关于线程之间发送消息，有很多种方法，如RunonUiThread，handler的post方法，AsyncTask ，view的post方法等等。大部分场景，都是可以通过handler传递一个message来实现的，现在我们具体看一下它是如何实现的。
这方面，其实涉及到Looper，Handler，MessageQueue.
我们如果在主线程去使用一个handler的话，只需要简单的创建一个handler对象就可以了，并且做好防止内存泄漏的准备。
//这么写是没有注意内存泄漏的哦 只是为了方便这么写
private Handler handler = new Handler(Looper.myLooper()){
@Override
public void dispatchMessage(Message msg) {
// todo
}
};之后，只需要在你想要发送信息的地方，调用handler的方法就可以了，post或者sendMessage都可以，本质上都是发送一个message，另外它自身维持了一个message池，所以用Message.obtain()获取一个message比new一个message来的更为合适一点。
//所谓的post了一个runnable过去，源码中也是从message池中拿到一个message，让这个runnable对象跟他的callback绑定而已，
//所以本质上跟sendMessage没两样，不过优先级还是这个callback更加高一点哦。
mHandler.postDelayed(new Runnable() {
@Override
public void run() {
//
}
}, 0);在主线程中创建handler是如此的简单，是因为ActivityThread中已经默认为我们调用了Looper的相关方法：
public static void main(String[] args) {
//去除无关代码
Looper.prepareMainLooper();//在这里 调用了prepare方法
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
Looper.loop(); //在这里 调用了loop方法 ，让messagequene开始无限循环
throw new RuntimeException("Main thread loop unexpectedly exited");
}抽取关键代码，即源码中为我们展示的demo：
* class LooperThread extends Thread {
* public Handler mHandler;
*
* public void run() {
* Looper.prepare();
*
* mHandler = new Handler() {
* public void handleMessage(Message msg) {
* // process incoming messages here
* }
* };
*
* Looper.loop();
* }查看源代码，看看他们究竟做了什么？
public static void prepare() {
prepare(true);
}
//ThreadLocal可以先简单的认为是一种工具类，我们发现，prepare每个线程只能调用一次，再次调用直接跑出异常
//如果第一次调用，那么将创建一个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));
}而Looper的构造方法中 会创建一个messageQuene。
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}这就是prepare方法的所有作用，在当前线程中创建一个与之绑定的Looper，Looper中创建一个messagequeue，也与之绑定。再看handler的构造方法：
public Handler(Callback callback, boolean async) {
//他会做判断，判断当前的looper是否存在，不存在直接抛出异常，
//所以在子线程创建handler的话，必须调用prepare方法，因为他不会默认为我们调用这个方法。
//如果looper已经被创建，他会将looper和messagequeue与自身绑定。
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;
}接着看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 block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
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);
}
}
// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
final long newIdent = Binder.clearCallingIdentity();
msg.recycleUnchecked();
}
}说重点，就是开始对messagequeue做无限循环，从队列当中拿出message，这个queue.next涉及到native的一些方法，并不是简单的直接拿下一个message，他必定能拿到一个next，它会判断message里面when属性去判断是否需要阻塞一下，当拿出来后，通过它的target，也就是handler，调用他的dispatchMessage方法，这个方法源代码中是空实现，所以一般需要我们去实现。那么如果队列当中不存在message了，那么他将会阻塞，直到有消息被放到队列中，再次开启以下循环。我们来看一下next方法：
Message next() {
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
// Next message is not ready. Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
// If first time idle, then get the number of idlers to run.
// Idle handles only run if the queue is empty or if the first message
// in the queue (possibly a barrier) is due to be handled in the future.
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null; // release the reference to the handler
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
pendingIdleHandlerCount = 0;
// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
nextPollTimeoutMillis = 0;
}
}nativePollOnce是个native方法，他根据传入的nextPollTimeoutMillis ，去判断要阻塞的时间，第一次传入0，所以一定不会阻塞，之后通过message的next方法，去拿到最远端的一个next，然后根据他的msg.when去判断需要阻塞的时间，在当前阻塞等待的时候，还需要去判断IdleHandler是否为空，不为空的话处理一下。当然，如果nextPollTimeoutMillis 为-1的情况下表示一直睡眠，直到有人唤醒。
最后我们看一下post方法，不管是post还是postdelay还是sendmessage什么的，最后都是调用sendMessageAtTime：
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);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}重点就在enqueueMessage方法中，它将msg放入到队列中
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
synchronized (this) {
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
msg.recycle();
return false;
}
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
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;
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}mMessages其实代表消息队列的头部，如果mMessages为空，说明还没有消息，如果当前插入的消息不需要延时，或者说延时比mMessages头消息的延时要小，那么当前要插入的消息就需要放在头部，如果不是，则开始循环判断当前的这个msg要插入的位置，插入进去，最后判断是否需要唤醒Loop线程，通过nativeWake唤醒。
总结：创建Looper，Looper当中创建MessageQueue，接着可以创建Handler对象，再调用loop方法让队列开始无限循环。之后通过handler的方法，将需要发送的msg放置到messagequeue当中，它会根据他的target属性找到对应的handler，执行dispatchMessage方法，他会判断是调用msg的callback方法还是执行handlermessage方法，如此消费一个msg。
