Handler延迟消息执行机制，会阻塞吗？

Handler调用过程简单说明

Handler在Android中使用频繁，主要用来进行线程间通信，子线程通过Handler可以操作UI。有send和post两种方法，send方法是在工作线程中处理完耗时操作后调用handler的sendMessage（message）把message对象发送给主线程，在主线程中重写handlerMessage（）方法，判断接收到的消息进行更新UI的操作；而post方法传递的是一个runnable对象，runnable的run方法最终执行在主线程中。

无论用那种方式最终都是调用sendMessageAtTime：

public final boolean postDelayed(Runnable r, Object token, long delayMillis)
{
    return sendMessageDelayed(getPostMessage(r, token), delayMillis);
}

public final boolean post(Runnable r)
{
   return  sendMessageDelayed(getPostMessage(r), 0);
}

Post方法最终调用还是send方法

public final boolean sendMessage(Message msg)
{
    return sendMessageDelayed(msg, 0);
}

public final boolean sendEmptyMessage(int what)
{
    return sendEmptyMessageDelayed(what, 0);
}

public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
    Message msg = Message.obtain();
    msg.what = what;
    return sendMessageDelayed(msg, delayMillis);
}

public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
    if (delayMillis < 0) {
        delayMillis = 0;
    }
    return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}

public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
    MessageQueue queue = mQueue;
    if (queue == null) {
        RuntimeException e = new RuntimeException(
                this + " sendMessageAtTime() called with no mQueue");
        Log.w("Looper", e.getMessage(), e);
        return false;
    }
    return enqueueMessage(queue, msg, uptimeMillis);
}

public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
    if (delayMillis < 0) {
        delayMillis = 0;
    }
    return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}

SendMessageAtTime第二个参数是利用系统开机时间加上delayMisllis后计算得到的时间。

2 延迟消息如何处理

Looper采用轮询的工作方式，发送信息之后，message加入到MessageQueue中，然后Looper不断从MessageQueue中读取Message进行处理，延迟消息现实使用中没有多大误差，如何从MessageQueue中读取延迟Message进行处理。

Handler不处理消息，Message放入MessageQueue：

public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
    MessageQueue queue = mQueue;
    if (queue == null) {
        RuntimeException e = new RuntimeException(
                this + " sendMessageAtTime() called with no mQueue");
        Log.w("Looper", e.getMessage(), e);
        return false;
    }
    return enqueueMessage(queue, msg, uptimeMillis);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
    msg.target = this;
    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis);
}

Handler没有自己处理消息，最终放入了MessageQueue 设置了target，target用于标识来源于哪个Handler，最终取出消息后调用特定Handler。

MessageQueue中的处理：

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) {
            
            msg.next = p;
            mMessages = msg;
            needWake = mBlocked;
        } else {
         
            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;
        }

        //最终是否唤醒
        if (needWake) {
            nativeWake(mPtr);
        }
    }
    return true;
}

在后面next()方法内部，如果有阻塞（没有消息了或者只有Delay的消息），会把mBlocked这个变量标记为true（下面会说明next方法），在下一个Message进队时会判断这个message的位置，如果在队首并且时间满足条件，会调用nativeWake()方法唤醒线程！

消息如何读取：
Looper内循环读取Message，looper的loop方法：

for (;;) {
    Message msg = queue.next(); // might block
    if (msg == null) {
        // No message indicates that the message queue is quitting.
        return;
    }

Queue.next方法可能被阻塞，所以最终调用还是queue的next方法。

Message next() {
        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;
            }

            // 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;
    }
}

取出消息，如果Message不满足执行条件，时间没到now < msg.when，计算下一个延迟 nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE)，继续阻塞;在循环开始的时候判断如果这个Message有延迟，就调用nativePollOnce(ptr, nextPollTimeoutMillis)进行阻塞，如果时间到了直接分发消息进行调用。

由于执行delay消息会阻塞，那么如果发送多个delay消息，是不是前一个消息阻塞执行完了，才会执行后一个，现实使用中肯定不是这样的。上面已经说了在enqueueMessage方法中，加入新的消息时会首先判断需不需要唤醒线程，这样线程就不会一直阻塞（加入新的消息，唤醒线程，就不需要等待上一个阻塞的消息）。

delay消息会一直阻塞线程，直到延迟走完，或者下一个消息到来。