平时用Handler比较多，对他的内部实现却不是很了解，只知道Handler用于线程间通讯。最后还是花了一点时间了解这个东西。
1、基本概念
Android的消息机制主要是指：Handler的运行机制、MessageQueue和Looper的工作过程。
平时使用Handler较多。Handler的主要作用是将一个任务切换到某个指定的线程中去执行。比如：子线程中不能直接访问UI，可以通过Handler将访问UI的操作切换到主线程中去。
Handler的post传递一个Runnable或者通过send方法发送一个Message最后都会在Looper中处理。
//从源码中可以看到post和send最后调用的都是sendMessageDelayed方法。
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
接下来我们看下post方法中的getPostMessage(r)。可以发现getPostMessage方法中将传入的Runnable封装到Message中，传给Looper的最终都是一个Message对象。
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
Looper发现有新的Message就会处理这个Message。最终Handler的dispatchMessage调用来处理post传入的Runnable或者send传入的Message。
最后画个图。
大概流程
2、消息队列的工作原理
消息队列在Android中指的是MessageQueue。它负责Message单链表的维护：Message的添加，读取删除操作。
关键的方法：enqueueMessage 、next。
enqueueMessage：往Message队列中插入一条Message。
next：从消息队列中读取消息并删除。
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;
//以下是对Message单链表的添加Message操作
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;
}
从源码中可以看出，enqueueMessage方法主要是对Message单链接进行插入数据操作。
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;
}
}
从源码中可以看出，next是一个无限循环（for( ; ; ){}）的方法，如果消息队列中没有消息，那么next会一直阻塞。当获取到消息时，next会返回这条消息并将其从单链表中移除。
3、Looper的工作原理
Looper在消息机制中用于循环的获取消息并处理，他会通过MessageQueue的next方法来获取新的消息并处理，如果没有获取到新的消息则会阻塞在那里。
关键方法：loop
loop：循环获取Message并处理
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();
}
}
从源码中可以看出，loop方法调用MessageQueue的next方法获取新的消息，如果没有消息next会阻塞，如果返回了新的消息就会通过 msg.target.dispatchMessage(msg);处理这条消息。
msg.target就是Handler对象。也就会调用Handler的dispatchMessage方法。
4、Handler的工作原理
Handler主要包含消息的发送和接收的过程。消息的发送可以通过post的一系列方法以及send的一系列方法来实现。
调用post最终都会通过send的方法来实现（基本概念中已经提到）。
send最终会调用sendMessageAtTime(）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);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
从源码中可知queue.enqueueMessage(msg, uptimeMillis)我们可以知道Handler发送消息的过程是向消息队列插入了一条Message。Looper通过MessageQueue的next()获取到这条消息并处理。最后Looper把消息交给Handler处理（Looper的工作原理中提到的msg.target）。Handler的dispatchMessage方法就会被调用。
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
private static void handleCallback(Message message) {
message.callback.run();
}
public void handleMessage(Message msg) {
}
从源码中我们可以看出。
如果是post一个Runnable，则会通过handleCallback方法处理，处理方法为：message.callback.run();
如果是send一个Message，则会通过handleMessage方法处理，这里应该比较熟悉，重写handleMessage方法处理Message在开发中会经常用到。
以上就是Android消息机制的基本理解。可能存在缺陷，日后再去补充了。
参考blog：http://www.jianshu.com/p/d5c9433345e7
http://www.jianshu.com/p/e266c1490598
参考书籍：安卓开发艺术探索
