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首先我们使用构造函数创建CountDownTimer对象,需要传递两个参数,并实现两个抽象方法
构造函数
public CountDownTimer(long millisInFuture, long countDownInterval) {
mMillisInFuture = millisInFuture;
mCountdownInterval = countDownInterval;
}
创建对象
mCountDownTimer = new CountDownTimer(Long.MAX_VALUE, 5000) {
@Override
public void onTick(long l) {
}
@Override
public void onFinish() {
}
};
mCountDownTimer.start();
onTick(long l):用于返回还剩余多长时间,每间隔一段时间回调一次
onFinish():当倒计时结束时回调
CountDownTimer通过start()来触发倒计时,那我们来看看里面是如何实现的
/**
* 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;
}
首先会判断设置的倒计时时间是否小于等于0,是的话直接回调onFinish()方法;反之计算倒计时结束时的时间(当前时间+总的倒计时时间);然后通过handle发送消息
// 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 {
long lastTickStart = SystemClock.elapsedRealtime();
onTick(millisLeft);
// take into account user's onTick taking time to execute
long lastTickDuration = SystemClock.elapsedRealtime() - lastTickStart;
long delay;
if (millisLeft < mCountdownInterval) {
// just delay until done
delay = millisLeft - lastTickDuration;
// special case: user's onTick took more than interval to
// complete, trigger onFinish without delay
if (delay < 0) delay = 0;
} else {
delay = mCountdownInterval - lastTickDuration;
// special case: user's onTick took more than interval to
// complete, skip to next interval
while (delay < 0) delay += mCountdownInterval;
}
sendMessageDelayed(obtainMessage(MSG), delay);
}
}
}
};
在Handle中我们首先会判断是否设置了mCancelled=true,若是的话直接不再处理,表示已经取消倒计时;反之,计算倒计时还剩余多少时间,若剩余时间<=0,则直接调用onFinish()表示倒计时结束;否则计算剩余时间时候大于设置每次减去的时间的时间,若是则发送延时消息,延时时间为mCountdownInterval,否则延时时间为剩余时间,这样就实现了倒计时的功能
CountDownTimer通过cancel()来取消倒计时
/**
* Cancel the countdown.
*/
public synchronized final void cancel() {
mCancelled = true;
mHandler.removeMessages(MSG);
}
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