倒计时器 CountDownTimer

使用介绍

开发中经常会遇到一些和倒计时有关的场景，比如发送验证码的按钮，会在点击发送后，显示倒计时间，倒计时结束后才能够刷新按钮，再次允许点击。为了不阻塞软件的运行，又要实时刷新界面，我们通常会用到 Handler 或者 AsyncTask 等技术，自己写逻辑实现。其实 Android 中已经封装好了一套 CountDownTimer 来实现这个功能需求。

CountDownTimer(long millisInFuture, long countDownInterval)

CountDownTimer的两个参数分别表示倒计时的总时间 millisInFuture 和间隔时间 countDownInterval。

具体的调用如下：

TextView vertifyBtn;
CountDownTimer timer = new CountDownTimer(60000, 1000) {

    @Override
    public void onTick(long millisUntilFinished) {
        vertifyBtn.setText((millisUntilFinished / 1000) + " second");
    }
    
    @Override
    public void onFinish() {
        vertifyBtn.setEnabled(true);
        vertifyBtn.setText("Send");
    }
};
timer.start();

上面的调用举例表示总计 60 秒，每 1 秒都会执行一次 onTick 方法，其参数 millisUntilFinished 表示倒计时剩余时间毫秒数，最后倒计时结束执行 onFinish 方法。

实现原理

下面是 CountDownTimer 的源码，代码非常少，很好理解。从源代码中可以看出，其实 CountDownTimer 也是利用 Handler 的消息处理机制来实现效果的。初始化设定好起始和终止时间后，每隔一定的间隔时间通过 Handler 给主线程发送消息，然后再在消息处理中回调方法。好好利用官方封装好的工具类，可以避免我们重复的造轮子，当然了解轮子的原理就更好了！

package android.os;

public abstract class CountDownTimer {
    private final long mMillisInFuture;
    private final long mCountdownInterval;
    private long mStopTimeInFuture;
    private boolean mCancelled = false;

    public CountDownTimer(long millisInFuture, long countDownInterval) {
        mMillisInFuture = millisInFuture;
        mCountdownInterval = countDownInterval;
    }

    public synchronized final void cancel() {
        mCancelled = true;
        mHandler.removeMessages(MSG);
    }

    public synchronized final CountDownTimer start() {
        mCancelled = false;
        if (mMillisInFuture <= 0) {
            onFinish();
            return this;
        }
        mStopTimeInFuture = SystemClock.elapsedRealtime() + mMillisInFuture;
        mHandler.sendMessage(mHandler.obtainMessage(MSG));
        return this;
    }

    public abstract void onTick(long millisUntilFinished);

    public abstract void onFinish();

    private static final int MSG = 1;
    
    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);
                }
            }
        }
    };
}