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Android的消息机制主要是指Handler的运行机制，Handler的运行机制需要底层MessageQueue和Looper的支撑。
- Handler：消息的执行者，也可以称之为异步任务的执行者
- Message：消息的封装者，把异步任务，消息码Handler对象等封装成Message对象
- MessageQueue：消息队列，用于保存当前线程的所有消息Message对象的一个列表,内部储存结构为单链表的数据结构
- Looper：循环者，能让工作线程变成循环线程，然后从消息队列中循环读取消息
- ThreadLocal：线程内部数据储存类，通过它可以获取每个线程的Looper，线程默认是没有Looper的，如果在线程内需要使用Handler就必须为线程创建Looper(详见 ThreadLocal工作原理)
1.MessageQueue（消息队列）的工作原理
这里说的消息队列即为MessageQueue，主要包含插入消息和读取消息两个操作，当然读取会伴随着删除。内部实现并不是队列，其实是单链表，单链表在插入和删除上有优势。
接下来看一下插入消息enqueueMessage
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("MessageQueue", 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;
} 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 (false) Log.v("MessageQueue", "Returning message: " + msg);
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("MessageQueue", "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;
}
}2.Looper的工作原理
Looper在消息机制中扮演着消息循环的角色，具体来说就是不停从MessageQueue中查看是否有新消息，有消息则立即处理，否则一致阻塞。
Looper的构造方法
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}我们知道Handler工作需要Looper，没有Looper则会报错，如何创建Looper呢，很简单Looper.prepare()来为当前线程创建Looper
public static void prepare() {
prepare(true);
}
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));
}
ActivityThread启动Activity之前，系统会调用Looper.prepareMainLooper方法为UI线程创建好Looper，这也是我们在主线程中使用Handler不需要先主动创建Looper的原因。
/**
* Initialize the current thread as a looper, marking it as an
* application's main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself. See also: {@link #prepare()}
*/
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
/** Returns the application's main looper, which lives in the main thread of the application.
*/
public static Looper getMainLooper() {
synchronized (Looper.class) {
return sMainLooper;
}
}
创建好Looper之后就可以创建Handler了，接着调用Looper.loop来开启消息循环。
/**
* 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
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();
}
} public void quit() {
mQueue.quit(false);
} public void quitSafely() {
mQueue.quit(true);
} void quit(boolean safe) {
if (!mQuitAllowed) {
throw new IllegalStateException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuitting) {
return;
}
mQuitting = true;
if (safe) {
removeAllFutureMessagesLocked();
} else {
removeAllMessagesLocked();
}
// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}Looper提供了quit和quitSafely两个方法：quit会直接退出Looper，而quitSafely设定了一个退出标记，处理完消息队列中已有的消息后会安全退出。Looper退出后Handler调用send方法会返回false，这里需要注意：在子线程中如果所有消息处理完成之后应该调用quit方法退出Looper否则子线程一直会处于等待状态，而退出Looper之后线程会立即终止。
在消息循环方法loop中如果有消息则调用 msg.target.dispatchMessage(msg);其中msg.target就是msg对应的Handler对象，Handler发送的消息最终由它的dispatchMessage方法处理了。而Handler的dispatchMessage方法是在创建Handler时使用的Looper中执行的，这样Handler的dispatchMessage代码逻辑切换到Looper所在线程中执行了。
3.Handler的工作原理
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);
}
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);
} 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();
}
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}Handler处理消息的逻辑为：
1.如果msg的callback不为空则调用msg的callback方法，
2.步骤1中msg的callback为空则检查Handler的MCallback是不是为空，如果不为空则执行mCallback方法，
3.步骤2中的mCallback为空，则执行子类复写的handlerMessage方法。
从上面步骤2可以追溯到 Handler可以这样创建：Handler handler = new Handler（callback）可以不用派生Handler的子类来创建一个Handler