IdleHandler类在android中的使用

介绍

在android的MessageQueue中有一个static的接口IdleHandler，这个接口用于在MessageQueue中没有可处理的Message的时候回调，这样就可以在UI线程中处理完所有的view事务之后，回调一些额外的操作而不会block UI线程。

    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();    }

使用

接口很简单，只有一个queueIdle方法，用于在进入Idle的时候执行额外的操作，返回值是ture的话，执行完queueIdle方法之后会保留这个IdleHandler，反之则删除这个IdleHandler。

使用起来也很简单。

mHandler.getLooper().getQueue().addIdleHandler(new MessageQueue.IdleHandler() {            @Override            public boolean queueIdle() {                Log.e(TAG, "----I am idle");                return false;            }        });

源码浅析

MessageQueue的Message都是在next方法中执行的，直接看next方法

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.                //-------------------第一部分---------------------------        final long ptr = mPtr;        if (ptr == 0) {            return null;        }        int pendingIdleHandlerCount = -1; // -1 only during first iteration        int 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.                    do {                        prevMsg = msg;                        msg = msg.next;                    } while (msg != null && !msg.isAsynchronous());                }                if (msg != null) {                    if (now < msg.when) {                        // Next message is not ready.  Set a timeout to wake up when it is ready.                        nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);                    } else {                        // Got a message.                        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 {                    // No more messages.                    nextPollTimeoutMillis = -1;                }                // Process the quit message now that all pending messages have been handled.                if (mQuitting) {                    dispose();                    return null;                }                //-----------第二部分-----------------------------------                // 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.                               //第一次进入，并且msg为null或者是当前的msg schedule到后面执行                if (pendingIdleHandlerCount < 0                        && (mMessages == null || now < mMessages.when)) {                    //获取当前IdleHandler的数量                    pendingIdleHandlerCount = mIdleHandlers.size();                }                 //没有需要处理的idleHandler，退出                if (pendingIdleHandlerCount <= 0) {                    // No idle handlers to run.  Loop and wait some more.                    mBlocked = true;                    continue;                }                if (mPendingIdleHandlers == null) {                    mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];                }                //用将所有的idleHandler存入mPendingIdleHandlers                mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);            }            // Run the idle handlers.            // We only ever reach this code block during the first iteration.            //迭代处理            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);                }                //根据返回值，选择是否remove这个idleHandler                if (!keep) {                    synchronized (this) {                        mIdleHandlers.remove(idler);                    }                }            }            // Reset the idle handler count to 0 so we do not run them again.            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;        }    }

这段代码可以分为两部分来看，第一部分用于从MessageQueue中取出可用的message，第二部分用于处理IdleHandler。我们直接从第二部分来看，首先会对当前的msg作一个判断，如果message为null或者是schedule到将来执行，那么就开始准备执行IdleHandler。所有的idleHandler放在mIdleHandlers中，将这些idleHandler转存到mPendingIdleHandlers,接下来就是迭代处理这些IdleHandler，如果返回值是false，就把这个idleHandler从mIdleHandlers中remove掉。

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