Android 定时器

对于一个Java开发者来说,想到定时器(计时器,倒计时)的应用 一般都会想到使用 java.util.Timer 和 java.util.TimerTask , 在Android中使用这2个类也可以实现计时的功能 但是使用起来还是有点麻烦的, 特别是在UI界面需要更新的时候, 例如 实现一个倒计时的界面, 在一个TextView中显示当前剩余的时间, 如果在TimerTask中计时,则无法更新TextView显示的剩余时间(在非UI线程中
不能访问UI组件),可以通过runOnUiThread函数来实现  但是多少有点烦琐

在Android
实现定时比较推荐的方式还是使用 android.os.Handler 中的 postXXX 和sendXXX 等方法. 细心的开发者可能已经注意到Android提供了一个倒计时的助手类 android.os.CountDownTimer 来方便实现倒计时的功能. 她就是通过handler的 sendMessageDelayed 来实现的. 使用该类来显示一个倒计时的TextView是很方便的.

new CountDownTimer(60000,1000){
@Override
public void onTick(long millisUntilFinished) {

}

@Override
public void onFinish() {

}
}.start();

CountDownTimer的源代码如下

Java

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);
}
}
}
};
}