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();
        }

    }
}