MessageQueue分析

/**
 * 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 {
    //中间先省略
}

MessageQueue(boolean quitAllowed) {
    mQuitAllowed = quitAllowed;
    mPtr = nativeInit();
}

private Looper(boolean quitAllowed) {
    mQueue = new MessageQueue(quitAllowed);
    mThread = Thread.currentThread();
}

boolean enqueueMessage(Message msg, long when) {
        synchronized (this) {
            msg.markInUse();
            msg.when = when;
            Message p = mMessages;
            boolean needWake;
            if (p == null || when == 0 || when < p.when) {
                //注释1
                // New head, wake up the event queue if blocked.
                msg.next = p;
                mMessages = msg;
                needWake = mBlocked;
            } else {
                //注释2
                // 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.
                // 把消息插入到队列之中，但是一般不会唤醒这个事件队列，除非队列前面有一个barrier(屏障)
                // 和这个消息是队列中最早的异步消息
                // p.target == null是一个barrier，判断是否为异步消息msg.isAsynchronous()
                // mBlocked == true目前是阻塞中，需要同时满足此三个条件才去唤醒
                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;
                    }
                }
                //就是一个插入队列的操作
                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;
    }

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;
    	//判断native层的MessageQueue是否存在，注释3
        if (ptr == 0) {
            return null;
        }
        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        int nextPollTimeoutMillis = 0;
        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();
            }
            //注释4
            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;
                //注释5
                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());
                }
                 //注释6
                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. 表示当前没有阻塞
                        mBlocked = false;
                        //如果prevMsg不为空，表示有消息屏障，
                        if (prevMsg != null) {
                            //异步消息要取出，让prevMsg的next指向msg的next
                            prevMsg.next = msg.next;
                        } else {
                            //prevMsg为空，表示没有消息屏障，更新mMessages指向mgs的next
                            mMessages = msg.next;
                        }
                        //取出消息，并断开在链表中的指向
                        msg.next = null;
                        //把这个消息标记为使用状态
                        msg.markInUse();
                        //返回消息到Looper处理
                        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;
                }
            }
            // 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;
        }
    }