在Android中，Android的消息机制多指Handler的运行机制，在Handler的运行过程中，Message，MessageQueue，Looper也是必不可少的一部分，接下来我们简单分析它们的工作过程。
一、Message
Message顾名思义就是消息的意思，他是Handler发送消息的载体。接下来我们来看Message中常用的几个参数。
/**
* User-defined message code so that the recipient can identify
* what this message is about. Each {@link Handler} has its own name-space
* for message codes, so you do not need to worry about yours conflicting
* with other handlers.
*/
public int what;
/**
* arg1 and arg2 are lower-cost alternatives to using
* {@link #setData(Bundle) setData()} if you only need to store a
* few integer values.
*/
public int arg1;
/**
* arg1 and arg2 are lower-cost alternatives to using
* {@link #setData(Bundle) setData()} if you only need to store a
* few integer values.
*/
public int arg2;
/**
* An arbitrary object to send to the recipient. When using
* {@link Messenger} to send the message across processes this can only
* be non-null if it contains a Parcelable of a framework class (not one
* implemented by the application). For other data transfer use
* {@link #setData}.
*/
public Object obj;
根据源码中的注释我们可以知道各个参数的含义；
what：用户定义的消息代码，以便接收人识别此消息的含义，简单来说就是标识符。
agr1/2：存储整型数值。
obj：发送的对象。
另外注释中还提到，setData方法。
public void setData(Bundle data) {
this.data = data;
}
通过setData方法，我们可以利用Bundle传递更多我们想要的参数。
1.1 Message的创建：
通常没阅读过源码的人都会通过new Message的方法来构造Message对象，但是我们来看源码。
/** Constructor (but the preferred way to get a Message is to call {@link #obtain() Message.obtain()}).
*/
public Message() {
}
/**
* Return a new Message instance from the global pool. Allows us to
* avoid allocating new objects in many cases.
*/
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}
通过代码中的注释，创建Message的首选方法是通过Message.obtain(); 此方法是从全局池中返回一个新的Message实例，避免了Message的重复创建。
二、MessageQueue
MessageQueue指的是消息队列，主要负责插入消息（enqueueMessage）和读取消息（next）。
2.1 插入消息
MessageQueue.enqueueMessage
boolean enqueueMessage(Message msg, long when) {
//如果target为空 target指的就是Handler
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
synchronized (this) {
//如果msg已经在使用
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
//如果退出 回收msg
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;
}
//标记msg的使用状态
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;
}
2.2 读取消息
MessageQueue.next
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;
}
}
MessageQueue是何时创建的呢？接下来我们看Looper；
三、Looper
Looper翻译为循环的意思，主要作用为不停的从MessageQueue中查看是否有新消息，如果有则会处理，否则会一直阻塞。首先看一下Looper的构造方法。
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
其中的mQueue指的就是MessageQueue，这恰好回答了我们上面的问题：MessageQueue是在创建Looper时创建的。
3.1 Looper的创建。
那么Looper又是何时创建的呢？ 我们先来看Handler的源码。
public Handler(@Nullable Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class<? extends Handler> klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());
}
}
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
看源码中抛出的异常是不是很熟悉？ 翻译为：不能在线程内创建Handler，还没有调用Looper.prepare()。
再加上面的 if (mLooper == null) 我们大概就可以知道，我们需要通过Looper.prepare()创建Looper。接下来继续看Looper的源码；
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));
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
通过Looper.prepare()方法，最后调用到了prepare(boolean quitAllowed)中的sThreadLocal.set(new Looper(quitAllowed));此时Looper就已经创建。如果我们在子线程中使用Handler，则必须先调用Looper.prepare()创建Looper。
3.2 Looper的启动。
其实Looper中最重要的是loop方法，只有调用了loop();消息循环才会生效，这也是为什么我们在子线程中使用Handler时，需要同时调用Looper.prepare()和Looper.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.");
}
if (me.mInLoop) {
Slog.w(TAG, "Loop again would have the queued messages be executed"
+ " before this one completed.");
}
me.mInLoop = true;
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();
// Allow overriding a threshold with a system prop. e.g.
// adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
final int thresholdOverride =
SystemProperties.getInt("log.looper."
+ Process.myUid() + "."
+ Thread.currentThread().getName()
+ ".slow", 0);
boolean slowDeliveryDetected = false;
//死循环
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {//如果msg为空 跳出
// 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
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
// Make sure the observer won't change while processing a transaction.
final Observer observer = sObserver;
final long traceTag = me.mTraceTag;
long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
if (thresholdOverride > 0) {
slowDispatchThresholdMs = thresholdOverride;
slowDeliveryThresholdMs = thresholdOverride;
}
final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);
final boolean needStartTime = logSlowDelivery || logSlowDispatch;
final boolean needEndTime = logSlowDispatch;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
final long dispatchEnd;
Object token = null;
if (observer != null) {
token = observer.messageDispatchStarting();
}
long origWorkSource = ThreadLocalWorkSource.setUid(msg.workSourceUid);
try {
msg.target.dispatchMessage(msg);
if (observer != null) {
observer.messageDispatched(token, msg);
}
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} catch (Exception exception) {
if (observer != null) {
observer.dispatchingThrewException(token, msg, exception);
}
throw exception;
} finally {
ThreadLocalWorkSource.restore(origWorkSource);
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (logSlowDelivery) {
if (slowDeliveryDetected) {
if ((dispatchStart - msg.when) <= 10) {
Slog.w(TAG, "Drained");
slowDeliveryDetected = false;
}
} else {
if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
msg)) {
// Once we write a slow delivery log, suppress until the queue drains.
slowDeliveryDetected = true;
}
}
}
if (logSlowDispatch) {
showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", 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();
}
}
可以看到，loop()方法是一个死循环，即一直都在循环消息，当MessageQueue的next方法返回的msg为null的时候才会跳出这个循环。
当我们调用Looper.quit方法时，就会调用MessageQueue的quit，此MessageQueue被标记为退出状态，next方法则返回为null。Looper才会退出。关键源码如下：
Looper.java
public void quit() {
mQueue.quit(false);
}
public void quitSafely() {
mQueue.quit(true);
}
MessageQueue.java
void quit(boolean safe) {
if (!mQuitAllowed) {
throw new IllegalStateException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuitting) {
return;
}
mQuitting = true; //标记为退出
if (safe) {
removeAllFutureMessagesLocked();
} else {
removeAllMessagesLocked();
}
// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}
MessageQueue.java next()方法
if (mQuitting) {
dispose();
return null;
}
所以说，当我们不在需要使用Handler时，需要调用Looper.quit来退出Looper，否则Looper.loop和MessageQueue.next均会阻塞。
到这里也会读者会问到，为什么子线程中需要调用Looper.prepare和Looper.loop，而主线程中却不用呢？
我们可以看到Looper中有这样一段代码。
/**
* Initialize the current thread as a looper, marking it as an
* application's main looper. See also: {@link #prepare()}
*
* @deprecated The main looper for your application is created by the Android environment,
* so you should never need to call this function yourself.
*/
@Deprecated
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
注释中提到：应用程序的主循环程序是由Android环境创建的，所以你无须调用它。那么prepareMainLooper()究竟是在哪里调用了呢? 我们通过查到引用。可以查到是在ActivityThread中的Main方法中调用的。部分代码如下：
public static void main(String[] args) {
Looper.prepareMainLooper();
Looper.loop();
}
由此可以判断出，ActivityThread就是我们APP的主线程了，且内部自动创建了Looper，而且主线程中的Looper不可退出。
至此，我们知道如果没有消息则会阻塞，如果有消息会怎样处理呢？我们来看loop方法中关键的一行代码。
msg.target.dispatchMessage(msg);
其中msg.target指的就是Handler，我们继续看Handler的dispatchMessage方法。
public void dispatchMessage(@NonNull Message msg) {
if (msg.callback != null) {
//如果callback不为空 则调用handleCallback
handleCallback(msg);
} else {
//如果mCallback不为空 则调用handlerMessage来处理消息
if (mCallback != null) {
//如果为true 则return
if (mCallback.handleMessage(msg)) {
return;
}
}
//如果mCallback为空 则调用Handler的handler方法
handleMessage(msg);
}
}
其中msg.callback是一个Runnable对象，实际上就是Handler的post方法传递的Runnable。
mCallback是一个接口，包含了 handleMessage方法。所以我们可以通过new Handler(Callback)的方式来创建一个Handler。
四、Handler工作原理
Handler的工作主要为发送消息和接收消息。通过一系列的send和post方法来实现发送消息。
图1
图2
就拿sendMessage来举例，我们先看下源码：
public final boolean sendMessage(@NonNull Message msg) {
return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(@NonNull 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(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
msg.target = this;
msg.workSourceUid = ThreadLocalWorkSource.getUid();
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
可以看到 最后还是调用了MessageQueue的enqueueMessage方法，也就是将这条消息插入到了队列中。再结合上面我们所认识的MessageQueue和Looper大致也就搞清楚Handler的工作原理了；大致如下：
Handler通过调用send/post等一系列方法，将Message插入到MessageQueue中，此时Looper通过MessageQueue的next方法拿到Message，当Looper拿到Message后，通过调用msg.target即Handler的dispatchMessage(msg)方法来处理消息。最终通过handleCallback或Handler的handleMessage将Message的处理交给开发者，这样大致就是整个Handler的运行机制了。
如果有哪里不对的地方，欢迎各位指正交流。一起进步。
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