Handler 系列二：如何通信

承接上一篇Handler系列一，上篇主要总结了Handler如何通信，这篇来介绍Handler怎么通信。

Handler的通信机制
Handler，Looper，MessageQueue如何关联

Handler 通信机制

创建Handler，并采用当前线程的Looper创建消息循环系统；
Handler通过sendMessage(Message)或Post(Runnable)发送消息，调用enqueueMessage把消息插入到消息链表中；
Looper循环检测消息队列中的消息，若有消息则取出该消息，并调用该消息持有的handler的dispatchMessage方法，回调到创建Handler线程中重写的handleMessage里执行。

Handler如何关联Looper、MessageQueue

Handler及其关联的类图

以上类图可以快速帮助我们理清Handler与Looper、MessageQueue的关系，以下从源码的角度慢慢分析：

1、Handler 发送消息

上一段很熟悉的代码：

 Message msg =Message.obtain();  //从全局池中返回一个message实例，避免多次创建message（如new Message）msg.obj = data;msg.what=1;   //标志消息的标志handler.sendMessage(msg);

从sendMessageQueue开始追踪，函数调用关系：sendMessage -> sendMessageDelayed ->sendMessageAtTime，在sendMessageAtTime中，携带者传来的message与Handler的mQueue一起通过enqueueMessage进入队列了。

对于postRunnable而言，通过post投递该runnable，调用getPostMessage，通过该runnable构造一个message，再通过 sendMessageDelayed投递，接下来和sendMessage的流程一样了。

2、消息入队列

在enqueueMessage中，通过MessageQueue入队列，并为该message的target赋值为当前的handler对象，记住msg.target很重要，之后Looper取出该消息时，还需要由msg.target.dispatchMessage回调到该handler中处理消息。

    private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {msg.target = this;if (mAsynchronous) {msg.setAsynchronous(true);}return queue.enqueueMessage(msg, uptimeMillis);}

在MessageQueue中,由Message的消息链表进行入队列

    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.nextprev.next = msg;}// We can assume mPtr != 0 because mQuitting is false.if (needWake) {nativeWake(mPtr);}}return true;}

3、Looper 处理消息

再说处理消息之前，先看Looper是如何构建与获取的：

构造Looper时，构建消息循环队列，并获取当前线程

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

但该函数是私有的，外界不能直接构造一个Looper，而是通过Looper.prepare来构造的：

      public static void prepare() {prepare(true);}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));}

这里创建Looper，并把Looper对象保存在sThreadLocal中，那sThreadLocal是什么呢？

static final ThreadLocal sThreadLocal = new ThreadLocal();

它是一个保存Looper的TheadLocal实例，而ThreadLocal是线程私有的数据存储类，可以来保存线程的Looper对象，这样Handler就可以通过ThreadLocal来保存于获取Looper对象了。

TheadLocal 如何保存与获取Looper？

      public void set(T value) {Thread currentThread = Thread.currentThread();Values values = values(currentThread);if (values == null) {values = initializeValues(currentThread);}values.put(this, value);}public T get() {// Optimized for the fast path.Thread currentThread = Thread.currentThread();Values values = values(currentThread);if (values != null) {Object[] table = values.table;int index = hash & values.mask;if (this.reference == table[index]) {return (T) table[index + 1];}} else {values = initializeValues(currentThread);}return (T) values.getAfterMiss(this);}

在 set 中都是通过 `values.put` 保存当前线程的 Looper 实例，通过 `values.getAfterMiss(this)`获取，其中`put`和`getAfterMiss`都有`key`和`value`，都是由Value对象的table数组保存的，那么在table数组里怎么存的呢？table[index] = key.reference;table[index + 1] = value;很显然在数组中，前一个保存着ThreadLocal对象引用的索引，后一个存储传入的Looper实例。

接下来看Looper在loop中如何处理消息

在loop中，一个循环，通过next取出MessageQueue中的消息

若取出的消息为null，则结束循环，返回。
- 设置消息为空，可以通过MessageQueue的quit和quitSafely方法通知消息队列退出。
若取出的消息不为空，则通过msg.target.dispatchMessage回调到handler中去。

    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 blockif (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 loggerPrinter 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();}}

4、handler处理消息

Looper把消息回调到handler的dispatchMessage中进行消息处理：

若该消息有callback，即通过Post(Runnable)的方式投递消息，因为在投递runnable时，把runnable对象赋值给了message的callback。
若handler的mCallback不为空，则交由通过callback创建handler方式去处理。
否则，由最常见创建handler对象的方式，在重写handlerMessage中处理。

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

总结

以一个时序图来总结handler的消息机制，包含上述如何关联Looper和MessageQueue的过程。

Handler-Looper-MessageQueue时序图