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MessageQueue，主要包含2個操作：插入和讀取。讀取操作會伴隨著刪除操作，插入和讀取對應的方法分別為enqueueMessage和next，其中enqueueMessage的作用是往消息隊列中插入一條消息，而next的作用是從消息隊列中取出一條消息並將其從消息隊列中移除。雖然MessageQueue叫消息隊列，但是它的內部實現並不是用的隊列，實際上它是通過一個單鏈表的數據結構來維護消息列表，單鏈表在插入和刪除上比較有優勢。

先看MessageQueue的定義：

/**
 * Low-level class holding the list of messages to be dispatched by a
 * {
 
@link Looper}.  Messages are not added directly to a MessageQueue,
 * but rather through {@link Handler} objects associated with the Looper.
 * 
 * <p>You can retrieve the MessageQueue for the current thread with
 * {@link Looper#myQueue() Looper.myQueue()}.
 */
public final class MessageQueue

通過源碼我們可以知道，MessageQueue維護了一個消息列表。Messgae並不是直接添加到MessageQueue中，而是通過和Looper相關聯的Handler來添加的。在當前線程中可以通過調用Looper.myQueue()方法來獲取當前線程的MessageQueue。

下面再看它的enqueueMessage插入方法：

    boolean enqueueMessage(Message msg, long when) {
        if (msg.target == null) {
            throw new IllegalArgumentException("Message must have a target.");
        }
        if (msg.isInUse()) {
            throw new IllegalStateException(msg + " This message is already in use.");
        }
 
        
 
synchronized (this) {
            if (mQuitting) {
                IllegalStateException e = new IllegalStateException(
                        msg.target + " sending message to a Handler on a dead thread");
                Log.w(TAG, e.getMessage(), e);
                msg.recycle();
                return false;
            }
 
            msg.markInUse();
            msg.when = when;
            Message p = mMessages;
            boolean needWake;
            if (p == null || when == 0 || when < p.when) {
                // New head, wake up the event queue if blocked.
                msg.next = p;
                mMessages = msg;
                needWake = mBlocked;
            } else {
                // Inserted within the middle of the queue.  Usually we don‘t have to wake
                // up the event queue unless there is a barrier at the head of the queue
                // and the message is the earliest asynchronous message in the queue.
                needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
                for (;;) {
                    prev = p;
                    p = p.next;
                    if (p == null || when < p.when) {
                        break;
                    }
                    if (needWake && p.isAsynchronous()) {
                        needWake = false;
                    }
                }
                msg.next = p; // invariant: p == prev.next
                prev.next = msg;
            }
 
            // We can assume mPtr != 0 because mQuitting is false.
            if (needWake) {
                nativeWake(mPtr);
            }
        }
        return true;
    }

在Message的源碼中定義了一個成員屬性target，其類型為Handler。由上面enqueuMessage的源碼，我們可以看到，當Message沒有處理其的Handler或該Message正在被處理的時候，都不能正常進入MessageQueue，這一點也是很容易理解的。當線程處於死亡狀態的時候，Message會被回收掉，而不再進入該線程對應的MessageQueue中。否則，一切正常，enqueMessage就執行單鏈表的插入操作，將Message插入到MessageQueue中。

再來看MessageQueue的next讀取操作：

    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 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.
                                  }
 
                // Process the quit message now that all pending messages have been handled.
                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.
                if (pendingIdleHandlerCount < 0
                        && (mMessages == null || now < mMessages.when)) {
                    pendingIdleHandlerCount = mIdleHandlers.size();
                }
                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)];
                }
                mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
            }
 
            // Run the idle handlers.
            // We only ever reach this code block during the first iteration.
            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) {
                    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;
        }
    }

通過源碼我們可以知道，next方法會不停地去循環讀取MessageQueue中的Message。若MessageQueue中沒有消息了，則next方法會暫時阻塞（ nextPollTimeoutMillis = -1）。有消息到來時，next會繼續讀取消息，返回該消息，並將其從單鏈表中移除。

【原創】源碼角度分析Android的消息機制系列（四）——MessageQueue的工作原理