都是ms。当调用start方法后，系统会每间隔一段时间（第二个参数指定）调用onTrick方法，倒计时结束会执行onFinish方法；那么到底是怎么实现的呢？

下面是源码以及对源码理解所做的注释，就不再解释了

package android.os;/* * * The calls to {@link #onTick(long)} are synchronized to this object so that * one call to {@link #onTick(long)} won't ever occur before the previous * callback is complete.  This is only relevant when the implementation of * {@link #onTick(long)} takes an amount of time to execute that is significant * compared to the countdown interval. */public abstract class CountDownTimer {    /**     * Millis since epoch when alarm should stop.     */    private final long mMillisInFuture;//倒计时总耗时间（单位:ms）    /**     * The interval in millis that the user receives callbacks     */    private final long mCountdownInterval;//间隔时间（单位:ms)    private long mStopTimeInFuture;//倒计时结束时间戳(单位:ms)    /**    * 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;        //判断构造器接受的参数是否合法，        //如果倒计时总耗时间<=0 直接调用onFinish方法,并立即返回该对象        if (mMillisInFuture <= 0) {            onFinish();            return this;        }        //保存倒计时结束时间戳        mStopTimeInFuture = SystemClock.elapsedRealtime() + mMillisInFuture;        //发送消息通知        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;    //注意此处mHandler成员变量的构造器没有传入Looper哦    // handles counting down    private Handler mHandler = new Handler() {        @Override        public void handleMessage(Message msg) {            synchronized (CountDownTimer.this) {                if (mCancelled) {                    return;                }                //计算还剩下多少时间需要倒计时                final long millisLeft = mStopTimeInFuture - SystemClock.elapsedRealtime();                if (millisLeft <= 0) {                    onFinish();                } else if (millisLeft < mCountdownInterval) {                    //如果还剩的时间比间隔时间还小，不再调用onTick方法，延迟剩余的时间发送通知后直接执行onFinish方法                    // no tick, just delay until done                    sendMessageDelayed(obtainMessage(MSG), millisLeft);                } else {                    long lastTickStart = SystemClock.elapsedRealtime();                    onTick(millisLeft);                    /**                     *delay = 间隔时间－onTick消耗的时间                     *如果delay >＝0 ,为保证onTick是严格间隔时间执行，延迟delay发送通知                     *如果delay < 0, 说明执行onTick方法消耗的时间比间隔时间大，只能跳过当前时序，直接进入下一个时序                     **/                    // take into account user's onTick taking time to execute                    long delay = lastTickStart + mCountdownInterval - SystemClock.elapsedRealtime();                    // special case: user's onTick took more than interval to                    // complete, skip to next interval                    while (delay < 0) delay += mCountdownInterval;                    sendMessageDelayed(obtainMessage(MSG), delay);                }            }        }    };}

总结：

1. 如果onTick涉及到UI修改，CountDownloadTimer对象创建确保在UI线程创建
2. CountDownTimer为了保持在间隔时间内准确通知onTick，会计算onTick时间，如果onTick消耗时间大于指定的间隔时间，只能在下个时间间隔内执行onTick了