安卓消息机制中的消息队列MessageQueue的插入轮询算法

1.MessageQueue的插入算法

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("MessageQueue", 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;
    } 

(1)当第一个消息到来时,此消息就是消息队列中的头消息。 mMessages  = msg;  mMessages  相当于头指针

(2)此后来的消息（假如消息的 when == 0或者小于头结点的when值 ）都会作为头消息插入(msg1表示的是上一个消息，本次消息是msg)

    i执行 Message p =  mMessages ;后

   ii执行 msg.next = p;后

 

  iii执行 mMessages  = msg;后

可以看到 mMessages 移动到了头部，新来的消息是从头部开始插入的，最后形成了一个单链表，方法执行完毕后p会被垃圾回收器回收

(3)若后面来的消息 when不等于0，或者when比头结点的 when大，即不满足 when == 0 || when < p.when。那么

走这段程序

for (;;) {
                    prev = p;
                    p = p.next;
                    if (p == null || when < p.when) {
                        break;
                    }
                    if (needWake && p.isAsynchronous()) {
                        needWake = false;
                    }
                }

进入无限循环,想跳出循环只能执行break,执行 break的条件是 p ==  null  || when < p.when,即到了MessageQueue的最后一个节点(前面节点的when值都比要插入的when值小，否则满足 when < p.when会直接break掉 )或者找到某个节点的when值比要插入message的when值大。跳出循环后执行:

                  msg.next = p;  // invariant: p == prev.next

               prev.next = msg;

将msg插入pre和p节点之间

所以消息队列是基于一个过期时间先后顺序排序的，需要尽快处理的消息排在前面。

1.MessageQueue的轮询算法

在 MessageQueue的 next()方法中,有个无限循环，一直取消息

。。。。。。
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 (false) Log.v("MessageQueue", "Returning message: " + msg);
                        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;
                }
                // 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);
            }  
。。。。。。

一般 msg.target !=  null, 所以取消息一般走的是下面的代码:

      mMessages  = msg.next;

       msg.next =  null ;

即取出头结点。因为消息队列是按照消息需要处理的时间进行排序的，所以取消息也就肯定会从消息队列的头开始。