概述
Android 的消息处理机制主要是指 Handler 的运行机制以及 Handler 所附带的 MessageQueue 和 Looper 的工作流程。
在 Handler 创建完毕之后，就可以通过 Handler.post 方法将一个 Runnable 转换成一个 Message 对象，它会调用 MessageQueue 的 enqueueMessage() 将其放入消息队列中：
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
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);
}
Looper 发现有新消息到来时，就会处理这个消息，最终消息中的 Runnable 或 Handler 的 handleMessage 方法会被调用。这样就切换到了创建 Handler 所在的线程中去执行了。
其工作流程图如下所示：
MessageQueue 的工作原理
MessageQueue 类主要包含两个操作：插入和读取，它通过一个单链表的数据结构来维护消息列表，在每个 Looper 中都持有一个 MessageQueue 对象。
enqueueMessage 方法主要就是往单链表中插入一条消息，我们来看一下 next 读取方法的代码：
Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
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;
}
}
尽管代码偏长，但可以看出 next 方法实际上是一个无限循环，直到 MessageQueue 中有消息则将其取出并删除，否则会一直阻塞在这里。
Looper 的工作原理
Looper 在消息处理机制中负责不断从 MessageQueue 中查看是否有新消息，如果有的话则进行处理，否则就一直阻塞在哪里。
在初始化 Looper 时会创建一个 MessageQueue：
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
当调用 prepare 方法时，可当前线程初始化并绑定一个 Looper，可以看到不能重复调用 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));
}
最后我们会调用 loop 方法来开启消息循环，这也是 Looper 中最重要的一个方法，我们先列出整体代码，再来依次进行分析：
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
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(msg);
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();
}
}
先取出与当前线程绑定的 Looper，并取出 Looper 中的 MessageQueue 消息队列，接着就进入了无限循环：
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;
for (;;) {
后面的代码都是在无限循环之内，这里取出消息，如果为 null 则退出循环：
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
注意当 MessageQueue 没有消息时，是不会返回 null 的，只会一直循环等待消息。只有当调用 quit 方法退出时，才会返回 null。
public void quit() {
mQueue.quit(false);
}
接下来会对消息进行处理：
msg.target.dispatchMessage(msg);
msg.target 就是发送该消息的 Handler，这里就会将线程切换到该 Handler 所在的线程去处理该 msg，这样就完成了线程的切换啦~
Handler 的工作原理
在概述中我们已经讲过了，Handler.post 方法会将消息插入到 Looper 中的消息队列，开启循环后又会将该消息转发到 Handler 所在线程进行处理，那么我们就来看一下 dispatchMessage 方法：
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
msg.callback 是 post 方法中传递进去的 Runnable 参数，如果不为空，则：
private static void handleCallback(Message message) {
message.callback.run();
}
如果为空，则判断 Handler 的 Callback 是否为空，这个 Callback 的定义如下：
public interface Callback {
public boolean handleMessage(Message msg);
}
我们可以用如下这种方式来创建一个 Handler：
public Handler(Callback callback) {
this(callback, false);
}
实际上是与我们重写 Handler.handleMessage 方法差不多的，只是另一种使用 Handler 的方式而已。
结语
那么以上就是，Android 的消息处理机制了，这一部分的代码比较简单易懂，所以推荐大家都去看看。
