要理解原理， read the fucking source
主要涉及到的类：Thread，Handler，Looper，MessageQueue，Message
这几个类的关系图：
发送消息流程图：
1、从HandlerThread入手。
HandlerThread是android系统提供的类。继承Thread，是一个线程，调用start()方法后。将运行run()方法。
public void run() {
mTid = Process.myTid();
Looper.prepare();// #1
synchronized (this) {
mLooper = Looper.myLooper();// #2
notifyAll();
}
Process.setThreadPriority(mPriority);
onLooperPrepared();
Looper.loop();// #3
mTid = -1;
}
第1个标示中，是调用了Looper的prepare方法。
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));// #4
}
在代码段的第4个标示中。将Looper实例存放在线程局部变量ThreadLocal中，将Looper和当前线程绑定。
在代码段的第2个标示中，获取了与当前线程绑定的Looper实例，当前线程就拥有了Looper实例。
在代码段的第3个标示中，调用了Looper的loop()方法。该方法是一个堵塞方法，loop()方法代码：
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
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 ＃５
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
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(msg);＃６
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.recycle();
}
}
for(;;)是一个死循环，在代码段的标示５中，官方凝视queue.next()方法是一个堵塞方法，也就是会一直等待消息。
代码段的标示６中调用handler的dispatchMessage方法。发送消息，这里的handler是怎样和Message关联上的？这里做个埋笔。往下分析。
也许有人会问，Message取完之后。在哪里唤醒该线程，然后继续循环获取Message呢？没错，是在调用Handler的sendMessage后。向MessageQueue中插入消息的时候唤醒，调用本地方法nativeWake(mPtr)。
２、開始发送消息
将Handler和Looper关联的代码：
handlerThread.start(); mLogicHandler = new LogicHandler(handlerThread.getLooper());
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}Handler就拥有了Looper和MessageQueue了。
Handler中的发送消息方法：
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);
}enqueueMessage方法：private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;#7
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);#8
}
代码段中的标示７。将当前handler实例赋值给message的targer，这是上面埋点的解答。
代码段中的标示８。将会调用MessageQueue的。MessageQueue的enqueueMessage方法
final boolean enqueueMessage(Message msg, long when) {
if (msg.isInUse()) {
throw new AndroidRuntimeException(msg + " This message is already in use.");
}
if (msg.target == null) {
throw new AndroidRuntimeException("Message must have a target.");
}
boolean needWake;
synchronized (this) {
if (mQuiting) {
RuntimeException e = new RuntimeException(
msg.target + " sending message to a Handler on a dead thread");
Log.w("MessageQueue", e.getMessage(), e);
return false;
}
msg.when = when;
Message p = mMessages;
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// Inserted within the middle of the queue. Usually we don't have to wake
// up the event queue unless there is a barrier at the head of the queue
// and the message is the earliest asynchronous message in the queue.
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
}
if (needWake) {
nativeWake(mPtr);// #９
}
return true;
}
在代码片段标示5中。调用了nativeWake(mPtr)方法，该方法是本地方法，用于唤醒Thread线程。
參考资料：