Handler消息机制主要涉及Looper、Handler、MessageQueue、Message。其中，Looper主要负责获取消息，Handler负责发送消息及处理消息，MessageQueue是消息队列，Message是消息类。
Looper循环获取消息
1、ActivityThread的main()方法:
public static void main(String[] args) {
...
//准备looper
Looper.prepareMainLooper();
...
//进入无限循环
Looper.loop();
//如果loop()循环退出，则抛出异常，整个应用退出
throw new RuntimeException("Main thread loop unexpectedly exited");
}
2、prepareMainLooper()方法:
/**
* 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() {
//初始化looper
prepare(false);
synchronized (Looper.class) {
//如果已经设置过sMainLooper，则抛出异常。每个线程中只允许存在一个looper。
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
//设置sMainLooper
sMainLooper = myLooper();
}
}
3、在prepareMainLooper()方法中，首先调用prepare(false)方法:
private static void prepare(boolean quitAllowed) {
//如果ThreadLocal中已经存在looper,则抛出异常
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
//如果没有初始化looper，则将looper保存到ThradLocal中。
sThreadLocal.set(new Looper(quitAllowed));
}
4、在prepare()方法中调用Looper的构造方法初始化MessageQueue:
private Looper(boolean quitAllowed) {
//初始化MessageQueue
mQueue = new MessageQueue(quitAllowed);
//设置当前线程给mThread变量
mThread = Thread.currentThread();
}
5、在prepareMainLooper()方法调用prepare(false)方法后，会调用myLooper()方法:
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static @Nullable Looper myLooper() {
//将保存在ThreadLocal中的looper返回
return sThreadLocal.get();
}
5、到这里，prepareMainLooper()方法执行完毕。然后执行Looper.loop()方法:
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
//获取looper
final Looper me = myLooper();
//如果looper为null,则抛出异常
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
//获取MessageQueue
final MessageQueue queue = me.mQueue;
...
//开启无限循环
for (;;) {
//从消息队列中取消息，如果没有消息，则会阻塞
Message msg = queue.next(); // might block
//如果消息为null，则表示退出循环
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
...
try {
//回调target，即Handler的dispatchMessage方法
msg.target.dispatchMessage(msg);
...
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
...
msg.recycleUnchecked();
}
}
首先，获取looper,如果没有设置过looper，则抛出异常。然后，开启无限循环，通过looper的MessageQueue，不停获取消息，如果没有消息，则阻塞。如果获取到了消息，则会回调Handler的dispatchMessage方法，方法执行会切换到Handler的线程。
6、MessageQueue的next()方法:
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;
}
}
在next()方法中，通过for(;;)开启无限循环去获取消息，如果获取到消息则返回。
Handler发送消息
1、sendMessage()方法:
/**
* Pushes a message onto the end of the message queue after all pending messages
* before the current time. It will be received in {@link #handleMessage},
* in the thread attached to this handler.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
2、sendMessage()方法会调用sendMessageDelayed()方法:
/**
* Enqueue a message into the message queue after all pending messages
* before (current time + delayMillis). You will receive it in
* {@link #handleMessage}, in the thread attached to this handler.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
3、sendMessageDelayed()方法会调用sendMessageAtTime():
/**
* Enqueue a message into the message queue after all pending messages
* before the absolute time (in milliseconds) <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* Time spent in deep sleep will add an additional delay to execution.
* You will receive it in {@link #handleMessage}, in the thread attached
* to this handler.
*
* @param uptimeMillis The absolute time at which the message should be
* delivered, using the
* {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
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);
}
4、在这个方法中，会通过MessageQueue的enqueueMessage()方法，将消息发送到消息队列中。
5、enqueueMessage()方法:
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
//设置Message的target为当前的Handler，以便获取到消息后能回调dispatchMessage方法。
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
6、MessageQueue的enqueueMessage()方法:
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;
}
sendMessageAtTime()方法中的mQueue是在Handler的构造方法中赋值的:
public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
//检测是否会有泄漏
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());
}
}
//获取looper
mLooper = Looper.myLooper();
//如果looper为null,则抛出异常
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
//设置mQueue
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
如果没有传入looper，则会通过Looper.myLooper()获取looper,如果没有在线程中设置过looper，则会抛出异常
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
如果传入了looper，则直接设置mQueue。
Handler处理消息
dispatchMessage()方法,如果Message设置了callBack,则会回调callBack的run()方法；如果Message没有设置callBack,在这种情况下，如果Handler的callBack不为null，则会回调handleMessage()方法;如果Handler没有设置callBack或者Handler的callBack处理了消息，并没有返回true,则会回调Handler的handleMessage()方法:
public void dispatchMessage(Message msg) {
//如果callBack不为null,则传给callBack处理。
if (msg.callback != null) {
handleCallback(msg);
} else {
//如果Handler的callBack不为空，则传给callBack处理
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
handleCallback()方法:
private static void handleCallback(Message message) {
//回调Runnable的run()方法
message.callback.run();
}