Handler机制中最重要的元素包括：Thread，Looper，Handler，MessageQueue，Message五大元素。
Android中主线程和子线程通信，常会使用到Handler，而Handler内部是如何工作呢？
首先，执行在子线程中的代码是指运行于run方法中的代码，而不是我们继承Thread中所有的代码。
在子线程中使用Handler机制有2个重要的方法Looper.prepare()和Looper.loop()。我们需要在线程中先调用Looper.prepare()才能创建Handler对象，然后在线程最后调用Looper.loop()消息才能执行。
Looper.prepare()中是执行Looper的创建并且使用ThreadLocal保存，限制一个线程中只能存在一个Looper对象。
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));
}
一旦在线程中重复创建Looper对象的话就会抛出异常。而在Looper的构造方法中
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
首先创建了一个MessageQueue对象，再讲线程和Looper挂上钩。
创建好Looper对象之后，才能创建Handler，那这是为什么呢？在Handler的构造方法中：
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;
当Looper对象为空直接抛异常，然后在将Hanler拿到MessageQueue的引用。
然后Looper.loop才能执行读取并且发送消息：
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.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loop中会先拿到线程中对应的Looper对象，然后获取MessageQueue并且进入无线循环读取Message并且执行Message的任务。
到此Looper对象的职责就清晰明了了，首先和线程一一对应，保证了线程中只有一个MessageQueue，以及进入一个无限循环读取执行消息。
Handler在创建的时候拿到线程中的Looper，而后获取MessageQueue对象，而在sendMessage以及postDelayed都会执行到enqueueMessage中向MessageQueue中添加消息。
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
在loop方法中，Message是通过target执行dispatchMessage方法处理消息。
msg.target.dispatchMessage(msg);
而target方法是Handler对象，在构建消息的时候，Message会在创建的时候传入target。
public static Message obtain(Handler h, Runnable callback) {
Message m = obtain();
m.target = h;
m.callback = callback;
return m;
}
当然这个传入的Handler没什么用，因为最终message的target在添加到对应的MessageQueue前都会将target指向发送Message的Handler。
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
最终回到消息处理回到Handler的dispatchMessage中。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
Handler的职责就是将Message添加到MessageQueue并且最终处理Message。
MessageQueue的职责是保存Message，并且在对应的时间给Thread处理Message，而在MessageQueue中没有即时消息，就会使线程进入阻塞。
Message就是一个消息体。
