Android nativePollOnce函数解析

static void android_os_MessageQueue_nativePollOnce(JNIEnv*env, jobject obj,
    jintptr, jint timeoutMillis)
   NativeMessageQueue*nativeMessageQueue =
              reinterpret_cast<NativeMessageQueue*>(ptr);
  //取出NativeMessageQueue对象，并调用它的pollOnce
  nativeMessageQueue->pollOnce(timeoutMillis);
}
//分析pollOnce函数
void NativeMessageQueue::pollOnce(inttimeoutMillis) {
  mLooper->pollOnce(timeoutMillis); //重任传递到Looper的pollOnce函数
}

inline int pollOnce(int timeoutMillis) {
    return pollOnce(timeoutMillis, NULL, NULL, NULL);
}

 
int epollHandle = epoll_create(10);


 structepoll_event listenEvent; //先定义一个event
 
 listenEvent.events= EPOLLIN;//指定该句柄的可读事件
 //epoll_event中有一个联合体叫data，用来存储上下文数据，本例的上下文数据就是句柄自己
 listenEvent.data.fd= listenEvent;

 epoll_ctl(epollHandle,EPOLL_CTL_ADD,listener,&listenEvent);
 
struct epoll_eventresultEvents[10];
 inttimeout = -1;
 while(1)
 {
  
 int nfds= epoll_wait(epollHandle, resultEvents, 10, timeout);
 if(nfds== -1)
 {
  // epoll_wait发生了错误
 }
 elseif(nfds == 0)
 {
  //发生超时，期间没有发生任何事件
 }
 else
 {
  //resultEvents用于返回那些发生了事件的信息
  for(int i = 0; i < nfds; i++)
  {
    struct epoll_event & event =resultEvents[i];
    if(event & EPOLLIN)
   {
     
   }
    .......//其他处理 
  }
 }

}

int Looper::pollOnce(int timeoutMillis, int*outFd, int* outEvents,
void** outData) {
  intresult = 0;
  for (;;){ //一个无限循环
  //mResponses是一个Vector，这里首先需要处理response
    while (mResponseIndex < mResponses.size()) {
      const Response& response = mResponses.itemAt(mResponseIndex++);
      ALooper_callbackFunc callback = response.request.callback;
      if (!callback) {//首先处理那些没有callback的Response
        int ident = response.request.ident; //ident是这个Response的id
        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;
        //实际上，对于没有callback的Response，pollOnce只是返回它的
       //ident，并没有实际做什么处理。因为没有callback，所以系统也不知道如何处理
        return ident;
      }
    }
 
    if(result != 0) {
     if (outFd != NULL) *outFd = 0;
      if (outEvents != NULL) *outEvents = NULL;
      if (outData != NULL) *outData = NULL;
      return result;
    }
    //调用pollInner函数。注意，它在for循环内部
    result = pollInner(timeoutMillis);
  }
}

int Looper::pollInner(int timeoutMillis) {
  
  if(timeoutMillis != 0 && mNextMessageUptime != LLONG_MAX) {
    nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
    ......//根据Native Message的信息计算此次需要等待的时间
    timeoutMillis= messageTimeoutMillis;
   }
  intresult = ALOOPER_POLL_WAKE;
  mResponses.clear();
  mResponseIndex = 0;
#ifdef LOOPER_USES_EPOLL //我们只讨论使用epoll进行I/O复用的方式
  structepoll_event eventItems[EPOLL_MAX_EVENTS];
  //调用epoll_wait，等待感兴趣的事件或超时发生
  inteventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS,
                   timeoutMillis);
#else
   ......//使用别的方式进行I/O复用
#endif
  //从epoll_wait返回，这时候一定发生了什么事情
  mLock.lock();
  if(eventCount < 0) { //返回值小于零，表示发生错误
    if(errno == EINTR) {
      goto Done;
    }
    //设置result为ALLOPER_POLL_ERROR,并跳转到Done
    result = ALOOPER_POLL_ERROR;
    gotoDone;
  }
 
  //eventCount为零，表示发生超时，因此直接跳转到Done
  if(eventCount == 0) {
   result = ALOOPER_POLL_TIMEOUT;
    gotoDone;
  }
#ifdef LOOPER_USES_EPOLL
  //根据epoll的用法，此时的eventCount表示发生事件的个数
  for (inti = 0; i < eventCount; i++) {
    intfd = eventItems[i].data.fd;
    uint32_t epollEvents = eventItems[i].events;
    
    if(fd == mWakeReadPipeFd) {
      if (epollEvents & EPOLLIN) {
        //awoken函数直接读取并清空管道数据，读者可自行研究该函数
        awoken();
      }
     ......
    }else {
      
      ssize_t requestIndex = mRequests.indexOfKey(fd);
      if (requestIndex >= 0) {
        int events = 0;
        //将epoll返回的事件转换成上层LOOPER使用的事件
        if (epollEvents & EPOLLIN) events |= ALOOPER_EVENT_INPUT;
        if (epollEvents & EPOLLOUT) events |= ALOOPER_EVENT_OUTPUT;
        if (epollEvents & EPOLLERR) events |= ALOOPER_EVENT_ERROR;
        if (epollEvents & EPOLLHUP) events |= ALOOPER_EVENT_HANGUP;
        //每处理一个Request，就相应构造一个Response
        pushResponse(events, mRequests.valueAt(requestIndex));
      } 
      ......
    }
  }
Done: ;
#else
   ......
#endif
  //除了处理Request外，还处理Native的Message
  mNextMessageUptime = LLONG_MAX;
  while(mMessageEnvelopes.size() != 0) {
    nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
    const MessageEnvelope& messageEnvelope =mMessageEnvelopes.itemAt(0);
    if(messageEnvelope.uptime <= now) {
      {
        sp<MessageHandler> handler = messageEnvelope.handler;
        Message message = messageEnvelope.message;
        mMessageEnvelopes.removeAt(0);
        mSendingMessage = true;
        mLock.unlock();
        //调用Native的handler处理Native的Message
        //从这里也可看出NativeMessage和Java层的Message没有什么关系
        handler->handleMessage(message);
      }
      mLock.lock();
      mSendingMessage = false;
      result = ALOOPER_POLL_CALLBACK;
    }else {
      mNextMessageUptime = messageEnvelope.uptime;
      break;
    }
  }
 
  mLock.unlock();
  //处理那些带回调函数的Response
  for(size_t i = 0; i < mResponses.size(); i++) {
    const Response& response = mResponses.itemAt(i);
    ALooper_callbackFunc callback = response.request.callback;
    if(callback) {//有了回调函数，就能知道如何处理所发生的事情了
      int fd = response.request.fd;
      int events = response.events;
      void* data = response.request.data;
      //调用回调函数处理所发生的事件
      int callbackResult = callback(fd, events, data);
      if (callbackResult == 0) {
        //callback函数的返回值很重要，如果为0，表明不需要再次监视该文件句柄
        removeFd(fd);
      }
      result = ALOOPER_POLL_CALLBACK;
    }
  }
  returnresult;
}

static jint
loadNativeCode_native(JNIEnv* env, jobject clazz,jstring path,
             jstring funcName,jobject messageQueue,
             jstring internalDataDir, jstring obbDir,
             jstring externalDataDir, int sdkVersion,
             jobject jAssetMgr, jbyteArraysavedState)
{
 ......
 
 code->looper->addFd(code->mainWorkRead,0,
             ALOOPER_EVENT_INPUT,mainWorkCallback, code);
 ......
}

int Looper::addFd(int fd, int ident, int events,
           ALooper_callbackFunccallback, void* data) {
  if (!callback) {
     //判断该Looper是否支持不带回调函数的文件句柄添加。一般不支持，因为没有回调函数
     //Looper也不知道如何处理该文件句柄上发生的事情
     if(! mAllowNonCallbacks) {
      return -1;
    }
   ......
  }
 
  #ifdefLOOPER_USES_EPOLL
  intepollEvents = 0;
  //将用户的事件转换成epoll使用的值
  if(events & ALOOPER_EVENT_INPUT) epollEvents |= EPOLLIN;
  if(events & ALOOPER_EVENT_OUTPUT) epollEvents |= EPOLLOUT;
 
  {
    AutoMutex _l(mLock);
    Request request; //创建一个Request对象
    request.fd = fd; //保存fd
    request.ident = ident; //保存id
    request.callback = callback; //保存callback
    request.data = data; //保存用户自定义数据
 
    struct epoll_event eventItem;
    memset(& eventItem, 0, sizeof(epoll_event));
    eventItem.events = epollEvents;
    eventItem.data.fd = fd;
    //判断该Request是否已经存在，mRequests以fd作为key值
    ssize_t requestIndex = mRequests.indexOfKey(fd);
    if(requestIndex < 0) {
      //如果是新的文件句柄，则需要为epoll增加该fd
      int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, fd, & eventItem);
      ......
      //保存Request到mRequests键值数组
      mRequests.add(fd, request);
    }else {
      //如果之前加过，那么就修改该监听句柄的一些信息
      int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_MOD, fd, &eventItem);
      ......
      mRequests.replaceValueAt(requestIndex, request);
    }
  }
#else
  ......
#endif
  return1;
}

void Looper::pushResponse(int events, constRequest& request) {
  Responseresponse;
  response.events = events;
  response.request = request; //其实很简单，就是保存所发生的事情和对应的Request
  mResponses.push(response); //然后保存到mResponse数组
}

void Looper::sendMessage(constsp<MessageHandler>& handler,
               constMessage& message) {
  //Native的sendMessage函数必须同时传递一个Handler
  nsecs_tnow = systemTime(SYSTEM_TIME_MONOTONIC);
  sendMessageAtTime(now, handler, message); //调用sendMessageAtTime
}
void Looper::sendMessageAtTime(nsecs_t uptime,
                   const sp<MessageHandler>& handler,
                   const Message& message) {
  size_t i= 0;
  { //acquire lock
    AutoMutex _l(mLock);
 
    size_t messageCount = mMessageEnvelopes.size();
    //按时间排序，将消息插入到正确的位置上
    while (i < messageCount &&
        uptime >= mMessageEnvelopes.itemAt(i).uptime) {
      i += 1;
    }
    
    MessageEnvelope messageEnvelope(uptime, handler, message);
    mMessageEnvelopes.insertAt(messageEnvelope, i, 1);
    //mSendingMessage和Java层中的那个mBlocked一样，是一个小小的优化措施
    if(mSendingMessage) {
      return;
    }
  }
  //唤醒epoll_wait，让它处理消息
  if (i ==0) {
    wake();
  }
}