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实现原理

拿CountDownTimer的源代码看一下，并不复杂，基本上是对Handler的封装，使用send/post delay。这套机制仍然首先于Handler的原理，所以在精度上也不能够保证很精确，只能保证不会早于预期执行。详见我另外一篇介绍Handler send/post delay的文章： Handler sendMessageDelayed()/postDelayed()机制详解。

源代码：

/*
 * 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;

/**
 * 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;
    
    /**
    * 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);
                }
            }
        }
    };
}

看其中两个关键点：

（1）Handler的创建

    // handles counting down
    private Handler mHandler = new Handler() {...};

作为一个私有的非静态成员变量，这段代码执行在构造方法中，也就是说，Handler是在调用构造方法（new）的线程中被创建，并且是通过不带参数的构造方法Handler()。从Handler的源代码可以看到，这种情况会调用到：

    /**
     * Use the {@link Looper} for the current thread with the specified callback interface
     * and set whether the handler should be asynchronous.
     *
     * Handlers are synchronous by default unless this constructor is used to make
     * one that is strictly asynchronous.
     *
     * Asynchronous messages represent interrupts or events that do not require global ordering
     * with respect to synchronous messages.  Asynchronous messages are not subject to
     * the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
     *
     * @param callback The callback interface in which to handle messages, or null.
     * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
     * each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
     *
     * @hide
     */
    public Handler(Callback callback, boolean async) {
        if (FIND_POTENTIAL_LEAKS) {
            final Class<? extends Handler> klass = getClass();
            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                    (klass.getModifiers() & Modifier.STATIC) == 0) {
                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                    klass.getCanonicalName());
            }
        }

        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

通过Looper.myLooper()得到Looper对象并且校验非空，空则抛出RE。Looper.myLooper():

    /**
     * Return the Looper object associated with the current thread.  Returns
     * null if the calling thread is not associated with a Looper.
     */
    public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }

由此可见，普通工作线程因为没有准备好Looper，所以显然是不能直接创建CountDownTimer对象的。

（2）Handler的消息处理/分发

    // 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);
                }
            }
        }
    };

handleMessage()方法：

首先做剩余时间计算：

final long millisLeft = mStopTimeInFuture – SystemClock.elapsedRealtime();

mStopTimeInFuture是在倒计时开始方法start()中计算得到：

mStopTimeInFuture = SystemClock.elapsedRealtime() + mMillisInFuture;

mMillisInFuture是CountDownTimer构造方法中传入的参数，即倒计时的时长。

所以millisLeft计算出的是当前handleMessage()被执行时候，距离倒计时结束的剩余时间。

接下来的逻辑：

（1）如果发现剩余时间<=0，意味着倒计时已经结束，直接调用onFinish()。这里注意到，可能是会超过最后时限（即<0的情况）才执行onFinish()。原因也可参见我另外一篇 Handler sendMessageDelayed()/postDelayed()机制详解。

（2）否则，如果剩余时间不足一次onTick间隔，直接以剩余时间sendMessageDelayed()。

（3）否则（这条逻辑分支是最关键的逻辑，实现onTick的循环调用）：

a)首先记下一个当前的时间戳，即本次执行onTick开始的时间；

b)执行onTick()，注意，这里是同步执行；

c)执行完onTick()以后，再看一下当前的时间情况

// take into account user’s onTick taking time to execute

long delay = lastTickStart + mCountdownInterval – SystemClock.elapsedRealtime();

因为不能够确认同步执行的onTick()会耗时多少，所以对当前时间需要校验：

// special case: user’s onTick took more than interval to
// complete, skip to next interval

while (delay < 0) delay += mCountdownInterval;

以onTick间隔作为时间单元向下跳，直到delay为非负值，也就是说还没有流失掉的onTick执行时间距离，并以这个值sendMessageDelayed()下一条Msg。

总结一下，可以看出：

（1）每次handleMessage的时候，实际上是以当前时间值为基础，计算一下剩余的时间并决定接下来的处理逻辑，这样做的目的显然是在现行Android Handler机制里面，无法保证以绝对准时的delay时间处理Msg。

（2）对于同步onTick的处理很严谨，考虑到了onTick可能的耗时操作。