CountDownTimer用法

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Section1

new CountDownTimer(10000, 1000) {    @Override    public void onTick(long millisUntilFinished) {            }    @Override    public void onFinish() {    }}.start();

看源码解释

/** * @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;}
第一个参数代表倒计时多少毫秒，第二个参数代表倒计时的间隔。
Section2
第一个问题，
是否延迟
final long starttime = System.currentTimeMillis();new CountDownTimer(10000, 1000) {    @Override    public void onTick(long millisUntilFinished) {        long tickTime = System.currentTimeMillis();        double betweenTime = tickTime-starttime;        betweenTime = (betweenTime/1000d);        Toast.makeText(MainActivity.this,betweenTime+"",Toast.LENGTH_SHORT).show();    }    @Override    public void onFinish() {    }}.start();
从Toast提示可以看出，代码逻辑上并不延迟，只不过代码运行需要点时间。
Section3
那么，如果不延迟的话，理论上来说
final long starttime = System.currentTimeMillis();new CountDownTimer(5000, 1000) {    @Override    public void onTick(long millisUntilFinished) {        long tickTime = System.currentTimeMillis();        double betweenTime = tickTime-starttime;        betweenTime = (betweenTime/1000d);        Toast.makeText(MainActivity.this,betweenTime+"",Toast.LENGTH_SHORT).show();    }    @Override    public void onFinish() {    }}.start();

应该一共弹 0,1,2,34,5

事实上只弹了 0,1,2,3
Section4
4和5 哪去了？
5应该是在onFinish里面
final long starttime = System.currentTimeMillis();new CountDownTimer(5000, 1000) {    @Override    public void onTick(long millisUntilFinished) {        long tickTime = System.currentTimeMillis();        double betweenTime = tickTime-starttime;        betweenTime = (betweenTime/1000d);        Toast.makeText(MainActivity.this,betweenTime+"",Toast.LENGTH_SHORT).show();    }    @Override    public void onFinish() {        long tickTime = System.currentTimeMillis();        double betweenTime = tickTime-starttime;        betweenTime = (betweenTime/1000d);        Toast.makeText(MainActivity.this,betweenTime+"",Toast.LENGTH_SHORT).show();    }}.start();

这次是 0,1,2,3,5
Section5
还是缺一个，
看源码
 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);    }}
else if (millisLeft < mCountdownInterval) 
这里小于时间间隔就不tick了
好吧，我们改进一下

final long starttime = System.currentTimeMillis();new CountDownTimer(5100, 1000) {    @Override    public void onTick(long millisUntilFinished) {        long tickTime = System.currentTimeMillis();        double betweenTime = tickTime-starttime;        betweenTime = (betweenTime/1000d);        Toast.makeText(MainActivity.this,betweenTime+"",Toast.LENGTH_SHORT).show();    }    @Override    public void onFinish() {        long tickTime = System.currentTimeMillis();        double betweenTime = tickTime-starttime;        betweenTime = (betweenTime/1000d);        Toast.makeText(MainActivity.this,betweenTime+"",Toast.LENGTH_SHORT).show();    }}.start();

这次终于0,1,2,3,4,5 都有了。。
这里关键点是把5000改为5100。把损耗时间算上。

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