Android6.0 消息机制原理解析

消息都是存放在一个消息队列中去，而消息循环线程就是围绕这个消息队列进入一个无限循环的，直到线程退出。如果队列中有消息，消息循环线程就会把它取出来，并分发给相应的Handler进行处理；如果队列中没有消息，消息循环线程就会进入空闲等待状态，等待下一个消息的到来。在编写Android应用程序时，当程序执行的任务比较繁重时，为了不阻塞UI主线程而导致ANR的发生，我们通常的做法的创建一个子线程来完成特定的任务。在创建子线程时，有两种选择，一种通过创建Thread对象来创建一个无消息循环的子线程；还有一种就是创建一个带有消息循环的子线程，而创建带有消息循环的子线程由于两种实现方法，一种是直接利用Android给我们封装好的HandlerThread类来直接生成一个带有消息循环的线程对象，另一种方法是在实现线程的run()方法内使用以下方式启动一个消息循环： 

一、消息机制使用 

通常消息都是有一个消息线程和一个Handler组成，下面我们看PowerManagerService中的一个消息Handler：        

 mHandlerThread = new ServiceThread(TAG,
        Process.THREAD_PRioRITY_DISPLAY, false );
    mHandlerThread.start();
    mHandler = new PowerManagerHandler(mHandlerThread.getLooper()); 

这里的ServiceThread就是一个HandlerThread，创建Handler的时候，必须把HandlerThread的looper传进去，否则就是默认当前线程的looper。 

而每个handler，大致如下：

   private final class PowerManagerHandler extends Handler {
    public PowerManagerHandler(Looper looper) {
      super(looper, null, true );
    }
    @Override
    public void handleMessage(Message msg) {
      switch (msg.what) {
        case MSG_USER_ACTIVITY_TIMEOUT:
          handleUserActivityTimeout();
          break;
        case MSG_SANDMAN:
          handleSandman();
          break;
        case MSG_SCREEN_BRIGHTNESS_BOOST_TIMEOUT:
          handleScreenBrightnessBoostTimeout();
          break;
        case MSG_CHECK_WAKE_LOCK_ACQUIRE_TIMEOUT:
          checkWakeLockAquireTooLong();
          Message m = mHandler.obtainMessage(MSG_CHECK_WAKE_LOCK_ACQUIRE_TIMEOUT);
          m.setAsynchronous(true);
          mHandler.sendMessageDelayed(m, WAKE_LOCK_ACQUIRE_TOO_LONG_TIMEOUT);
          break;
      }
    }
  }

二、消息机制原理
那我们先来看下HandlerThread的主函数run函数： 

 public void run() {
    mTid = Process.myTid();
    Looper.prepare();
    synchronized (this) {
      mLooper = Looper.myLooper();//赋值后notifyall，主要是getLooper函数返回的是mLooper
      notifyAll();
    }
    Process.setThreadPriority(mPriority);
    onLooperPrepared();
    Looper.loop();
    mTid = -1;
  }

再来看看Lopper的prepare函数，最后新建了一个Looper对象，并且放在线程的局部变量中。

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));
  } 

Looper的构造函数中创建了MessageQueue

   private Looper(boolean quitAllowed) {
    MQueue = new MessageQueue(quitAllowed);
    mThread = Thread.currentThread();
  } 

我们再来看下MessageQueue的构造函数，其中nativeInit是一个native方法，并且把返回值保存在mPtr显然是用long型变量保存的指针

MessageQueue(boolean quitAllowed) {
    mQuitAllowed = quitAllowed;
    mPtr = nativeInit();
  } 

native函数中主要创建了NativeMessageQueue对象，并且把指针变量返回了。

 static jlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) {
  NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();
  if (!nativeMessageQueue) {
    jniThrowRuntimeException(env, "Unable to allocate native queue");
    return 0;
  }
  nativeMessageQueue->incStrong(env);
  return reinterpret_cast<jlong>(nativeMessageQueue);
} 

NativeMessageQueue构造函数就是获取mLooper，如果没有就是新建一个Looper 

NativeMessageQueue::NativeMessageQueue() :
    mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) {
  mLooper = Looper::getForThread();
  if (mLooper == NULL) {
    mLooper = new Looper(false);
    Looper::setForThread(mLooper);
  }
}

然后我们再看下Looper的构造函数，显示调用了eventfd创建了一个fd，eventfd它的主要是用于进程或者线程间的通信，我们可以看下这篇博客eventfd介绍

 Looper::Looper(bool allowNonCallbacks) :
    mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),
    mPolling(false), mEpollFd(-1), mEpollRebuildRequired(false),
    mNextRequestSeq(0), mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {
  mWakeEventFd = eventfd(0, EFD_NONBLOCK);
  LOG_ALWAYS_FATAL_IF(mWakeEventFd < 0, "Could not make wake event fd. errno=%d", errno);
  AutoMutex _l(mLock);
  rebuildEpollLocked();
}

2.1 c层创建epoll 

我们再来看下rebuildEpollLocked函数，创建了epoll，并且把mWakeEventFd加入epoll，而且把mRequests的fd也加入epoll

 void Looper::rebuildEpollLocked() {
  // Close old epoll instance if we have one.
  if (mEpollFd >= 0) {
#if DEBUG_CALLBACKS
    ALOGD("%p ~ rebuildEpollLocked - rebuilding epoll set", this);
#endif
    close(mEpollFd);
  }
  // Allocate the new epoll instance and reGISter the wake pipe.
  mEpollFd = epoll_create(EPOLL_SIZE_HINT);
  LOG_ALWAYS_FATAL_IF(mEpollFd < 0, "Could not create epoll instance. errno=%d", errno);
  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;
  int result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem);
  LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake event fd to epoll instance. errno=%d",
      errno);
  for (size_t i = 0; i < mRequests.size(); i++) {
    const Request& request = mRequests.valueAt(i);
    struct epoll_event eventItem;
    request.initEventItem(&eventItem);
    int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, request.fd, & eventItem);
    if (epollResult < 0) {
      ALOGE("Error adding epoll events for fd %d while rebuilding epoll set, errno=%d",
          request.fd, errno);
    }
  }
} 

继续回到HandlerThread的run函数，我们继续分析Looper的loop函数

public void run() {
    mTid = Process.myTid();
    Looper.prepare();
    synchronized (this) {
      mLooper = Looper.myLooper();
      notifyAll();
    }
    Process.setThreadPriority(mPriority);
    onLooperPrepared();
    Looper.loop();
    mTid = -1;
  } 

我们看看Looper的loop函数：

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;//得到Looper的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这个函数会阻塞，阻塞主要是epoll_wait
      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;//自己打的打印
      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();
    }
  }

MessageQueue类的next函数主要是调用了nativePollOnce函数，后面就是从消息队列中取出一个Message

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;//之前保留的指针
    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); 

下面我们主要看下nativePollOnce这个native函数，把之前的指针强制转换成NativeMessageQueue，然后调用其pollOnce函数

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);
}

2.2 c层epoll_wait阻塞 

pollOnce函数，这个函数前面的while一般都没有只是处理了indent大于0的情况，这种情况一般没有，所以我们可以直接看pollInner函数

 int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
  int result = 0;
  for (;;) {
    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 DEBUG_POLL_AND_WAKE
        ALOGD("%p ~ pollOnce - returning signalled identifier %d: "
            "fd=%d, events=0x%x, data=%p",
            this, ident, fd, events, data);
#endif
        if (outFd != NULL) *outFd = fd;
        if (outEvents != NULL) *outEvents = events;
        if (outData != NULL) *outData = data;
        return ident;
      }
    }
    if (result != 0) {
#if DEBUG_POLL_AND_WAKE
      ALOGD("%p ~ pollOnce - returning result %d", this, result);
#endif
      if (outFd != NULL) *outFd = 0;
      if (outEvents != NULL) *outEvents = 0;
      if (outData != NULL) *outData = NULL;
      return result;
    }
    result = pollInner(timeoutMillis);
  }
} 

pollInner函数主要就是调用epoll_wait阻塞，并且java层会计算每次阻塞的时间传到c层，等待有mWakeEventFd或者之前addFd的fd有事件过来，才会epoll_wait返回。 

int Looper::pollInner(int timeoutMillis) {
#if DEBUG_POLL_AND_WAKE
  ALOGD("%p ~ pollOnce - waiting: timeoutMillis=%d", this, timeoutMillis);
#endif
  // 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;
    }
#if DEBUG_POLL_AND_WAKE
    ALOGD("%p ~ pollOnce - next message in %" PRId64 "ns, adjusted timeout: timeoutMillis=%d",
        this, mNextMessageUptime - now, timeoutMillis);
#endif
  }
  // Poll.
  int result = POLL_WAKE;
  mResponses.clear();//清空mResponses
  mResponseIndex = 0;
  // We are about to idle.
  mPolling = true;
  struct epoll_event eventItems[EPOLL_MAX_EVENTS];
  int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);//epoll_wait主要线程阻塞在这，这个阻塞的时间也是有java层传过来的
  // No longer idling.
  mPolling = false;
  // Acquire lock.
  mLock.lock();
  // 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, errno=%d", errno);
    result = POLL_ERROR;
    goto Done;
  }
  // Check for poll timeout.
  if (eventCount == 0) {
#if DEBUG_POLL_AND_WAKE
    ALOGD("%p ~ pollOnce - timeout", this);
#endif
    result = POLL_TIMEOUT;
    goto Done;
  }
  // Handle all events.
#if DEBUG_POLL_AND_WAKE
  ALOGD("%p ~ pollOnce - handling events from %d fds", this, eventCount);
#endif
  for (int i = 0; i < eventCount; i++) {
    int fd = eventItems[i].data.fd;
    uint32_t epollEvents = eventItems[i].events;
    if (fd == mWakeEventFd) {//通知唤醒线程的事件
      if (epollEvents & EPOLLIN) {
        awoken();
      } else {
        ALOGW("Ignoring unexpected epoll events 0x%x on wake event fd.", epollEvents);
      }
    } else {
      ssize_t requestIndex = mRequests.indexOfKey(fd);//之前addFd的事件
      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;
        pushResponse(events, mRequests.valueAt(requestIndex));//放在mResponses中
      } else {
        ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "
            "no longer registered.", epollEvents, fd);
      }
    }
  }
Done: ;
  // Invoke pending message callbacks.
  mNextMessageUptime = LLONG_MAX;
  while (mMessageEnvelopes.size() != 0) {// 这块主要是c层的消息，java层的消息是自己管理的
    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();
#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS
        ALOGD("%p ~ pollOnce - sending message: handler=%p, what=%d",
            this, handler.get(), message.what);
#endif
        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;
    }
  }
  // Release lock.
  mLock.unlock();
  // Invoke all response callbacks.
  for (size_t i = 0; i < mResponses.size(); i++) {//这是之前addFd的事件的处理，主要是遍历mResponses，然后调用其回调
    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;
#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS
      ALOGD("%p ~ pollOnce - invoking fd event callback %p: fd=%d, events=0x%x, data=%p",
          this, response.request.callback.get(), fd, events, data);
#endif
      // 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;
} 

继续分析Looper的loop函数，可以增加自己的打印来调试代码，之前调用Message的target的dispatchMessage来分配消息

     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;//自己的打印
      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();
    }
  }

2.3 增加调试打印 

我们先来看自己添加打印，可以通过Lopper的setMessageLogging函数来打印

public void setMessageLogging(@Nullable Printer printer) {
    mLogging = printer;
  } 
Printer就是一个interface
public interface Printer {
  
  void println(String x);
}

2.4 java层消息分发处理 

再来看消息的分发，先是调用Handler的obtainMessage函数               

 Message msg = mHandler.obtainMessage(MSG_CHECK_WAKE_LOCK_ACQUIRE_TIMEOUT);
 msg.setAsynchronous(true);
 mHandler.sendMessageDelayed(msg, WAKE_LOCK_ACQUIRE_TOO_LONG_TIMEOUT); 

先看obtainMessage调用了Message的obtain函数

public final Message obtainMessage(int what)
  {
    return Message.obtain(this, what);
  } 

Message的obtain函数就是新建一个Message，然后其target就是设置成其Handler

public static Message obtain(Handler h, int what) {
    Message m = obtain();//就是新建一个Message
    m.target = h;
    m.what = what;
    return m;
  }

我们再联系之前分发消息 

msg.target.dispatchMessage(msg);最后就是调用Handler的dispatchMessage函数，最后在Handler中，最后会根据不同的情况对消息进行处理。

   public void dispatchMessage(Message msg) {
    if (msg.callback != null) {
      handleCallback(msg);//这种就是用post形式发送，带Runnable的
    } else {
      if (mCallback != null) {//这种是handler传参的时候就是传入了mCallback回调了
        if (mCallback.handleMessage(msg)) {
          return;
        }
      }
      handleMessage(msg);//最后就是在自己实现的handleMessage处理
    }
  }

2.3 java层 消息发送 

我们再看下java层的消息发送，主要也是调用Handler的sendMessage post之类函数，最终都会调用下面这个函数

   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);
  } 

我们再来看java层发送消息最终都会调用enqueueMessage函数

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
    msg.target = this;
    if (mAsynchronous) {
      msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis);
  } 

最终在enqueueMessage中，把消息加入消息队列，然后需要的话就调用c层的nativeWake函数

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;
  } 

我们看下这个native方法，最后也是调用了Looper的wake函数

 static void android_os_MessageQueue_nativeWake(JNIEnv* env, jclass clazz, jlong ptr) {
  NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
  nativeMessageQueue->wake();
}
void NativeMessageQueue::wake() {
  mLooper->wake();
} 

Looper类的wake，函数只是往mWakeEventfd中写了一些内容，这个fd只是通知而已，类似pipe，最后会把epoll_wait唤醒，线程就不阻塞了继续先发送c层消息，然后处理之前addFd的事件，然后处理java层的消息。 

void Looper::wake() {
#if DEBUG_POLL_AND_WAKE
  ALOGD("%p ~ wake", this);
#endif
  uint64_t inc = 1;
  ssize_t nWrite = TEMP_FAILURE_RETRY(write(mWakeEventFd, &inc, sizeof(uint64_t)));
  if (nWrite != sizeof(uint64_t)) {
    if (errno != EAGAIN) {
      ALOGW("Could not write wake signal, errno=%d", errno);
    }
  }
}

2.4 c层发送消息 

在c层也是可以发送消息的，主要是调用Looper的sendMessageAtTime函数，参数有有一个handler是一个回调，我们把消息放在mMessageEnvelopes中。

 void Looper::sendMessageAtTime(nsecs_t uptime, const sp<MessageHandler>& handler,
    const Message& message) {
#if DEBUG_CALLBACKS
  ALOGD("%p ~ sendMessageAtTime - uptime=%" PRId64 ", handler=%p, what=%d",
      this, uptime, handler.get(), message.what);
#endif
  size_t i = 0;
  { // acquire lock
    AutoMutex _l(mLock);
    size_t messageCount = mMessageEnvelopes.size();
    while (i < messageCount && uptime >= mMessageEnvelopes.itemAt(i).uptime) {
      i += 1;
    }
    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;
    }
  } // release lock
  // Wake the poll loop only when we enqueue a new message at the head.
  if (i == 0) {
    wake();
  }
} 

当在pollOnce中，在epoll_wait之后，会遍历mMessageEnvelopes中的消息，然后调用其handler的handleMessage函数

   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();
#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS
        ALOGD("%p ~ pollOnce - sending message: handler=%p, what=%d",
            this, handler.get(), message.what);
#endif
        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;
    }
  } 

有一个Looper_test.cpp文件，里面介绍了很多Looper的使用方法，我们来看下

   sp<StubMessageHandler> handler = new StubMessageHandler();
  mLooper->sendMessageAtTime(now + ms2ns(100), handler, Message(MSG_TEST1)); 
StubMessageHandler继承MessageHandler就必须实现handleMessage方法
class StubMessageHandler : public MessageHandler {
public:
  Vector<Message> messages;
  virtual void handleMessage(const Message& message) {
    messages.push(message);
  }
}; 

我们再顺便看下Message和MessageHandler类

 struct Message {
  Message() : what(0) { }
  Message(int what) : what(what) { }
  
  int what;
};

class MessageHandler : public virtual RefBase {
protected:
  virtual ~MessageHandler() { }
public:
  
  virtual void handleMessage(const Message& message) = 0;
};

2.5 c层addFd 

我们也可以在Looper.cpp的addFd中增加fd放入线程epoll中，当fd有数据来我们也可以处理相应的数据，下面我们先来看下addFd函数，我们注意其中有一个callBack回调

 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) {
#if DEBUG_CALLBACKS
  ALOGD("%p ~ addFd - fd=%d, ident=%d, events=0x%x, callback=%p, data=%p", this, fd, ident,
      events, callback.get(), data);
#endif
  if (!callback.get()) {
    if (! mAllowNonCallbacks) {
      ALOGE("Invalid attempt to set NULL callback but not allowed for this looper.");
      return -1;
    }
    if (ident < 0) {
      ALOGE("Invalid attempt to set NULL callback with ident < 0.");
      return -1;
    }
  } else {
    ident = POLL_CALLBACK;
  }
  { // acquire lock
    AutoMutex _l(mLock);
    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);
    ssize_t requestIndex = mRequests.indexOfKey(fd);
    if (requestIndex < 0) {
      int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, fd, & eventItem);//加入epoll
      if (epollResult < 0) {
        ALOGE("Error adding epoll events for fd %d, errno=%d", fd, errno);
        return -1;
      }
      mRequests.add(fd, request);//放入mRequests中
    } else {
      int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_MOD, fd, & eventItem);//更新
      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. This error may occur naturally
          // when a callback has the side-effect of closing the file descriptor
          // before returning and unregistering itself. Callback sequence number
          // checks further ensure that the race is benign.
          //
          // Unfortunately due to kernel limitations we need to rebuild the epoll
          // set from scratch because it may contain an old file handle that we are
          // now unable to remove since its file descriptor is no longer valid.
          // No such problem would have occurred if we were using the poll system
          // call instead, but that approach carries others disadvantages.
#if DEBUG_CALLBACKS
          ALOGD("%p ~ addFd - EPOLL_CTL_MOD failed due to file descriptor "
              "being recycled, falling back on EPOLL_CTL_ADD, errno=%d",
              this, errno);
#endif
          epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, fd, & eventItem);
          if (epollResult < 0) {
            ALOGE("Error modifying or adding epoll events for fd %d, errno=%d",
                fd, errno);
            return -1;
          }
          scheduleEpollRebuildLocked();
        } else {
          ALOGE("Error modifying epoll events for fd %d, errno=%d", fd, errno);
          return -1;
        }
      }
      mRequests.replaceValueAt(requestIndex, request);
    }
  } // release lock
  return 1;
} 

在pollOnce函数中，我们先寻找mRequests中匹配的fd，然后在pushResponse中新建一个Response，然后把Response和Request匹配起来。

     } else {
      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;
        pushResponse(events, mRequests.valueAt(requestIndex));
      } else {
        ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "
            "no longer registered.", epollEvents, fd);
      }
    } 

下面我们就会遍历mResponses中的Response，然后调用其request中的回调

   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;
#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS
      ALOGD("%p ~ pollOnce - invoking fd event callback %p: fd=%d, events=0x%x, data=%p",
          this, response.request.callback.get(), fd, events, data);
#endif
      // 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;
    }
  } 

同样我们再来看看Looper_test.cpp是如何使用的？

   Pipe pipe;
  StubCallbackHandler handler(true);
  handler.setCallback(mLooper, pipe.receiveFd, Looper::EVENT_INPUT); 

我们看下handler的setCallback函数

class CallbackHandler {
public:
  void setCallback(const sp<Looper>& looper, int fd, int events) {
    looper->addFd(fd, 0, events, staticHandler, this);//就是调用了looper的addFd函数，并且回调
  }
protected:
  virtual ~CallbackHandler() { }
  virtual int handler(int fd, int events) = 0;
private:
  static int staticHandler(int fd, int events, void* data) {//这个就是回调函数
    return static_cast<CallbackHandler*>(data)->handler(fd, events);
  }
};
class StubCallbackHandler : public CallbackHandler {
public:
  int nextResult;
  int callbackCount;
  int fd;
  int events;
  StubCallbackHandler(int nextResult) : nextResult(nextResult),
      callbackCount(0), fd(-1), events(-1) {
  }
protected:
  virtual int handler(int fd, int events) {//这个是通过回调函数再调到这里的
    callbackCount += 1;
    this->fd = fd;
    this->events = events;
    return nextResult;
  }
}; 

我们结合Looper的addFd一起来看，当callback是有的，我们新建一个SimpleLooperCallback

 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);
} 

这里的Looper_callbackFunc是一个typedef
typedef int (*Looper_callbackFunc)(int fd, int events, void* data);

我们再来看SimpleLooperCallback

 class SimpleLooperCallback : public LooperCallback {
protected:
  virtual ~SimpleLooperCallback();
public:
  SimpleLooperCallback(Looper_callbackFunc callback);
  virtual int handleEvent(int fd, int events, void* data);
private:
  Looper_callbackFunc mCallback;
};SimpleLooperCallback::SimpleLooperCallback(Looper_callbackFunc callback) :
    mCallback(callback) {
}
SimpleLooperCallback::~SimpleLooperCallback() {
}
int SimpleLooperCallback::handleEvent(int fd, int events, void* data) {
  return mCallback(fd, events, data);
} 

最后我们是调用callback->handleEvent(fd, events, data)，而callback就是SimpleLooperCallback，这里的data，之前传进来的就是CallbackHandler 的this指针
 因此最后就是调用了staticHandler，而data->handler，就是this->handler，最后是虚函数就调用到了StubCallbackHandler 的handler函数中了。 

当然我们也可以不用这么复杂，直接使用第二个addFd函数，当然callBack我们需要自己定义一个类来实现LooperCallBack类就行了，这样就简单多了。
 int addFd(int fd, int ident, int events, const sp<LooperCallback>& callback, void* data);

2.6 java层addFd 

一直以为只能在c层的Looper中才能addFd，原来在java层也通过jni做了这个功能。 

我们可以在MessageQueue中的addOnFileDescriptorEventListener来实现这个功能

   public void addOnFileDescriptorEventListener(@NonNull FileDescriptor fd,
      @OnFileDescriptorEventListener.Events int events,
      @NonNull OnFileDescriptorEventListener listener) {
    if (fd == null) {
      throw new IllegalArgumentException("fd must not be null");
    }
    if (listener == null) {
      throw new IllegalArgumentException("listener must not be null");
    }
    synchronized (this) {
      updateOnFileDescriptorEventListenerLocked(fd, events, listener);
    }
  }

我们再来看看OnFileDescriptorEventListener 这个回调

   public interface OnFileDescriptorEventListener {
    public static final int EVENT_INPUT = 1 << 0;
    public static final int EVENT_OUTPUT = 1 << 1;
    public static final int EVENT_ERROR = 1 << 2;
    
    @Retention(RetentionPolicy.SOURCE)
    @IntDef(flag=true, value={EVENT_INPUT, EVENT_OUTPUT, EVENT_ERROR})
    public @interface Events {}
    @Events int onFileDescriptorEvents(@NonNull FileDescriptor fd, @Events int events);
  }

接着调用了updateOnFileDescriptorEventListenerLocked函数

 private void updateOnFileDescriptorEventListenerLocked(FileDescriptor fd, int events,
      OnFileDescriptorEventListener listener) {
    final int fdNum = fd.getInt$();
    int index = -1;
    FileDescriptorRecord record = null;
    if (mFileDescriptorRecords != null) {
      index = mFileDescriptorRecords.indexOfKey(fdNum);
      if (index >= 0) {
        record = mFileDescriptorRecords.valueAt(index);
        if (record != null && record.mEvents == events) {
          return;
        }
      }
    }
    if (events != 0) {
      events |= OnFileDescriptorEventListener.EVENT_ERROR;
      if (record == null) {
        if (mFileDescriptorRecords == null) {
          mFileDescriptorRecords = new SparseArray<FileDescriptorRecord>();
        }
        record = new FileDescriptorRecord(fd, events, listener);//fd保存在FileDescriptorRecord对象
        mFileDescriptorRecords.put(fdNum, record);//mFileDescriptorRecords然后保存在
      } else {
        record.mListener = listener;
        record.mEvents = events;
        record.mSeq += 1;
      }
      nativeSetFileDescriptorEvents(mPtr, fdNum, events);//调用native函数
    } else if (record != null) {
      record.mEvents = 0;
      mFileDescriptorRecords.removeAt(index);
    }
  } 

native最后调用了NativeMessageQueue的setFileDescriptorEvents函数 

static void android_os_MessageQueue_nativeSetFileDescriptorEvents(JNIEnv* env, jclass clazz,
    jlong ptr, jint fd, jint events) {
  NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
  nativeMessageQueue->setFileDescriptorEvents(fd, events);
}

setFileDescriptorEvents函数，这个addFd就是调用的第二个addFd，因此我们可以肯定NativeMessageQueue继承了LooperCallback

 void NativeMessageQueue::setFileDescriptorEvents(int fd, int events) {
  if (events) {
    int looperEvents = 0;
    if (events & CALLBACK_EVENT_INPUT) {
      looperEvents |= Looper::EVENT_INPUT;
    }
    if (events & CALLBACK_EVENT_OUTPUT) {
      looperEvents |= Looper::EVENT_OUTPUT;
    }
    mLooper->addFd(fd, Looper::POLL_CALLBACK, looperEvents, this,
        reinterpret_cast<void*>(events));
  } else {
    mLooper->removeFd(fd);
  }
}

果然是，需要实现handleEvent函数

 class NativeMessageQueue : public MessageQueue, public LooperCallback {
public:
  NativeMessageQueue();
  virtual ~NativeMessageQueue();
  virtual void raiseException(JNIEnv* env, const char* msg, jthrowable exceptionObj);
  void pollOnce(JNIEnv* env, jobject obj, int timeoutMillis);
  void wake();
  void setFileDescriptorEvents(int fd, int events);
  virtual int handleEvent(int fd, int events, void* data);

handleEvent就是在looper中epoll_wait之后，当我们增加的fd有数据就会调用这个函数

 int NativeMessageQueue::handleEvent(int fd, int looperEvents, void* data) {
  int events = 0;
  if (looperEvents & Looper::EVENT_INPUT) {
    events |= CALLBACK_EVENT_INPUT;
  }
  if (looperEvents & Looper::EVENT_OUTPUT) {
    events |= CALLBACK_EVENT_OUTPUT;
  }
  if (looperEvents & (Looper::EVENT_ERROR | Looper::EVENT_HANGUP | Looper::EVENT_INVALID)) {
    events |= CALLBACK_EVENT_ERROR;
  }
  int oldWatchedEvents = reinterpret_cast<intptr_t>(data);
  int newWatchedEvents = mPollEnv->CallIntMethod(mPollObj,
      gMessageQueueClassInfo.dispatchEvents, fd, events); //调用回调
  if (!newWatchedEvents) {
    return 0; // unregister the fd
  }
  if (newWatchedEvents != oldWatchedEvents) {
    setFileDescriptorEvents(fd, newWatchedEvents);
  }
  return 1;
}

最后在java的MessageQueue中的dispatchEvents就是在jni层反调过来的，然后调用之前注册的回调函数

// Called from native code.
  private int dispatchEvents(int fd, int events) {
    // Get the file descriptor record and any state that might change.
    final FileDescriptorRecord record;
    final int oldWatchedEvents;
    final OnFileDescriptorEventListener listener;
    final int seq;
    synchronized (this) {
      record = mFileDescriptorRecords.get(fd);//通过fd得到FileDescriptorRecord 
      if (record == null) {
        return 0; // spurious, no listener registered
      }
      oldWatchedEvents = record.mEvents;
      events &= oldWatchedEvents; // filter events based on current watched set
      if (events == 0) {
        return oldWatchedEvents; // spurious, watched events changed
      }
      listener = record.mListener;
      seq = record.mSeq;
    }
    // Invoke the listener outside of the lock.
    int newWatchedEvents = listener.onFileDescriptorEvents(//listener回调
        record.mDescriptor, events);
    if (newWatchedEvents != 0) {
      newWatchedEvents |= OnFileDescriptorEventListener.EVENT_ERROR;
    }
    // Update the file descriptor record if the listener changed the set of
    // events to watch and the listener itself hasn't been updated since.
    if (newWatchedEvents != oldWatchedEvents) {
      synchronized (this) {
        int index = mFileDescriptorRecords.indexOfKey(fd);
        if (index >= 0 && mFileDescriptorRecords.valueAt(index) == record
            && record.mSeq == seq) {
          record.mEvents = newWatchedEvents;
          if (newWatchedEvents == 0) {
            mFileDescriptorRecords.removeAt(index);
          }
        }
      }
    }
    // Return the new set of events to watch for native code to take care of.
    return newWatchedEvents;
  }

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