Android的消息机制

一、简介

　　Android的消息机制主要是指Handler的运行机制，那么什么是Handler的运行机制那？通俗的来讲就是，使用Handler将子线程的Message放入主线程的Messagequeue中，在主线程使用。

二、学习内容

　　学习Android的消息机制，我们需要先了解如下内容。

1. 消息的表示：Message
2. 消息队列：MessageQueue
3. 消息循环，用于循环取出消息进行处理：Looper
4. 消息处理，消息循环从消息队列中取出消息后要对消息进行处理：Handler

　　平常我们接触的大多是Handler和Message，今天就让我们来深入的了解一下他们。

三、代码详解

　　一般而言我们都是这样使用Handler的

　　

 xxHandler.sendEmptyMessage(xxx);

　　当然还有其他表示方法，但我们深入到源代码中，会发现，他们最终都调用了一个方法

 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);
    }

　　sendMessageAtTime()方法，但这依然不是结束，我们可以看到最后一句enqueueMessage(queue, msg, uptimeMillis);按字面意思来说插入一条消息，那么疑问来了，消息插入了哪里。

　　

  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;
    }

　　进入源代码，我们发现，我们需要了解一个新类Messagequeue。

　　虽然我们一般把他叫做消息队列，但是通过研究，我们发下，它实际上是一种单链表的数据结构，而我们对它的操作主要是插入和读取。

　　看代码33-44，学过数据结构，我们可以轻松的看出，这是一个单链表的插入末尾的操作。

　　这样就明白了，我们send方法实质就是向Messagequeue中插入这么一条消息，那么另一个问题随之而来，我们该如何处理这条消息。

　　处理消息我们离不开一个重要的，Looper。那么它在消息机制中又有什么样的作用那？

　　Looper扮演着消息循环的角色，具体而言它会不停的从MessageQueue中查看是否有新消息如果有新消息就会立刻处理，否则就已知阻塞在那里，现在让我们来看一下他的代码实现。

　　首先是构造方法

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

      可以发现，它将当前线程对象保存了起来。我们继续

　　Looper在新线程创建过程中有两个重要的方法looper.prepare() looper.loop

　　

new Thread(){
    public void run(){
        Looper.prepare();
        Handler handler = new Handler()；
        Looper.loop();
    }
}.start();
        

我们先来看prepare()方法

private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper(quitAllowed));
    }

咦，我们可以看到这里面又有一个ThreadLocal类，我们在这简单了解一下，他的特性，set()，get()方法。

    首先ThreadLocal是一个线程内部的数据存储类，通过它可以在指定的线程中存储数据，数据存储后，只有在制定线程中可以获取存储的数据，对于其他线程而言则无法获取到数据。简单的来说。套用一个列子：

private ThreadLocal<Boolean> mBooleanThreadLocal = new                ThreadLocal<Boolean>();//


mBooleanThreadLocal.set(true);
Log.d(TAH,"Threadmain"+mBooleanThreadLocal.get());
new Thread("Thread#1"){
    public void run(){
        mBooleanThreadLocal.set(false);
        Log.d(TAH,"Thread#1"+mBooleanThreadLocal.get());
    }；      
}.start();

new Thread("Thread#2"){
    public void run(){
        Log.d(TAH,"Thread#2"+mBooleanThreadLocal.get());
    }；      
}.start();

上面的代码运行后，我们会发现，每一个线程的值都是不同的，即使他们访问的是同意个ThreadLocal对象。

那么我们接下来会在之后分析源码，为什么他会不一样。现在我们跳回prepare()方法那一步，loop()方法源码贴上

public static void loop() {
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        final MessageQueue queue = me.mQueue;

        // Make sure the identity of this thread is that of the local process,
        // and keep track of what that identity token actually is.
        Binder.clearCallingIdentity();
        final long ident = Binder.clearCallingIdentity();

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

            // This must be in a local variable, in case a UI event sets the logger
            Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            msg.target.dispatchMessage(msg);

            if (logging != null) {
                logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
            }

            // Make sure that during the course of dispatching the
            // identity of the thread wasn't corrupted.
            final long newIdent = Binder.clearCallingIdentity();
            if (ident != newIdent) {
                Log.wtf(TAG, "Thread identity changed from 0x"
                        + Long.toHexString(ident) + " to 0x"
                        + Long.toHexString(newIdent) + " while dispatching to "
                        + msg.target.getClass().getName() + " "
                        + msg.callback + " what=" + msg.what);
            }

            msg.recycleUnchecked();
        }
    }

首先loop()方法，获得这个线程的Looper，若没有抛出异常。再获得新建的Messagequeue，在这里我们有必要补充一下Messagequeue的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.
        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;
        }
    }

从24-30我们可以看到，他遍历了整个queue找到msg，若是msg为null，我们可以看到50，他把nextPollTimeoutMillis = -1;实际上是等待enqueueMessage的nativeWake来唤醒。较深的源码涉及了native层代码，有兴趣可以研究一下。简单来说next()方法，在有消息是会返回这条消息，若没有，则阻塞在这里。

我们回到loop()方法27msg.target.dispatchMessage(msg);我们看代码

 public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

msg.target实际上就是发送这条消息的Handler，我们可以看到它将msg交给dispatchMessage(),最后调用了我们熟悉的方法handleMessage(msg);

 

三、总结

　　到目前为止，我们了解了android的消息机制流程，但它实际上还涉及了深层的native层方法，这里有一篇博客专门讲解这个转载一下

http://www.cnblogs.com/angeldevil/p/3340644.html。