自我理解Looper、Handler、Message的关系，一般面试的必答题，涉及到Android的异步消息处理机制。
- 简单的来说，我们一般用Handler实现异步消息处理。为什么会用到Handler？
子线程不允许访问 UI（在多线程情况下，UI控件会处于不可预期的状态）
Handler采用单线程模型，切换到UI线程去操作UI，解决以上问题。所以要明白Handler的消息机制。 - Looper：在主线程启动中（ActivityThread）中会调用
Looper.prepareMainLooper();方法：
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>");
//这里直接掉用Looper类的静态方法
Looper.prepareMainLooper();
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);
//调用Looper类静态方法
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}先看Looper.prepareMainLooper()接着往里面看有：
public static void prepareMainLooper() {
prepare(false);
//加锁保证每个线程初始化的唯一性
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}主要看下prepare(false);方法里面：
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));
}方法中new Looper（boolean） 执行Looper的构造方法，创建了Looper。
sThreadLocal是一个ThreadLocal对象，可以在一个线程中存储变量。可以看到，将一个Looper的实例放入了ThreadLocal，并且在之前判断了sThreadLocal是否为null，否则抛出异常。这也就说明了Looper.prepare()方法不能被调用两次，同时也保证了一个线程中只有一个Looper实例。
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}这里创建了MessageQueue的实例。
回看ActivityThread中还调用Looper.loop（）方法，众所周知的Looper类中的两个方法在ActivityThread中间接调用（不单单只有ActivityThread才能调用），现在看看Looper.loop方法做了什么：
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
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.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
msg.target.dispatchMessage(msg);
} 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();
}
}首先看下第二行final Looper me = myLooper();myLooper做了什么。
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}还记得前面初始化的时候prepare()中将一个Looper实例放入到ThreadLocal中，这里是取出Looper。
再看第四行final MessageQueue queue = me.mQueue; 初始化中调用构造函数中，创建了MessageQueue的实例，这里赋值给局部变量 queue。
再往下有一个for（；；）的死循环，有什么作用了？
循环体内第一行，调用了message.next方法，取出消息队列中的消息，为空没有消息就阻塞。
有消息的时候就执行msg.target.dispatchMessage(msg); 把消息交给msg.target去执行dispatchMessage方法处理消息，而msg.target就是Handle了。查看Message源码中Handler target; target就是指的Handler，我们经常使用handler的时候，回调用handler.sendMessager方法，最后深入会走Handler类如下代码：
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;//handler = msg.target
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}当我们使用Handler时候，Handler mHandler = new Handler() {……}看看Handler的构造方法：
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;
}其中调用了比较熟悉的mLooper = Looper.myLooper(); 获取存储的Looper实例赋值。mQueue = mLooper.mQueue; 获取存储的MessageQueue实例赋值。比如使用Handler方法：
private Handler mHandler = new Handler()
{
public void handleMessage(android.os.Message msg)
{
switch (msg.what)
{
case value:
break;
default:
break;
}
};
}; 看到这里是不是发现Looper、Handler、Message已经绑定在一起了。
总结如下：
Looper：是一个消息分发器，在主线程创建的时候就会创建一个Looper对象；
MessageQueue ：消息队列，是由Message组成的一个队列；
Handler ：获取到Message，然后执行动作，可以在主线程和子线程中互相传递数据；
用一张很常见的图总结：
