1、API

1
CountDownTimer (long millisInFuture, long countDownInterval)

参数1，设置倒计时的总时间（毫秒）
参数2，设置每次减去多少毫秒

2、基本用法

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
private Button btn;// 略写初始化

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    btn.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            timer.start();
        }
    });
}
 
private CountDownTimer timer = new CountDownTimer(60000, 1000) {  

    @Override  
    public void onTick(long millisUntilFinished) {  
        btn.setText((millisUntilFinished / 1000) + "秒后可重发");  
    }  

    @Override  
    public void onFinish() {  
        btn.setText("重新获取");  
    }  
};  

@Override
protected void onDestroy() {
    super.onDestroy();
    if (timer != null){
        timer.cancel();
    }
}

3、存在的问题

注意onDestroy() ，不然会存在内存泄漏的问题，源码很简单：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
package android.os;

/**
 *  ...
 */
public abstract class CountDownTimer {
    /**
     *  ...
     */
    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;
    }

    .......

    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);
                }
            }
        }
    };
}

从源码中可以看出，CountDownTimer的内部实现是采用Handler机制，通过sendMessageDelayed延迟发送一条message到主线程的looper中，然后在自身中收到之后判断剩余时间，并发出相关回调，然后再次发出message的方式。
如果在Activity或者Fragment被回收时并未调用CountDownTimer的cancel()方法结束自己，这个时候CountDownTimer的Handler方法中如果判断到当前的时间未走完，那么会继续调用sendMessageDelayed(obtainMessage(MSG), delay);触发onTick(millisLeft);，当回调了Activity或者fragment中CountDownTimer的onTick方法时，Activity或者Fragment已经被系统回收，从而里面的变量被设置为null，再调用btn.setText((millisUntilFinished / 1000) + "秒后可重发");btn为空，也就空指针了，而CountDownTimer中的Handler方法还在继续执行，这一块空间始终无法被系统回收也就造成了内存泄漏问题。