本文主要是介绍深入理解Looper，MessageQueue和Handler，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
这几乎是每次面试必问的问题了吧，然而并没有什么高深，之所以每次面试问，是由于Handler在平时工作中用的比较多，记得当年去vivo面试，问了两个技术点，这是其中一个，先从整体说一下，每个线程只能有一个Looper，Looper里面会创建一个消息队列MessageQueue，Handler在发消息的时候找到当前Looper的MessageQueue，然后把消息传进去，Looper里面有个无限循环，不断从MessageQueue里读消息，并交还给发消息的Handler进行处理。。。有点绕？看源码，我们一句一句来解释
一 每个线程只能有一个Looper，Looper里面会创建一个消息队列MessageQueue
private Looper(boolean quitAllowed) {mQueue = new MessageQueue(quitAllowed);mThread = Thread.currentThread();}构造函数创建了一个MessageQueue，没什么好说的，接下来是大家平时用的最多的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));}看到这里，quitAllowed是什么呢，找到MessageQueue的构造函数
MessageQueue(boolean quitAllowed) {mQuitAllowed = quitAllowed;mPtr = nativeInit();}void quit(boolean safe) {if (!mQuitAllowed) {throw new IllegalStateException("Main thread not allowed to quit.");}看到这里明白了，主线程的消息队列是不允许退出，由于主线程的Looper创建不需要我们管，我们一般创建的Looper默认这个值就是true，回过头来看prepare，里面new了一个Looper存到sThreadLocal中，并且第二次进入，sThreadLocal.get()不为空就会报异常，每个线程只能创建一个Looper对象，这里应该清晰了吧，接下来看Handler：
二 Handler在发消息的时候找到当前Looper的MessageQueue，然后把消息传进去
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;}Handler通过Looper.myLooper()获得当前线程的Looper对象
public static Looper myLooper() {return sThreadLocal.get();}然后再通过mLooper.mQueue获取到Looper里创建的MessageQueue，好，接下来看我们常用的sendMessage方法：
public final boolean sendMessage(Message msg){return sendMessageDelayed(msg, 0);}里面其实是调用了sendMessageDelayed方法
public final boolean sendMessageDelayed(Message msg, long delayMillis){if (delayMillis < 0) {delayMillis = 0;}return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);}再往里调用了sendMessageAtTime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);}最终调用enqueueMessage把消息放到消息队列中
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {msg.target = this;if (mAsynchronous) {msg.setAsynchronous(true);}return queue.enqueueMessage(msg, uptimeMillis);}注意msg.target = this;这句话，把当前handler对象赋给msg.target，这个待会会用到
三 Looper里面有个无限循环，不断从MessageQueue里读消息，并交还给发消息的Handler进行处理。。。
看Looper里的loop方法：
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();}}final Looper me = myLooper(); 获取当前Looper，final MessageQueue queue = me.mQueue;获取到当前MessageQueue，接下来就是个无限循环for (;;) {Message msg = queue.next();不断的从消息队列中取出消息，然后调用msg.target.dispatchMessage(msg);处理，前面说了msg.target里存的就是发消息的Handler，这样的话直接调用了Handler的dispatchMessage方法:
public void dispatchMessage(Message msg) {if (msg.callback != null) {handleCallback(msg);} else {if (mCallback != null) {if (mCallback.handleMessage(msg)) {return;}}handleMessage(msg);}}最终就会回调到我们重写的handlerMessage处理消息了。
总结一下，还是那三句话
1 每个线程只能有一个Looper，Looper里面会创建一个消息队列MessageQueue
2 Handler在发消息的时候找到当前Looper的MessageQueue，然后把消息传进去
3 Looper里面有个无限循环，不断从MessageQueue里读消息，并交还给发消息的Handler进行处理。。。
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