Android6.0 消息機制原理解析

消息都是存放在一個消息隊列中去，而消息循環線程就是圍繞這個消息隊列進入一個無限循環的，直到線程退出。如果隊列中有消息，消息循環線程就會把它取出來，並分發給相應的Handler進行處理；如果隊列中沒有消息，消息循環線程就會進入空閒等待狀態，等待下一個消息的到來。在編寫Android應用程序時，當程序執行的任務比較繁重時，為了不阻塞UI主線程而導致ANR的發生，我們通常的做法的創建一個子線程來完成特定的任務。在創建子線程時，有兩種選擇，一種通過創建Thread對象來創建一個無消息循環的子線程；還有一種就是創建一個帶有消息循環的子線程，而創建帶有消息循環的子線程由於兩種實現方法，一種是直接利用Android給我們封裝好的HandlerThread類來直接生成一個帶有消息循環的線程對象，另一種方法是在實現線程的run()方法內使用以下方式啟動一個消息循環： 

一、消息機制使用 

通常消息都是有一個消息線程和一個Handler組成，下面我們看PowerManagerService中的一個消息Handler：        

 mHandlerThread = new ServiceThread(TAG,
        Process.THREAD_PRIORITY_DISPLAY, false /*allowIo*/);
    mHandlerThread.start();
    mHandler = new PowerManagerHandler(mHandlerThread.getLooper()); 

這裡的ServiceThread就是一個HandlerThread，創建Handler的時候，必須把HandlerThread的looper傳進去，否則就是默認當前線程的looper。 

而每個handler，大致如下：

   private final class PowerManagerHandler extends Handler {
    public PowerManagerHandler(Looper looper) {
      super(looper, null, true /*async*/);
    }

    @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、消息機制原理
那我們先來看下HandlerThread的主函數run函數： 

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

再來看看Lopper的prepare函數，最後新建了一個Looper對象，並且放在線程的局部變量中。

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的構造函數中創建了MessageQueue

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

我們再來看下MessageQueue的構造函數，其中nativeInit是一個native方法，並且把返回值保存在mPtr顯然是用long型變量保存的指針

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

native函數中主要創建了NativeMessageQueue對象，並且把指針變量返回了。

 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構造函數就是獲取mLooper，如果沒有就是新建一個Looper 

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

然後我們再看下Looper的構造函數，顯示調用了eventfd創建了一個fd，eventfd它的主要是用於進程或者線程間的通信，我們可以看下這篇博客eventfd介紹

 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層創建epoll 

我們再來看下rebuildEpollLocked函數，創建了epoll，並且把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);
    }
  }
} 

繼續回到HandlerThread的run函數，我們繼續分析Looper的loop函數

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

我們看看Looper的loop函數：

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這個函數會阻塞，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類的next函數主要是調用了nativePollOnce函數，後面就是從消息隊列中取出一個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;//之前保留的指針
    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); 

下面我們主要看下nativePollOnce這個native函數，把之前的指針強制轉換成NativeMessageQueue，然後調用其pollOnce函數

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層epoll_wait阻塞 

pollOnce函數，這個函數前面的while一般都沒有只是處理了indent大於0的情況，這種情況一般沒有，所以我們可以直接看pollInner函數

 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函數主要就是調用epoll_wait阻塞，並且java層會計算每次阻塞的時間傳到c層，等待有mWakeEventFd或者之前addFd的fd有事件過來，才會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主要線程阻塞在這，這個阻塞的時間也是有java層傳過來的

  // 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) {//通知喚醒線程的事件
      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) {// 這塊主要是c層的消息，java層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++) {//這是之前addFd的事件的處理，主要是遍歷mResponses，然後調用其回調
    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;
} 

繼續分析Looper的loop函數，可以增加自己的打印來調試代碼，之前調用Message的target的dispatchMessage來分配消息

     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 增加調試打印 

我們先來看自己添加打印，可以通過Lopper的setMessageLogging函數來打印

public void setMessageLogging(@Nullable Printer printer) {
    mLogging = printer;
  } 
Printer就是一個interface
 
public interface Printer {
  /**
   * Write a line of text to the output. There is no need to terminate
   * the given string with a newline.
   */
  void println(String x);
}

2.4 java層消息分發處理 

再來看消息的分發，先是調用Handler的obtainMessage函數               

 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調用了Message的obtain函數

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

Message的obtain函數就是新建一個Message，然後其target就是設置成其Handler

public static Message obtain(Handler h, int what) {
    Message m = obtain();//就是新建一個Message
    m.target = h;
    m.what = what;

    return m;
  }
 

我們再聯系之前分發消息 

msg.target.dispatchMessage(msg);最後就是調用Handler的dispatchMessage函數，最後在Handler中，最後會根據不同的情況對消息進行處理。

   public void dispatchMessage(Message msg) {
    if (msg.callback != null) {
      handleCallback(msg);//這種就是用post形式發送，帶Runnable的
    } else {
      if (mCallback != null) {//這種是handler傳參的時候就是傳入了mCallback回調了
        if (mCallback.handleMessage(msg)) {
          return;
        }
      }
      handleMessage(msg);//最後就是在自己實現的handleMessage處理
    }
  }

2.3 java層 消息發送 

我們再看下java層的消息發送，主要也是調用Handler的sendMessage post之類函數，最終都會調用下面這個函數

   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);
  } 

我們再來看java層發送消息最終都會調用enqueueMessage函數

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

最終在enqueueMessage中，把消息加入消息隊列，然後需要的話就調用c層的nativeWake函數

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;
  } 

我們看下這個native方法，最後也是調用了Looper的wake函數

 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類的wake，函數只是往mWakeEventfd中寫了一些內容，這個fd只是通知而已，類似pipe，最後會把epoll_wait喚醒，線程就不阻塞了繼續先發送c層消息，然後處理之前addFd的事件，然後處理java層的消息。 

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層發送消息 

在c層也是可以發送消息的，主要是調用Looper的sendMessageAtTime函數，參數有有一個handler是一個回調，我們把消息放在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();
  }
} 

當在pollOnce中，在epoll_wait之後，會遍歷mMessageEnvelopes中的消息，然後調用其handler的handleMessage函數

   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;
    }
  } 

有一個Looper_test.cpp文件，裡面介紹了很多Looper的使用方法，我們來看下

   sp<StubMessageHandler> handler = new StubMessageHandler();
  mLooper->sendMessageAtTime(now + ms2ns(100), handler, Message(MSG_TEST1)); 
StubMessageHandler繼承MessageHandler就必須實現handleMessage方法
 
class StubMessageHandler : public MessageHandler {
public:
  Vector<Message> messages;

  virtual void handleMessage(const Message& message) {
    messages.push(message);
  }
}; 

我們再順便看下Message和MessageHandler類

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

  /* The message type. (interpretation is left up to the handler) */
  int what;
};


/**
 * Interface for a Looper message handler.
 *
 * The Looper holds a strong reference to the message handler whenever it has
 * a message to deliver to it. Make sure to call Looper::removeMessages
 * to remove any pending messages destined for the handler so that the handler
 * can be destroyed.
 */
class MessageHandler : public virtual RefBase {
protected:
  virtual ~MessageHandler() { }

public:
  /**
   * Handles a message.
   */
  virtual void handleMessage(const Message& message) = 0;
};

2.5 c層addFd 

我們也可以在Looper.cpp的addFd中增加fd放入線程epoll中，當fd有數據來我們也可以處理相應的數據，下面我們先來看下addFd函數，我們注意其中有一個callBack回調

 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函數中，我們先尋找mRequests中匹配的fd，然後在pushResponse中新建一個Response，然後把Response和Request匹配起來。

     } 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);
      }
    } 

下面我們就會遍歷mResponses中的Response，然後調用其request中的回調

   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;
    }
  } 

同樣我們再來看看Looper_test.cpp是如何使用的？

   Pipe pipe;
  StubCallbackHandler handler(true);

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

我們看下handler的setCallback函數

class CallbackHandler {
public:
  void setCallback(const sp<Looper>& looper, int fd, int events) {
    looper->addFd(fd, 0, events, staticHandler, this);//就是調用了looper的addFd函數，並且回調
  }

protected:
  virtual ~CallbackHandler() { }

  virtual int handler(int fd, int events) = 0;

private:
  static int staticHandler(int fd, int events, void* data) {//這個就是回調函數
    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) {//這個是通過回調函數再調到這裡的
    callbackCount += 1;
    this->fd = fd;
    this->events = events;
    return nextResult;
  }
}; 

我們結合Looper的addFd一起來看，當callback是有的，我們新建一個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);
} 

這裡的Looper_callbackFunc是一個typedef
typedef int (*Looper_callbackFunc)(int fd, int events, void* data);

我們再來看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);
} 

最後我們是調用callback->handleEvent(fd, events, data)，而callback就是SimpleLooperCallback，這裡的data，之前傳進來的就是CallbackHandler 的this指針
 因此最後就是調用了staticHandler，而data->handler，就是this->handler，最後是虛函數就調用到了StubCallbackHandler 的handler函數中了。 

當然我們也可以不用這麼復雜，直接使用第二個addFd函數，當然callBack我們需要自己定義一個類來實現LooperCallBack類就行了，這樣就簡單多了。
 int addFd(int fd, int ident, int events, const sp<LooperCallback>& callback, void* data);

2.6 java層addFd 

一直以為只能在c層的Looper中才能addFd，原來在java層也通過jni做了這個功能。 

我們可以在MessageQueue中的addOnFileDescriptorEventListener來實現這個功能

   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);
    }
  }

我們再來看看OnFileDescriptorEventListener 這個回調

   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;

    /** @hide */
    @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);
  }

接著調用了updateOnFileDescriptorEventListenerLocked函數

 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對象
        mFileDescriptorRecords.put(fdNum, record);//mFileDescriptorRecords然後保存在
      } else {
        record.mListener = listener;
        record.mEvents = events;
        record.mSeq += 1;
      }
      nativeSetFileDescriptorEvents(mPtr, fdNum, events);//調用native函數
    } else if (record != null) {
      record.mEvents = 0;
      mFileDescriptorRecords.removeAt(index);
    }
  } 

native最後調用了NativeMessageQueue的setFileDescriptorEvents函數 

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函數，這個addFd就是調用的第二個addFd，因此我們可以肯定NativeMessageQueue繼承了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，需要實現handleEvent函數

 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之後，當我們增加的fd有數據就會調用這個函數

 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); //調用回調
  if (!newWatchedEvents) {
    return 0; // unregister the fd
  }
  if (newWatchedEvents != oldWatchedEvents) {
    setFileDescriptorEvents(fd, newWatchedEvents);
  }
  return 1;
}

最後在java的MessageQueue中的dispatchEvents就是在jni層反調過來的，然後調用之前注冊的回調函數

// 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);//通過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回調
        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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