Android 关于倒计时功能的说说

关于倒计时的实现，可以说有很多的方法，比较常见的就是Timer+TimerTask+Handler了，或者还可以配合Runnable。例如下面的代码：

/* * Copyright (C) 2008 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * *      http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */package android.os;import android.util.Log;/** * Schedule a countdown until a time in the future, with * regular notifications on intervals along the way. * * Example of showing a 30 second countdown in a text field: * * <pre class="prettyprint"> * new CountDownTimer(30000, 1000) { * *     public void onTick(long millisUntilFinished) { *         mTextField.setText("seconds remaining: " + millisUntilFinished / 1000); *     } * *     public void onFinish() { *         mTextField.setText("done!"); *     } *  }.start(); * </pre> * * The calls to {@link #onTick(long)} are synchronized to this object so that * one call to {@link #onTick(long)} won't ever occur before the previous * callback is complete.  This is only relevant when the implementation of * {@link #onTick(long)} takes an amount of time to execute that is significant * compared to the countdown interval. */public abstract 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;    /**     * @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 final void cancel() {        mHandler.removeMessages(MSG);    }    /**     * Start the countdown.     */    public synchronized final CountDownTimer start() {        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) {                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);                }            }        }    };}

所以，如果你的程序需要执行一些周期性的任务，就可以考虑使用CountDownTimer这个类了。需要注意的是，在上面的这个例子中，最后显示时间是1，也就是说其实上执行了29次。所以这个地方一定要注意，如果你的任务次数是n，那么设置的时候一定要注意设置成n+1的时间。

最后，欢迎大家评论交流，谢谢。