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为线程本地存储区；每个线程仅有一个Looper
 sThreadLocal.set(new Looper(quitAllowed));
}

private Looper(boolean quitAllowed) {
 //创建出MessageQueue
 mQueue = new MessageQueue(quitAllowed);
 mThread = Thread.currentThread();
}

MessageQueue(boolean quitAllowed) {
 mQuitAllowed = quitAllowed;
 //mPtr的类型为long？
 mPtr = nativeInit();
}

static jlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) {
 //MessageQueue的Native层实体
 NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();
 ............
 //这里应该类似与将指针转化成long类型，放在Java层保存；估计Java层使用时，会在native层将long变成指针，就可以操作队列了
 return reinterpret_cast<jlong>(nativeMessageQueue);
}

NativeMessageQueue::NativeMessageQueue() :
 mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) {
 //创建一个Native层的Looper，也是线程唯一的
 mLooper = Looper::getForThread();
 if (mLooper == NULL) {
 mLooper = new Looper(false);
 Looper::setForThread(mLooper);
 }
}

Looper::Looper(bool allowNonCallbacks) :
 mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),
 mPolling(false), mEpollFd(-1), mEpollRebuildRequired(false),
 mNextRequestSeq(0), mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {
 //此处创建了个fd
 mWakeEventFd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
 .......
 rebuildEpollLocked();
}

void Looper::rebuildEpollLocked() {
 // Close old epoll instance if we have one.
 if (mEpollFd >= 0) {
 close(mEpollFd);
 }

 // Allocate the new epoll instance and register the wake pipe.
 mEpollFd = epoll_create(EPOLL_SIZE_HINT);
 ............
 struct epoll_event eventItem;
 memset(& eventItem, 0, sizeof(epoll_event)); // zero out unused members of data field union
 eventItem.events = EPOLLIN;
 eventItem.data.fd = mWakeEventFd;
 //在mEpollFd上监听mWakeEventFd上是否有数据到来
 int result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem);
 ...........
 for (size_t i = 0; i < mRequests.size(); i++) {
 const Request& request = mRequests.valueAt(i);
 struct epoll_event eventItem;
 request.initEventItem(&eventItem);
 //监听request对应fd上数据的到来
 int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, request.fd, & eventItem);
 ............
 }
}

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) {
  .....
  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;
  //在头部插入数据，如果之前MessageQueue是阻塞的，那么现在需要唤醒
  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;
  }
  //不是第一个异步消息时，needWake置为false
  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；
}

void NativeMessageQueue::wake() {
 mLooper->wake();
}

void Looper::wake() {
 uint64_t inc = 1;
 //就是向mWakeEventFd写入数据
 ssize_t nWrite = TEMP_FAILURE_RETRY(write(mWakeEventFd, &inc, sizeof(uint64_t)));
 .............
}

void Looper::sendMessage(const sp<MessageHandler>& handler, const Message& message) {
 nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
 sendMessageAtTime(now, handler, message);
}

void Looper::sendMessageAtTime(nsecs_t uptime, const sp<MessageHandler>& handler,
 const Message& message) {
 size_t i = 0;
 {
 AutoMutex _l(mLock);

 //同样需要按时间插入
 size_t messageCount = mMessageEnvelopes.size();
 while (i < messageCount && uptime >= mMessageEnvelopes.itemAt(i).uptime) {
  i += 1;
 }

 //将message包装成一个MessageEnvelope对象
 MessageEnvelope messageEnvelope(uptime, handler, message);
 mMessageEnvelopes.insertAt(messageEnvelope, i, 1);

 // Optimization: If the Looper is currently sending a message, then we can skip
 // the call to wake() because the next thing the Looper will do after processing
 // messages is to decide when the next wakeup time should be. In fact, it does
 // not even matter whether this code is running on the Looper thread.
 if (mSendingMessage) {
  return;
 }
 }
 // Wake the poll loop only when we enqueue a new message at the head.
 if (i == 0) {
 //若插入在队列头部，同样利用wake函数触发epoll唤醒
 wake();
 }
}

public static void loop() {
 final Looper me = myLooper();
 .......
 for (;;) {
 Message msg = queue.next(); // might block
 .......
 try {
  //调用Message的处理函数进行处理
  msg.target.dispatchMessage(msg);
 }........
 }
}

Message next() {
 //mPtr保存了NativeMessageQueue的指针
 final long ptr = mPtr;
 .......
 int pendingIdleHandlerCount = -1; // -1 only during first iteration
 int nextPollTimeoutMillis = 0;

 for (;;) {
 if (nextPollTimeoutMillis != 0) {
  //会调用Native函数，最终调用IPCThread的talkWithDriver，将数据写入Binder驱动或者读取一次数据
  //不知道在此处进行这个操作的理由？
  Binder.flushPendingCommands();
 }

 //处理native层的数据，此处会利用epoll进行blocked
 nativePollOnce(ptr, nextPollTimeoutMillis);

 synchronized (this) {
  final long now = SystemClock.uptimeMillis();
  Message prevMsg = null;
  Message msg = mMessages;
  //下面其实就是找出下一个异步处理类型的消息；异步处理类型的消息，才含有对应的执行函数
  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;
   //完成next记录的存储
   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;
  }

  //MessageQueue中引入了IdleHandler接口，即当MessageQueue没有数据处理时，调用IdleHandler进行一些工作

  //pendingIdleHandlerCount表示待处理的IdleHandler，初始为-1
  if (pendingIdleHandlerCount < 0
   && (mMessages == null || now < mMessages.when)) {
  //mIdleHandlers的size默认为0，调用接口addIdleHandler才能增加
  pendingIdleHandlerCount = mIdleHandlers.size();
  }

  if (pendingIdleHandlerCount <= 0) {
  // No idle handlers to run. Loop and wait some more.
  mBlocked = true;
  continue;
  }

  //将待处理的IdleHandler加入到PendingIdleHandlers中
  if (mPendingIdleHandlers == null) {
  mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
  }
  //调用ArrayList.toArray(T[])节省每次分配的开销；毕竟对于Message.Next这样调用频率较高的函数，能省一点就是一点
  mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
 }

 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 {
  //执行实现类的queueIdle函数，返回值决定是否继续保留
  keep = idler.queueIdle();
  } catch (Throwable t) {
  Log.wtf(TAG, "IdleHandler threw exception", t);
  }

  if (!keep) {
  synchronized (this) {
   mIdleHandlers.remove(idler);
  }
  }
 }
 pendingIdleHandlerCount = 0;
 nextPollTimeoutMillis = 0;
 }
}

static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj,
 jlong ptr, jint timeoutMillis) {
 //果然Java层调用native层MessageQueue时，将long类型的ptr变为指针
 NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
 nativeMessageQueue->pollOnce(env, obj, timeoutMillis);
}

void NativeMessageQueue::pollOnce(JNIEnv* env, jobject pollObj, int timeoutMillis) {
 mPollEnv = env;
 mPollObj = pollObj;
 //最后还是进入到Native层looper的pollOnce函数
 mLooper->pollOnce(timeoutMillis);
 mPollObj = NULL;
 mPollEnv = NULL;

 if (mExceptionObj) {
 .........
 }
}

//timeoutMillis为超时等待时间。值为-1时，表示无限等待直到有事件到来；值为0时，表示无需等待
//outFd此时为null，含义是：存储产生事件的文件句柄
//outEvents此时为null，含义是：存储outFd上发生了哪些事件，包括可读、可写、错误和中断
//outData此时为null，含义是：存储上下文数据，其实调用时传入的参数
int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
 int result = 0;
 for (;;) {
 //处理response，目前我们先不关注response的内含
 while (mResponseIndex < mResponses.size()) {
  const Response& response = mResponses.itemAt(mResponseIndex++);
  int ident = response.request.ident;
  if (ident >= 0) {
  int fd = response.request.fd;
  int events = response.events;
  void* data = response.request.data;

  if (outFd != NULL) *outFd = fd;
  if (outEvents != NULL) *outEvents = events;
  if (outData != NULL) *outData = data;
  return ident;
  }
 }

 //根据pollInner的结果，进行操作
 if (result != 0) {
  if (outFd != NULL) *outFd = 0;
  if (outEvents != NULL) *outEvents = 0;
  if (outData != NULL) *outData = NULL;
  return result;
 }

 //主力还是靠pollInner
 result = pollInner(timeoutMillis);
 }
}

int Looper::pollInner(int timeoutMillis) {
 // Adjust the timeout based on when the next message is due.
 //timeoutMillis是Java层事件等待事件
 //native层维持了native message的等待时间
 //此处其实就是选择最小的等待时间
 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;
 }
 }

 int result = POLL_WAKE;
 //pollInner初始就清空response
 mResponses.clear();
 mResponseIndex = 0;

 // We are about to idle.
 mPolling = true;

 //利用epoll等待mEpollFd监控的句柄上事件到达
 struct epoll_event eventItems[EPOLL_MAX_EVENTS];
 int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);

 // No longer idling.
 mPolling = false;

 // Acquire lock.
 mLock.lock();

 //重新调用rebuildEpollLocked时，将使得epoll句柄能够监听新加入request对应的fd
 if (mEpollRebuildRequired) {
 mEpollRebuildRequired = false;
 rebuildEpollLocked();
 goto Done;
 }

 // Check for poll error.
 if (eventCount < 0) {
 if (errno == EINTR) {
  goto Done;
 }
 ......
 result = POLL_ERROR;
 goto Done;
 }

 // Check for poll timeout.
 if (eventCount == 0) {
 result = POLL_TIMEOUT;
 goto Done;
 }

 for (int i = 0; i < eventCount; i++) {
 if (fd == mWakeEventFd) {
  if (epollEvents & EPOLLIN) {
  //前面已经分析过，当java层或native层有数据写入队列时，将写mWakeEventFd，以触发epoll唤醒
  //awoken将读取并清空mWakeEventFd上的数据
  awoken();
  } else {
  .........
  }
 } else {
  //epoll同样监听的request对应的fd
  ssize_t requestIndex = mRequests.indexOfKey(fd);
  if (requestIndex >= 0) {
  int events = 0;
  if (epollEvents & EPOLLIN) events |= EVENT_INPUT;
  if (epollEvents & EPOLLOUT) events |= EVENT_OUTPUT;
  if (epollEvents & EPOLLERR) events |= EVENT_ERROR;
  if (epollEvents & EPOLLHUP) events |= EVENT_HANGUP;
  //存储这个fd对应的response
  pushResponse(events, mRequests.valueAt(requestIndex));
  } else {
  ..........
  }
 }
 }

 // Invoke pending message callbacks.
 mNextMessageUptime = LLONG_MAX;
 //处理Native层的Message
 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.
  {
  sp<MessageHandler> handler = messageEnvelope.handler;
  Message message = messageEnvelope.message;
  mMessageEnvelopes.removeAt(0);
  mSendingMessage = true;
  mLock.unlock();

  //处理Native Message
  handler->handleMessage(message);
  }
  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;
 }
 }

 // Release lock.
 mLock.unlock();

 //处理带回调函数的response
 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;

  //调用response的callback
  int callbackResult = response.request.callback->handleEvent(fd, events, data);
  if (callbackResult == 0) {
  removeFd(fd, response.request.seq);
  }

  response.request.callback.clear();
  result = POLL_CALLBACK;
 }
 }
 return result;
}

//fd表示需要监听的句柄
//ident的含义还没有搞明白
//events表示需要监听的事件，例如EVENT_INPUT、EVENT_OUTPUT、EVENT_ERROR和EVENT_HANGUP中的一个或多个
//callback为事件发生后的回调函数
//data为回调函数对应的参数
int Looper::addFd(int fd, int ident, int events, Looper_callbackFunc callback, void* data) {
 return addFd(fd, ident, events, callback ? new SimpleLooperCallback(callback) : NULL, data);
}

int Looper::addFd(int fd, int ident, int events, const sp<LooperCallback>& callback, void* data) {
 ........
 {
 AutoMutex _l(mLock);

 //利用参数构造一个request
 Request request;
 request.fd = fd;
 request.ident = ident;
 request.events = events;
 request.seq = mNextRequestSeq++;
 request.callback = callback;
 request.data = data;
 if (mNextRequestSeq == -1) mNextRequestSeq = 0; // reserve sequence number -1

 struct epoll_event eventItem;
 request.initEventItem(&eventItem);

 //判断之前是否已经利用该fd构造过Request
 ssize_t requestIndex = mRequests.indexOfKey(fd);
 if (requestIndex < 0) {
  //mEpollFd新增一个需监听fd
  int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, fd, & eventItem);
  .......
  mRequests.add(fd, request);
 } else {
  //mEpollFd修改旧的fd对应的监听事件
  int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_MOD, fd, & eventItem);
  if (epollResult < 0) {
  if (errno == ENOENT) {
   // Tolerate ENOENT because it means that an older file descriptor was
   // closed before its callback was unregistered and meanwhile a new
   // file descriptor with the same number has been created and is now
   // being registered for the first time. 
   epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, fd, & eventItem);
   .......
  }
  //发生错误重新加入时，安排EpollRebuildLocked，将让epollFd重新添加一次待监听的fd
  scheduleEpollRebuildLocked();
  }
  mRequests.replaceValueAt(requestIndex, request);
 }
 }
}