Android c++层 Handler分析和使用

1. C++层的Looper分析

文件：

platform/system/core/include/utils/Looper.h
platform/system/core/libutils/Looper.cpp

• 前面的文章 Android Looper与Message 中最后
  可以看到，Java层的MessageQueue的等待/唤醒机制其实是由底层的Looper
  中对epoll进行操作来实现的。
• 其实在同一个逻辑流程中， 也对C++层的Message做了通知处理。
  (C++中的Looper对Java层MessageQueue的影响就只有等待唤醒机制，没有其他的逻辑影响)

1. C++层的Handler： MessageHandler

1. MessageHandler

//system/core/include/utils/Looper.h
/**
 * 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. Message

/**
 * A message that can be posted to a Looper.
 */
struct Message {
    Message() : what(0) { }
    Message(int what) : what(what) { }

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

---

2. C++ Messagehandler的使用

Messagehandler在使用上其实也很简单：

//1. 导入Looper.h
#include 

//2. 获取当前线程的Looper对象：
sp<Looper> looper = Looper::getForThread();

//3. 发送Message到当前线程的Looper队列中，等待回调：
mHandler = new MHandler();
msg = new Message(1/*what*/);
 looper ->sendMessage(delay, mHandler/*MessageHandler*/, msg/*Message*/);

//-------------------------------------------------------
//mHandler
class MHandler: public MessageHandler {
public:
  virtual void handleMessage(const Message& message) override;
}

 void MHandler::handleMessage(const Message& message) {
     //最后会调用到这里
 }

---

3. Looper::sendMessage 流程

1. 将事件封装入队

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) {
#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; //按照嗲用时间进行排序，计算要插入的位置
        }

        //将Handler和Message封装成MessageEnvelope对象， 然后插入到mMessageEnvelopes队列中；
        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();
    }
}

2. pollOnce之后，进行对Handler和Message的处理：

int Looper::pollInner(int timeoutMillis) {
    ...
//-----------------1. 对Java层MQ阻塞事件和addFd函数添加的fd中的事件进行处理---------------------------------------------------------
	for (int i = 0; i < eventCount; i++) {
        int fd = eventItems[i].data.fd;
        uint32_t epollEvents = eventItems[i].events;
        if (fd == mWakeEventFd) { //mWakeEventFd 用来解除Java层MQ的阻塞状态；
            if (epollEvents & EPOLLIN) {
                awoken();
            } else {
                ALOGW("Ignoring unexpected epoll events 0x%x on wake event fd.", epollEvents);
            }
        } else {
            //这里是将addFd中的Request事件封装为Resonse事件， 并添加Response事件，即对addFd函数添加的fd中的事件响应；
            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)); //添加Request的Response事件
            } else {
                ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "
                        "no longer registered.", epollEvents, fd);
            }
        }
    }
Done: ;

//----------------------2. 对C++层Handler（MessageHandler）事件进行处理-----------------------------------------------

    // Invoke pending message callbacks.
    mNextMessageUptime = LLONG_MAX;
    while (mMessageEnvelopes.size() != 0) {
        nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
        const MessageEnvelope& messageEnvelope = mMessageEnvelopes.itemAt(0);//取出队列中的MessageEnvelope
        if (messageEnvelope.uptime <= now) { //已经到达运行时间
            { // 获取Handler和Message
                sp<MessageHandler> handler = messageEnvelope.handler;
                Message message = messageEnvelope.message;
                //移除队列中的首位MessageEnvelope项
                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
                //回调handleMessage方法；
                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();

//--------------------------3. 对addFd函数中request注册的回调进行处理(Response事件处理)---------------------------------------------------
    // Invoke all response callbacks.
    for (size_t i = 0; i < mResponses.size(); i++) {
        //取出每个Response对象
        Response& response = mResponses.editItemAt(i);
        if (response.request.ident == POLL_CALLBACK) {
            //获取回调需要的参数
            int fd = response.request.fd;
            int events = response.events;
            void* data = response.request.data;

            ...
            
            //回调handleEvent方法；
            int callbackResult = response.request.callback->handleEvent(fd, events, data);
            if (callbackResult == 0) {
                removeFd(fd, response.request.seq);
            }
            //清除CallBack
            response.request.callback.clear();
            result = POLL_CALLBACK;
        }
    }

4. addFd函数

addFd函数，在调用时， 传入一个需要添加的fd到对应Looper的epoll事件监听池中，对fd中感兴趣的事件进行监听，监听的结果会返回到传入的监听器中，见上面代码中分析的第三点；

1. addFd时需要传入的CallBack监听器：

///system/core/include/utils/Looper.h
/**
 * A looper callback.
 */
class LooperCallback : public virtual RefBase {
protected:
    virtual ~LooperCallback() { }

public:
    /**
     * Handles a poll event for the given file descriptor.
     * It is given the file descriptor it is associated with,
     * a bitmask of the poll events that were triggered (typically EVENT_INPUT),
     * and the data pointer that was originally supplied.
     *
     * Implementations should return 1 to continue receiving callbacks, or 0
     * to have this file descriptor and callback unregistered from the looper.
     */
    virtual int handleEvent(int fd, int events, void* data) = 0;
};

2. SimpleLooperCallback

SimpleLooperCallback 时对LooperCallback的简单封装， 目的主要是为了addFd的一个重载，用于函数式调用

/**
 * Wraps a Looper_callbackFunc function pointer.
 */
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;
};

4. addFd函数：

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); //SimpleLooperCallback即用于该重载， 为了简单化callBack的传入类型， 之间穿入一个函数指针；
}

//最终调用的这一个addFd
int Looper::addFd(int fd, int ident, int events, const sp<LooperCallback>& callback, void* data) {
    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);

        //构造Request对象
        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);
      
        //处理对fd事件的监听， 并将其加入到epoll事件池中；
        if (requestIndex < 0) {
            //如果不存在，则做添加操作
            int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, fd, & eventItem);
            if (epollResult < 0) {
                ALOGE("Error adding epoll events for fd %d: %s", fd, strerror(errno));
                return -1;
            }
            mRequests.add(fd, request);
        } else {
            //如果已经存在，则做修改操作
            int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_MOD, fd, & eventItem);
            if (epollResult < 0) {
                if (errno == ENOENT) {
                   /*...*/
                   //修改失败，则做添加操作
                    epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, fd, & eventItem);
                    if (epollResult < 0) {
                        ALOGE("Error modifying or adding epoll events for fd %d: %s",
                                fd, strerror(errno));
                        return -1;
                    }
                    scheduleEpollRebuildLocked();
                } else {
                    ALOGE("Error modifying epoll events for fd %d: %s", fd, strerror(errno));
                    return -1;
                }
            }
            //更新Request；
            mRequests.replaceValueAt(requestIndex, request);
        }
    } // release lock
    return 1;
}

2. 总结：

1. 逻辑相关：
   Looper中的事件队列机制， 集成了两种事件处理监听机制：

1. addFd： 对添加的文件描述符中的(写入/写出)事件仅从监听处理；
2. MessageHandler： C++层的Handler机制；

2.非逻辑相关：

• Looper中的mWakeEventFd只是对Java层MQ的阻塞/唤醒机制做控制，而不影响Java MQ的逻辑。
• Java MQ只是借助了该Looper进行阻塞实现。Looper中的mWakeEventFd只是对Java层MQ的阻塞/唤醒机制做控制，而不影响- - – Java MQ的逻辑。
• Java MQ只是借助了该Looper进行阻塞实现。