android addIdleHandler 空闲线程解析

转载：android addIdleHandler 空闲线程源码分析

描述
在项目中偶然看到其他人有在用

Looper.myQueue().addIdleHandler(new IdleHandler() {    @Override    public boolean queueIdle() {        ...        return false;    }}

之前一直不知道这么写是怎么回事, 今天特意研究了一下源码, 搞清楚了IdleHandler的机制.

源码分析
我们先来看下IdleHandler的add跟remove方法

/** * Callback interface for discovering when a thread is going to block * waiting for more messages. */public static interface IdleHandler {    /**     * Called when the message queue has run out of messages and will now     * wait for more.  Return true to keep your idle handler active, false     * to have it removed.  This may be called if there are still messages     * pending in the queue, but they are all scheduled to be dispatched     * after the current time.     */    boolean queueIdle();}/** * Add a new {@link IdleHandler} to this message queue.  This may be * removed automatically for you by returning false from * {@link IdleHandler#queueIdle IdleHandler.queueIdle()} when it is * invoked, or explicitly removing it with {@link #removeIdleHandler}. * * This method is safe to call from any thread. * * @param handler The IdleHandler to be added. */public void addIdleHandler(@NonNull IdleHandler handler) {    if (handler == null) {        throw new NullPointerException("Can't add a null IdleHandler");    }    synchronized (this) {        mIdleHandlers.add(handler);    }}/** * Remove an {@link IdleHandler} from the queue that was previously added * with {@link #addIdleHandler}.  If the given object is not currently * in the idle list, nothing is done. * * This method is safe to call from any thread. * * @param handler The IdleHandler to be removed. */public void removeIdleHandler(@NonNull IdleHandler handler) {    synchronized (this) {        mIdleHandlers.remove(handler);    }}

IdleHandler接口表示当MessageQueue发现当前没有更多消息可以处理的时候, 则顺便干点别的事情的callback函数(即如果发现idle了,
那就找点别的事干). callback函数有个boolean的返回值, 表示是否keep. 如果返回false, 则它会在调用完毕之后从mIdleHandlers中移除.

接下来看MessageQueue的核心方法—next()方法, 源码如下, 相关说明写在注释中:

Message next() {// Return here if the message loop has already quit and been disposed.// This can happen if the application tries to restart a looper after// quit// which is not supported.final long ptr = mPtr;if (ptr == 0) {    return null;}int pendingIdleHandlerCount = -1; // -1 only during first iterationint nextPollTimeoutMillis = 0;for (;;) {    if (nextPollTimeoutMillis != 0) {        Binder.flushPendingCommands();    }    nativePollOnce(ptr, nextPollTimeoutMillis);    synchronized (this) {        // Try to retrieve the next message. Return if found.        final long now = SystemClock.uptimeMillis();        Message prevMsg = null;        Message msg = mMessages;        if (msg != null && msg.target == null) {            // Stalled by a barrier. Find the next asynchronous message            // in the queue.            // 这里执行的操作是忽略所有的同步消息, 知道找出queue中的异步消息            // 我理解是这个的同步消息会造成线程的阻塞, 所以忽略同步的消息            do {                prevMsg = msg;                msg = msg.next;            } while (msg != null && !msg.isAsynchronous());        }        // 走到这一步, 有两种可能,        // 一种是遍历到队尾没有发现异步消息,        // 另一种是找到queue中的第一个异步消息        if (msg != null) {            // 找到queue中的第一个异步消息            if (now < msg.when) {                // Next message is not ready. Set a timeout to wake up                // when it is ready.                // 没有到消息的执行时间                nextPollTimeoutMillis = (int) Math.min(msg.when - now,                        Integer.MAX_VALUE);            } else {                // Got a message.                // 当前消息到达可以执行的时间, 直接返回这个msg                mBlocked = false;                if (prevMsg != null) {                    prevMsg.next = msg.next;                } else {                    mMessages = msg.next;                }                msg.next = null;                if (DEBUG)                    Log.v(TAG, "Returning message: " + msg);                msg.markInUse();                return msg;            }        } else {            // 遍历到队尾, 没有发现异步消息或者没有消息了            // No more messages.            nextPollTimeoutMillis = -1;        }        // Process the quit message now that all pending messages have        // been handled.        // 检查当前的线程是否退出        if (mQuitting) {            dispose();            return null;        }        // If first time idle, then get the number of idlers to run.        // Idle handles only run if the queue is empty or if the first        // message        // in the queue (possibly a barrier) is due to be handled in the        // future.        // 如果queue中没有msg, 或者msg没到可执行的时间,        // 那么现在线程就处于空闲时间了, 可以执行IdleHandler了        if (pendingIdleHandlerCount < 0                && (mMessages == null || now < mMessages.when)) {            // pendingIdleHandlerCount在进入for循环之前是被初始化为-1的            // 并且没有更多地消息要进行处理            pendingIdleHandlerCount = mIdleHandlers.size();        }        if (pendingIdleHandlerCount <= 0) {            // No idle handlers to run. Loop and wait some more.            // 如果没有IdleHandler要进行处理, 则直接进入下次循环            mBlocked = true;            continue;        }        if (mPendingIdleHandlers == null) {            mPendingIdleHandlers = new IdleHandler[Math.max(                    pendingIdleHandlerCount, 4)];        }        mPendingIdleHandlers = mIdleHandlers                .toArray(mPendingIdleHandlers);    }    // Run the idle handlers.    // We only ever reach this code block during the first iteration.    // 退出同步块, 接下来就可以执行IdleHandler的相关操作了    for (int i = 0; i < pendingIdleHandlerCount; i++) {        final IdleHandler idler = mPendingIdleHandlers[i];        mPendingIdleHandlers[i] = null; // release the reference to the                                        // handler        boolean keep = false;        try {            keep = idler.queueIdle();        } catch (Throwable t) {            Log.wtf(TAG, "IdleHandler threw exception", t);        }        if (!keep) {            // 如果之前addIdleHandler中返回为false,            // 就在执行完这个IdleHandler的callback之后, 将这个idler移除掉            synchronized (this) {                mIdleHandlers.remove(idler);            }        }    }    // Reset the idle handler count to 0 so we do not run them again.    // 全部执行完, 重新设置这个值为0, 以便下次可以再次执行    pendingIdleHandlerCount = 0;    // While calling an idle handler, a new message could have been    // delivered    // so go back and look again for a pending message without waiting.    nextPollTimeoutMillis = 0;}}

引用自：android中IdleHandler的使用
IdleHandler 在API上面的解释如下：

public final void addIdleHandler (MessageQueue.IdleHandler handler)

向消息队列中添加一个新的MessageQueue.IdleHandler。当调用IdleHandler.queueIdle()返回false时，此MessageQueue.IdleHandler会自动的从消息队列中移除。或者调用removeIdleHandler(MessageQueue.IdleHandler)也可以从消息队列中移除MessageQueue.IdleHandler。

此方法是线程安全的。

  参数        handler              要添加的IdleHandler。

具体用法我是用在 android初始化activty界面时使用的。如果想用android做一个播放器，如果下面包括播放进度条，暂停、停止等按钮的控件用popWindow实现的话。就是在程序一起来然后下面的popWindow就要显示在activty上。用这个是比较好的，当然你可以用myHandler.sendEmptyMessage() 去你想要的操作。

把IdleHandler用在onCreate ( )里面，用法很简单如下：

Looper.myQueue().addIdleHandler(new IdleHandler()     {            @Override            public boolean queueIdle()             {                // TODO Auto-generated method stub                //你想做的任何事情                //........                //........                return false;              }     });

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