学习过程
跟着鸿洋_的博客的思路,结合7.0的源码进行学习,同时参考其他好的文章。
概述
主要涉及四个类:Looper、Handler、Message、MessageQueue。
Message是消息对象,MessageQueue是消息队列。Looper负责创建消息队列,并进入无限循环不断从消息队列中读取消息。而Handler负责发送消息到消息队列,以及消息的回调处理。
Looper
1. Looper类的作用
源码的类注释中已经把Looper类的作用和使用方法说得很清楚了。
/**
* Class used to run a message loop for a thread. Threads by default do
* not have a message loop associated with them; to create one, call
* {@link #prepare} in the thread that is to run the loop, and then
* {@link #loop} to have it process messages until the loop is stopped.
*
* <p>Most interaction with a message loop is through the
* {@link Handler} class.
*
* <p>This is a typical example of the implementation of a Looper thread,
* using the separation of {@link #prepare} and {@link #loop} to create an
* initial Handler to communicate with the Looper.
*
* <pre>
* class LooperThread extends Thread {
* public Handler mHandler;
*
* public void run() {
* Looper.prepare();
*
* mHandler = new Handler() {
* public void handleMessage(Message msg) {
* // process incoming messages here
* }
* };
*
* Looper.loop();
* }
* }</pre>
*/
Looper类的作用就是让线程进行消息循环。如果一个线程需要消息循环,只需要调用Looper类的prepare方法和loop方法就可以了。因此,Looper类中我们主要关注prepare和loop这两个方法,它们都是static方法。
2. 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));
}
这里创建了一个Looper对象,然后保存到一个ThreadLocal的静态变量中。当sThreadLocal中取出的对象不为null时,会抛出异常,保证一个线程中只有一个Looper对象。ThreadLocal后面再研究。
然后看Looper的构造方法。
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
创建了一个MessageQueue(消息队列)。
3. loop()方法
/**
* 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
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
final long traceTag = me.mTraceTag;
if (traceTag != 0) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
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();
}
}
其中,Binder.clearCallingIdentity()的作用不清楚,先忽略。
执行流程:
- myLooper方法获取sThreadLocal中保存的Looper实例。因此再loop方法执行前应该先执行prepare方法。
- 进入无限循环。
- 从消息队列中取出一条消息,如果没有消息则会阻塞;如果消息队列已释放,则会返回null,退出消息循环。
- 调用msg.target的dispatchMessage方法处理消息。target就是Handler的实例,负责接收处理这个消息。
- 回收msg资源。
4. Looper类小结
- 每个线程最多只能有一个Looper对象。每个Looper对象创建并持有一个MessageQueue对象。
- 通过调用Looper.loop方法使当前线程进入消息循环。当前线程的Looper对象循环从消息队列中取出消息,交由相应的Handler对象去处理。
Handler
1. Handler类的作用
还是看源码中的类注释。
/**
* A Handler allows you to send and process {@link Message} and Runnable
* objects associated with a thread's {@link MessageQueue}. Each Handler
* instance is associated with a single thread and that thread's message
* queue. When you create a new Handler, it is bound to the thread /
* message queue of the thread that is creating it -- from that point on,
* it will deliver messages and runnables to that message queue and execute
* them as they come out of the message queue.
*
* <p>There are two main uses for a Handler: (1) to schedule messages and
* runnables to be executed as some point in the future; and (2) to enqueue
* an action to be performed on a different thread than your own.
*
* <p>Scheduling messages is accomplished with the
* {@link #post}, {@link #postAtTime(Runnable, long)},
* {@link #postDelayed}, {@link #sendEmptyMessage},
* {@link #sendMessage}, {@link #sendMessageAtTime}, and
* {@link #sendMessageDelayed} methods. The <em>post</em> versions allow
* you to enqueue Runnable objects to be called by the message queue when
* they are received; the <em>sendMessage</em> versions allow you to enqueue
* a {@link Message} object containing a bundle of data that will be
* processed by the Handler's {@link #handleMessage} method (requiring that
* you implement a subclass of Handler).
*
* <p>When posting or sending to a Handler, you can either
* allow the item to be processed as soon as the message queue is ready
* to do so, or specify a delay before it gets processed or absolute time for
* it to be processed. The latter two allow you to implement timeouts,
* ticks, and other timing-based behavior.
*
* <p>When a
* process is created for your application, its main thread is dedicated to
* running a message queue that takes care of managing the top-level
* application objects (activities, broadcast receivers, etc) and any windows
* they create. You can create your own threads, and communicate back with
* the main application thread through a Handler. This is done by calling
* the same <em>post</em> or <em>sendMessage</em> methods as before, but from
* your new thread. The given Runnable or Message will then be scheduled
* in the Handler's message queue and processed when appropriate.
*/
Handler用于发送和处理Message和Runnable对象。Handler对象在创建时关联当前线程的MessageQueue,且每个Handler对象只能关联一个MessageQueue。Handler对象发送Message和Runnable对象到关联的MessageQueue,然后当它们从MessageQueue中移出时,负责执行它们。
Handler的主要用途有两个:
- 定时执行message或runnable。
- 让其他线程执行某个操作。比如,在非UI线程发送一个消息,让UI线程更新界面。
Handler中重要的有以下几组方法:
- 构造方法
- sendMessage方法
- post方法
- dispatchMessage方法
2. 构造方法
Handler中有很多构造方法,但是最终分别进入到两个构造方法中。先来看下这两个构造方法有什么不同。
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
public Handler(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 that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
第二个构造方法前面那段主要是当Handler的之类不是static类时,警告可能会导致内存泄漏。忽略这个,两个方法的区别就在于mLooper这个成员变量的来源。第一个方法由参数传入,而第二个方法是获取当前线程的Looper。可以看到,构造方法里面就是对几个成员变量赋值而已。这里先了解一下这几个成员变量的作用。
- mLooper:Looper,消息循环
- mQueue:MessageQueue,消息队列,就是mLooper中持有的那个
- mCallback:Handler.CallBack,Handler中声明的接口,用于处理消息
- mAsynchronous:boolean,发送的消息是否是异步的。那么,这个异步到底是什么意思呢?我们在后面MessageQueue的next方法中再去详细了解。
3. sendMessage方法
sendMessage有一系列的方法:
- sendMessage(Message msg): boolean
- sendEmptyMessage(int what): boolean
- sendEmptyMessageDelayed(int what, long delayMillis): boolean
- sendMessageDelayed(Message msg, long delayMillis): boolean
- sendEmptyMessageAtTime(int what, long uptimeMillis): boolean
- sendMessageAtTime(Message msg, long uptimeMillis): boolean
- sendMessageAtFrontOfQueue(Message msg): boolean
其中,sendMessage、sendEmptyMessage、sendMessageDelayed、sendEmptyMessageDelayed和sendEmptyMessageAtTime方法都调用sendMessageAtTime方法。所以,我们重点看sendMessageAtTime方法和sendMessageAtFrontOfQueue方法。
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);
}
enquueMessage方法只是调用MessageQueue的enqueueMessage方法。
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
uptimeMillis是消息分发的时间,基于SystemClock.uptimeMillis()。比如,sendMessageDelayed方法中使用SystemClock.uptimeMillis()加上延迟的时间。在消息队列中,消息是按分发时间的先后排列的。因此,这里的msg会插入到所有分发时间在uptimeMillis之前的消息后面。
sendMessageAtFrontOfQueue方法
public final boolean sendMessageAtFrontOfQueue(Message msg) {
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, 0);
}
顾名思义,sendMessageAtFrontOfQueue方法就是把消息插入到队列的最前面。和sendMessageAtTime唯一的不同是,在调用enqueueMessage方法时传的uptimeMillis参数是0。用0来表示插入到消息队列最前面,也是比较自然的做法。
4. post方法
post方法把Runable对象添加到消息队列中,也有一系列方法。
- post(Runnable r): boolean
- postAtTime(Runnable r, long uptimeMillis): boolean
- postAtTime(Runnable r, Object token, long uptimeMillis): boolean
- postDelayed(Runnable r, long delayMillis): boolean
- postAtFrontOfQueue(Runnable r): boolean
这些方法实现都类似,都是通过getPostMessage方法,获取一个Message,同时把Runnable存到Message中,然后调用sendMessage方法。
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
比较特殊的是带有token参数的postAtTime方法,这里的token是传给消息接收者的数据,会保存到Message的成员变量obj中。
private static Message getPostMessage(Runnable r, Object token) {
Message m = Message.obtain();
m.obj = token;
m.callback = r;
return m;
}
5. dispatchMessage方法
dispatchMessage方法就是前面Looper中调用的处理消息的方法。
msg.target.dispatchMessage(msg);
先看dispatchMessage方法的源码。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
如果消息中带有callback,直接执行callback(就是Message带的Runnable);如果Handler中设置了CallBack,则调用CallBack来处理消息;前面两个都没有的话,才是调用handleMessage方法。handleMessage是空方法,子类可以重写该方法来处理消息。
6. Handler类小结
- Handler创建时会关联一个Looper和Looper中持有的MessageQueue。
- Handler可以在任意线程发送消息到关联的MessageQueue。
- Handler在关联的Looper所在线程处理自己发送的消息。
- Hander主要用于定时任务和在其他线程执行任务。
MessageQueue
看完了Looper和Handler,已经基本理清了消息机制。再来看一下消息队列是怎么实现的。主要看一下把消息加入队列和从队列中取消息的实现。
1. 消息加入队列
MessageQueue的enqueueMessage方法负责把消息加入队列中。就是Handler中添加消息到消息队列调用的方法。下面贴的代码中省略了一下不影响主要逻辑的部分。
boolean enqueueMessage(Message msg, long when) {
......
synchronized (this) {
......
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
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;
prev.next = msg;
}
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
成员变量mMessages是消息队列的头部,是一个Message对象。MessageQueue中通过Message的next形成一个链表。如果待插入的消息的分发时间是0,表示直接插入到队列头部。如果队列头部Message为空,或者待插入消息的分发时间小于队列头部Message,也把消息插入到队列头部。如果不满足插入到头部的条件的话,就遍历消息队列,按分发时间找到合适的插入位置。
2. 从队列中获取下一条消息
在Looper中已经看到,MessageQueue的next方法负责从队列中取下一条消息。
Message next() {
...
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// 找下一个消息。找到的话就返回这个消息。
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// 被Barrier阻塞。找队列中的下一个异步消息。
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
// 下一个消息的分发时间还没到。设置一个时间,等到消息准备分发时再唤醒。
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// 有一个可以分发的消息。
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 {
// 没有消息,也可能是消息都被Barrier阻塞了。
nextPollTimeoutMillis = -1;
}
// 消息循环准备退出。释放资源,并且返回null。
if (mQuitting) {
dispose();
return null;
}
// pendingIdleHandlerCount初始值为-1,所以第一次会去获取IdleHandlers的数量。
// IdleHandler在需要等待下一条消息时去运行,因为这时是空闲的。
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);
}
// 执行IdleHandler
// 只在第一次迭代时会执行,因为后面会把pendingIdleHandlerCount设成0
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);
}
}
}
// 把pendingIdleHandlerCount设成0,后面的迭代不会再去执行IdleHandlers。
pendingIdleHandlerCount = 0;
// 执行完IdleHandler,可能已经有新的消息了,所以不需要再等待了。
nextPollTimeoutMillis = 0;
}
}
这个方法还是比较复杂的,所以我画了个流程图,能够看得清楚一些。
MessageQueue的next方法流程图.png通过流程图已经能理清next方法的执行过程。但是,还有两个需要解释的地方。一个是Barrier和异步消息,另一个是nativePollOnce这个方法。
Barrier和异步消息
Barrier是什么?
Barrier是阻塞器,会阻塞消息队列。它也是一个Message对象,只不过它的target是null。从next方法中可以看到,在Barrier后面所有消息,除了异步消息外都无法执行。
MessageQueue中对外提供了post和remove的方法。
public int postSyncBarrier();
public void removeSyncBarrier(int token);
调用post方法时,会创建一个空的Message对象,时间设为当前的系统时间,同时生成一个token,保存在Message中。这个Message对象会像普通的消息一样被插入到消息队列中。调用remove方法时,根据token从消息队列中找到相应的Barrier,然后移除。
看一个具体的例子,这是ViewRootImpl中的一段代码。
void scheduleTraversals() {
...
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
...
}
void unscheduleTraversals() {
...
mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
...
}
当要阻塞消息队列时,通过Handler获取到MessageQueue,然后直接调用MessageQueue的postSyncBarrier方法,保存下token。需要取消消息队列的阻塞时,通过先前保存的token去移除Barrier。
nativePollOnce方法
nativePollOnce是一个native方法。它的实现在frameworks/base/code/jni目录下的android_os_MessageQueue.cpp中。想要了解怎么找到这个实现的,可以阅读这篇文章:Android JNI原理分析。
static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj, jlong ptr, jint timeoutMillis) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->pollOnce(env, obj, timeoutMillis);
}
这里去调用了NativeMessageQueue的pollOnce方法。NativeMessageQueue的对象是在Java层的MessageQueue创建时,同时创建的。
void NativeMessageQueue::pollOnce(JNIEnv* env, jobject pollObj, int timeoutMillis) {
...
mLooper->pollOnce(timeoutMillis);
...
}
这里调用mLooper的pollOnce方法。这里的mLooper是JNI层的Looper,是在创建NativeMessageQueue时创建的。这个类的实现在system/core/libutils/Looper.cpp中。
int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
int result = 0;
for (;;) {
...
if (result != 0) {
...
return result;
}
result = pollInner(timeoutMillis);
}
}
我们把pollOnce方法中不太重要的部分都去掉,只留下最主要的部分。实际上就是循环去调用pollInner方法,当pollInner方法的返回结果不为0时,这个方法就可以返回了。下面来看一下pollInner方法的实现。
int Looper::pollInner(int timeoutMillis) {
// Adjust the timeout based on when the next message is due.
if (timeoutMillis != 0 && mNextMessageUptime != LLONG_MAX) {
nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
int messageTimeoutMillis = toMillisecondTimeoutDelay(now, mNextMessageUptime);
if (messageTimeoutMillis >= 0
&& (timeoutMillis < 0 || messageTimeoutMillis < timeoutMillis)) {
timeoutMillis = messageTimeoutMillis;
}
}
// Poll.
int result = POLL_WAKE;
...
struct epoll_event eventItems[EPOLL_MAX_EVENTS];
int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
// Rebuild epoll set if needed.
if (mEpollRebuildRequired) {
mEpollRebuildRequired = false;
rebuildEpollLocked();
goto Done;
}
// Check for poll error.
if (eventCount < 0) {
if (errno == EINTR) {
goto Done;
}
ALOGW("Poll failed with an unexpected error: %s", strerror(errno));
result = POLL_ERROR;
goto Done;
}
// Check for poll timeout.
if (eventCount == 0) {
result = POLL_TIMEOUT;
goto Done;
}
for (int i = 0; i < eventCount; i++) {
int fd = eventItems[i].data.fd;
uint32_t epollEvents = eventItems[i].events;
if (fd == mWakeEventFd) {
if (epollEvents & EPOLLIN) {
awoken();
} else {
ALOGW("Ignoring unexpected epoll events 0x%x on wake event fd.", epollEvents);
}
} else {
...
}
}
Done: ;
// Invoke pending message callbacks.
mNextMessageUptime = LLONG_MAX;
while (mMessageEnvelopes.size() != 0) {
nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
const MessageEnvelope& messageEnvelope = mMessageEnvelopes.itemAt(0);
if (messageEnvelope.uptime <= now) {
// Remove the envelope from the list.
// We keep a strong reference to the handler until the call to handleMessage
// finishes. Then we drop it so that the handler can be deleted *before*
// we reacquire our lock.
{ // obtain handler
sp<MessageHandler> handler = messageEnvelope.handler;
Message message = messageEnvelope.message;
mMessageEnvelopes.removeAt(0);
mSendingMessage = true;
mLock.unlock();
handler->handleMessage(message);
} // release handler
mLock.lock();
mSendingMessage = false;
result = POLL_CALLBACK;
} else {
// The last message left at the head of the queue determines the next wakeup time.
mNextMessageUptime = messageEnvelope.uptime;
break;
}
}
...
// Invoke all response callbacks.
for (size_t i = 0; i < mResponses.size(); i++) {
Response& response = mResponses.editItemAt(i);
if (response.request.ident == POLL_CALLBACK) {
int fd = response.request.fd;
int events = response.events;
void* data = response.request.data;
// Invoke the callback. Note that the file descriptor may be closed by
// the callback (and potentially even reused) before the function returns so
// we need to be a little careful when removing the file descriptor afterwards.
int callbackResult = response.request.callback->handleEvent(fd, events, data);
if (callbackResult == 0) {
removeFd(fd, response.request.seq);
}
// Clear the callback reference in the response structure promptly because we
// will not clear the response vector itself until the next poll.
response.request.callback.clear();
result = POLL_CALLBACK;
}
}
return result;
}
pollInner方法中调用了epoll_wait(),等待消息到来,或者等到超时返回。如果有消息到来,则进行处理。
poolInner方法的流程:(摘自[M0]Android Native层Looper详解)
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调整timeout:
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mNextMessageUptime 是 消息队列 mMessageEnvelopes 中最近一个即将要被处理的message的时间点。
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所以需要根据mNextMessageUptime 与 调用者传下来的timeoutMillis 比较计算出一个最小的timeout,这将决定epoll_wait() 可能会阻塞多久才会返回。
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epoll_wait():
epoll_wait()这里会阻塞,在三种情况下回返回,返回值eventCount为上来的epoll event数量。 -
出现错误返回, eventCount < 0;
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timeout返回,eventCount = 0,表明监听的文件描述符中都没有事件发生,将直接进行native message的处理;
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监听的文件描述符中有事件发生导致的返回,eventCount > 0; 有eventCount 数量的epoll event 上来。
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处理epoll_wait() 返回的epoll events.
判断epoll event 是哪个fd上发生的事件 -
如果是mWakeEventFd,则执行awoken(). awoken() 只是将数据read出来,然后继续往下处理了。其目的也就是使epoll_wait() 从阻塞中返回。
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如果是通过Looper.addFd() 接口加入到epoll监听队列的fd,并不是立马处理,而是先push到mResponses,后面再处理。
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处理消息队列 mMessageEnvelopes 中的Message.
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如果还没有到处理时间,就更新一下mNextMessageUptime
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处理刚才放入mResponses中的事件.
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只处理 ident 为POLL_CALLBACK的事件。其他事件在 pollOnce 中处理
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