Android CountDownTimer倒计时

这是一个Google 已经封装好的倒计时抽象类，在开发过程中可以快速地实现倒计时功能，如：获取手机验证码的倒计时实现；该类封装的源码和使用都比较简单。

一、源码

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 if (millisLeft < mCountdownInterval) {                    // no tick, just delay until done                    sendMessageDelayed(obtainMessage(MSG), millisLeft);                } else {                    long lastTickStart = SystemClock.elapsedRealtime();                    onTick(millisLeft);                    // 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);                }            }        }    };}

1、实现原理

通过 Handler 间隔性地发送消息来实现倒计时。

2、方法介绍：

从源码可以看到 CountDownTimer类共有5个方法，分别是：构造方法：CountDownTimer(long,long)取消倒计时：cancel()开始倒计时：start()倒计时间隔执行：onTick(long)倒计时结束：onFinish()（1）构造方法：CountDownTimer(long,long)   传入两个long类型的参数，第一个参数表示倒计时的总时长，第二个参数表示倒计时的间隔时间；（2）取消倒计时方法：cancel()  方法由final关键字修饰，可被继承，不可重写；移除消息队列中的消息，倒计时停止；（3）开始倒计时：start()  方法由final关键字修饰，可被继承，不可重写；synchronized 保证线程安全；启动Handler 开始倒计时；（4）倒计时间隔执行：onTick(long)  抽象方法，具体实现内容由子类重写，按时间间隔执行；由 final long millisLeft = mStopTimeInFuture - SystemClock.elapsedRealtime();看出该方法的参数为倒计时剩余时间；（5）倒计时结束：onFinish()  抽象方法，具体实现内容由子类重写；在倒计时完成时执行；注：handleMessage(Message msg) 中实现主要的倒计时逻辑判断。

二、简单的使用Demo

1.实现效果

  倒计时中：

倒计时结束：

2.实现代码

public class MainActivity extends Activity {    private Button button;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        button=(Button)findViewById(R.id.btn);        button.setOnClickListener(new OnClickListener() {            @Override            public void onClick(View v) {                CountDownTime countDown=new CountDownTime(60*1000L, 1*1000L);                countDown.start();            }        });    }    class CountDownTime extends CountDownTimer{        /*         *  millisInFuture: 倒计时总时间         *  countDownInterval: 间隔时间         */        public CountDownTime(long millisInFuture, long countDownInterval) {            super(millisInFuture, countDownInterval);        }        @Override        public void onTick(long millisUntilFinished) {            // 每间隔1s 执行一次            button.setText((int)(millisUntilFinished/1000)+"s");            button.setTextColor(Color.BLUE);        }        @Override        public void onFinish() {            // 倒计时结束时执行            button.setText("重新倒计时");            button.setTextColor(Color.BLACK);            cancel();        }    }}