在Android中可以通过Handler来更新主线程中UI的变化，更新UI只能在主线程中进行更新，而为了让其他线程也能控制UI的变化，Android提供了一种机制Handler、Looper与MessageQueue一同协作来达到其他线程更新UI的目的。

一般我们会在主线程中通过如下方法定义一个Handler

private Handler mHandler = new Handler() {@Overridepublic void handleMessage(Message msg) {tv.setText("mHandler change UI");super.handleMessage(msg);}};

一般都见不到Looper与MessageQueue的，那么它们都是在哪里调用与如何协作的呢？在主线程不会显式的调用Looper而是会在ActivityThread.main方法中默认调用。

ActivityThread.main

public static void main(String[] args) {Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");SamplingProfilerIntegration.start();// CloseGuard defaults to true and can be quite spammy.  We// disable it here, but selectively enable it later (via// StrictMode) on debug builds, but using DropBox, not logs.CloseGuard.setEnabled(false);Environment.initForCurrentUser();// Set the reporter for event logging in libcoreEventLogger.setReporter(new EventLoggingReporter());// Make sure TrustedCertificateStore looks in the right place for CA certificatesfinal File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());TrustedCertificateStore.setDefaultUserDirectory(configDir);Process.setArgV0("<pre-initialized>");Looper.prepareMainLooper();//创建LooperActivityThread thread = new ActivityThread();thread.attach(false);if (sMainThreadHandler == null) {sMainThreadHandler = thread.getHandler();}if (false) {Looper.myLooper().setMessageLogging(newLogPrinter(Log.DEBUG, "ActivityThread"));}// End of event ActivityThreadMain.Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);Looper.loop();//开启Looper循环throw new RuntimeException("Main thread loop unexpectedly exited");}

如上代码，调用了Looper.prepareMainLooper()方法，在主线程中创建了一个Looper，不信的话我们再查看该方法做了什么

Looper

prepare

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));//创建Looper并赋给sThreadLocal}/*** Initialize the current thread as a looper, marking it as an* application's main looper. The main looper for your application* is created by the Android environment, so you should never need* to call this function yourself.  See also: {@link #prepare()}*/public static void prepareMainLooper() {prepare(false);synchronized (Looper.class) {if (sMainLooper != null) {throw new IllegalStateException("The main Looper has already been prepared.");}sMainLooper = myLooper();}}public static @Nullable Looper myLooper() {return sThreadLocal.get();}

在prepareMainLooper方法中调用了prepare而通过prepare会发现它其实就是创建了一个Looper，并把它赋给了sThreadLocal。同时可以通过myLooper方法获取当前线程中的Looper。再来看下new Looper(quitAllowed)初始化了什么

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

在这里我们终于看到了MessageQueue了，它创建了一个MessageQueue。该消息队列就是用来保存后续的Message。再回到ActivityThread.main方法中，发现它调用了Looper.loop()是用来开启Looper循环的，监听消息队列MessageQueue中的消息。

loop

我们来看下Looper.loop()的源码：

public static void loop() {final Looper me = myLooper();//获取Looperif (me == null) {throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");}final MessageQueue queue = me.mQueue;//获取消息队列// 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 loggerfinal Printer logging = me.mLogging;if (logging != null) {logging.println(">>>>> Dispatching to " + msg.target + " " +msg.callback + ": " + msg.what);}final long traceTag = me.mTraceTag;if (traceTag != 0) {Trace.traceBegin(traceTag, msg.target.getTraceName(msg));}try {msg.target.dispatchMessage(msg);//通过Handler分发消息} finally {if (traceTag != 0) {Trace.traceEnd(traceTag);}}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();}}

在loop中首先获取了当前所在线程的Looper，同时也获取到了Looper中的MessageQueue，说明Looper已经与当前的线程进行了绑定。在后面开启了一个for的死循环，发现它做的事件是不断的从消息队列中取出消息，最后都交给msg.target调用它的dispatchMessage方法，那么target又是什么呢？我们进入Message

Message

/*package*/ int flags;/*package*/ long when;/*package*/ Bundle data;/*package*/ Handler target;/*package*/ Runnable callback;// sometimes we store linked lists of these things/*package*/ Message next;

发现它就是我们熟悉的Handler，说明最后调用的就是Handler中的dispatchMessage方法，对消息的分发处理。这样一来Handler就通过Looper联系上了Looper所绑定的线程，即为主线程。

Handler

public Handler(Callback callback, boolean async) {if (FIND_POTENTIAL_LEAKS) {final Class<? extends Handler> klass = getClass();if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&(klass.getModifiers() & Modifier.STATIC) == 0) {Log.w(TAG, "The following Handler class should be static or leaks might occur: " +klass.getCanonicalName());}}mLooper = Looper.myLooper();if (mLooper == null) {throw new RuntimeException("Can't create handler inside thread that has not called Looper.prepare()");}mQueue = mLooper.mQueue;mCallback = callback;mAsynchronous = async;}

通过Handler的初始化，它获取了它所处线程的Looper，同时也获取了Looper中的消息队列。当然如果所处线程的Looper为空的话就会抛出异常，这就解释了为什么在非主线程中创建Handler要分别调用Looper.prepare与Looper.loop而主线程则不需要，因为它默认已经调用了。

dispatchMessage

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

回到前面，对于dispatchMessage的处理，首先判断msg.callback是否为空，这里callback通过上面的Message应该能知道他就是一个Runnable，如果不为空则直接调用Runnable的run方法。否则调用Handler的handleMessage方法.而这个方法相信大家已经很熟悉了，对事件的处理都是在这个方法中执行的。因为通过前面我们已经知道了Handler已经联系上了主线程，所以handleMessage中的处理自然相对于在主线程中进行，自然也能更新UI了。通过这里我们能把Looper比作是一个桥梁，来连接Looper所在的线程与Handler之间的通信，同时管理消息队列MessageQueue中的消息。那么前面的Runnable又是如何不为空的呢？我们使用Handler有两种方法，一种是直接创建一个Handler并且重写它的handleMessage方法，而另一种可以通过Handler.post(Runnable)来使用，这样事件的处理自然就在run方法中实现。

上面介绍了Handler是如何联系上了需要操作的线程与对消息是如何取出与处理的。下面来谈谈消息是如何放入到Looper中的MessageQueue中的。

sendMessageAtTime

通过Handler发送消息的方式很多，例如：sendMessage、sendEmptyMessage与sendMessageDelayed等，其实到最后他们调用的都是sendMessageAtTime方法。所以还是来看下sendMessageAtTime方法中的实现。

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);}

而sendMessageAtTime则就是调用了enqueueMessage操作，看这方法名就知道是入队列操作了。

enqueueMessage

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

果不其然直接调用了MessageQueue中的queue.enqueueMessage(msg, uptimeMillis)将消息加入消息队列，同时这段代码msg.target = this 将当前的Handler赋给了msg.target,这就是前面所说的Looper.loop方法中调用的Handler。这样就把消息放到了MessageQueue中，进而通过前面所讲的loop来取出消息进行相应的处理，这样就构成了整个对消息进行处理的系统。这也是使用Handler内部所发生的原理。好了Handler、Looper与MessageQueue它们之间的联系基本就是这些了。我也简单画了张图希望有所帮助

总结

来总结下它们之间的流程。首先创建Handler而在Handler所处的线程中必须要有一个Looper，如果在主线程中默认帮我们实现了，其他线程必须调用Looper.prepare来创建Looper同时调用Looper.loop开启对消息的处理。每个Looper中都有一个MessageQueue它是用来存储Message的，Handler通过post或者send..等一系列操作通过Looper将消息放入到消息队列中，而Looper通过开启一个无限的循环来一直监听着消息的处理，不断从MessageQueue中取出消息，并交给与当前Looper所绑定的handler的dispatchMessage进行分发，最后根据情况调用Runnable的run或者Handler的HandlerMessage方法对消息进行最后的处理。

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