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() {
@Override
public 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 libcore
EventLogger.setReporter(new EventLoggingReporter());
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();//创建Looper
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(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Looper
if (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 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
final 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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android
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