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:
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TextView mTv;
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@Override
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protected void onCreate(Bundle savedInstanceState) {
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super .onCreate(savedInstanceState);
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setContentView(R.layout.main);
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mTv = (TextView) findViewById(R.id.tv);
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mTv.setText( "test" );
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new CountDownTimer( 30000 , 100 ) {
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@Override
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public void onTick( long millisUntilFinished) {
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mTv.setText( "seconds remaining: " +
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millisUntilFinished / 1000 + " S " +(millisUntilFinished % 1000 )/ 100 );
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}
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@Override
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public void onFinish() {
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mTv.setText( "Done!" );
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}
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}.start();
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}
CountDownTimer的源代码如下:
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public abstract class CountDownTimer {
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/**
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* Millis since epoch when alarm should stop.
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*/
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private final long mMillisInFuture;
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/**
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* The interval in millis that the user receives callbacks
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*/
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private final long mCountdownInterval;
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private long mStopTimeInFuture;
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/**
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* @param millisInFuture The number of millis in the future from the call
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* to {@link #start()} until the countdown is done and {@link #onFinish()}
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* is called.
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* @param countDownInterval The interval along the way to receive
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* {@link #onTick(long)} callbacks.
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*/
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public CountDownTimer( long millisInFuture, long countDownInterval) {
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mMillisInFuture = millisInFuture;
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mCountdownInterval = countDownInterval;
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}
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/**
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* Cancel the countdown.
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*/
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public final void cancel() {
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mHandler.removeMessages(MSG);
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}
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/**
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* Start the countdown.
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*/
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public synchronized final CountDownTimer start() {
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if (mMillisInFuture <= 0 ) {
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onFinish();
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return this ;
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}
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mStopTimeInFuture = SystemClock.elapsedRealtime() + mMillisInFuture;
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mHandler.sendMessage(mHandler.obtainMessage(MSG));
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return this ;
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}
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/**
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* Callback fired on regular interval.
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* @param millisUntilFinished The amount of time until finished.
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*/
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public abstract void onTick( long millisUntilFinished);
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/**
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* Callback fired when the time is up.
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*/
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public abstract void onFinish();
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private static final int MSG = 1 ;
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// handles counting down
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private Handler mHandler = new Handler() {
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@Override
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public void handleMessage(Message msg) {
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synchronized (CountDownTimer. this ) {
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final long millisLeft = mStopTimeInFuture - SystemClock.elapsedRealtime();
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if (millisLeft <= 0 ) {
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onFinish();
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} else if (millisLeft < mCountdownInterval) {
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// no tick, just delay until done
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sendMessageDelayed(obtainMessage(MSG), millisLeft);
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} else {
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long lastTickStart = SystemClock.elapsedRealtime();
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onTick(millisLeft);
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// take into account user's onTick taking time to execute
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long delay = lastTickStart + mCountdownInterval - SystemClock.elapsedRealtime();
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// special case: user's onTick took more than interval to
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// complete, skip to next interval
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while (delay < 0 ) delay += mCountdownInterval;
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sendMessageDelayed(obtainMessage(MSG), delay);
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}
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}
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}
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};
}
here:http://www.chengyunfeng.com/2010/11/android-timer