Android消息机制不完全解析（下）

接着上一篇文章Android消息机制不完全解析（上），接着看C++部分的实现。

首先，看看在/frameworks/base/core/jni/android_os_MessageQueue.cpp文件中看看android.os.MessageQueue类中的四个原生函数的实现：

static void android_os_MessageQueue_nativeInit(JNIEnv* env, jobject obj) {    NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();//构造NativeMessageQueue实例    if (!nativeMessageQueue) {        jniThrowRuntimeException(env, "Unable to allocate native queue");        return;    }    nativeMessageQueue->incStrong(env);//强引用+1    android_os_MessageQueue_setNativeMessageQueue(env, obj, nativeMessageQueue);}static void android_os_MessageQueue_nativeDestroy(JNIEnv* env, jobject obj) {    NativeMessageQueue* nativeMessageQueue =            android_os_MessageQueue_getNativeMessageQueue(env, obj);    if (nativeMessageQueue) {        android_os_MessageQueue_setNativeMessageQueue(env, obj, NULL);        nativeMessageQueue->decStrong(env);//强引用-1，实际上会导致释放NativeMessageQueue实例    }}static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj,        jint ptr, jint timeoutMillis) {    NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);//指针强制转换    nativeMessageQueue->pollOnce(env, timeoutMillis);//调用nativeMessageQueue的pollonce函数}static void android_os_MessageQueue_nativeWake(JNIEnv* env, jobject obj, jint ptr) {    NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);    return nativeMessageQueue->wake();//调用nativeMessageQueue的wake函数}

从代码中，可以看到这四个函数的实现都是依赖于NativeMessageQueue类。不过，在开始解析NativeMessageQueue之前，我们再看一些有意思的代码：

static void android_os_MessageQueue_setNativeMessageQueue(JNIEnv* env, jobject messageQueueObj,        NativeMessageQueue* nativeMessageQueue) {    env->SetIntField(messageQueueObj, gMessageQueueClassInfo.mPtr,             reinterpret_cast<jint>(nativeMessageQueue));//把nativeMessageQueue的实例地址强转为java的int类型并保存到gMessageQueueClassInfo.mPtr中}

那么gMessageQueueClassInfo.mPtr是什么呢？

static JNINativeMethod gMessageQueueMethods[] = {    /* name, signature, funcPtr */    { "nativeInit", "()V", (void*)android_os_MessageQueue_nativeInit },    { "nativeDestroy", "()V", (void*)android_os_MessageQueue_nativeDestroy },    { "nativePollOnce", "(II)V", (void*)android_os_MessageQueue_nativePollOnce },    { "nativeWake", "(I)V", (void*)android_os_MessageQueue_nativeWake }};#define FIND_CLASS(var, className) \        var = env->FindClass(className); \        LOG_FATAL_IF(! var, "Unable to find class " className);#define GET_FIELD_ID(var, clazz, fieldName, fieldDescriptor) \        var = env->GetFieldID(clazz, fieldName, fieldDescriptor); \        LOG_FATAL_IF(! var, "Unable to find field " fieldName);//这个函数在Android启动的时候，会被系统调用int register_android_os_MessageQueue(JNIEnv* env) {    int res = jniRegisterNativeMethods(env, "android/os/MessageQueue",            gMessageQueueMethods, NELEM(gMessageQueueMethods));//关联MessageQueueQueue的原生函数    LOG_FATAL_IF(res < 0, "Unable to register native methods.");    jclass clazz;    FIND_CLASS(clazz, "android/os/MessageQueue");//获取MessageQueue的class    GET_FIELD_ID(gMessageQueueClassInfo.mPtr, clazz,            "mPtr", "I");//获取MessageQueue class的mPtr field的Id        return 0;}

上面的代码很像java的反射有木有？

    Class cls = Class.forName("android.os.MessageQueue");    Field feild = cls.getField("mPtr");

到这里，我们就明白了android_os_MessageQueue_setNativeMessageQueue函数实际上把android.os.MessageQueue实例的mPtr值设置为nativeMessageQueue实例的地址。虽然Java语言没有指针的说法，但是，这里的mPtr却的的确确是作为一个指针使用的。现在，我们也就理解了，为什么mPtr可以被强制转换为nativeMessageQueue了。

小结：

android_os_MessageQueue_nativeInit和android_os_MessageQueue_nativeDestory两个函数做了些什么：

android_os_MessageQueue_nativeInit：构造NativeMessageQueue实例
android_os_MessageQueue_nativeDestory:销毁NativeMessageQeue实例

NativeMessageQueue

先来看看NativeMessageQueue的声明：

class NativeMessageQueue : public MessageQueue {public:    NativeMessageQueue();    virtual ~NativeMessageQueue();    virtual void raiseException(JNIEnv* env, const char* msg, jthrowable exceptionObj);    void pollOnce(JNIEnv* env, int timeoutMillis);    void wake();private:    bool mInCallback;    jthrowable mExceptionObj;};

NativeMessageQueue继承自MessageQueue（不是java中的android.os.MessageQueue哦），关于MessageQueue我们只需要了解，它包含了一个成员mLooper即可（有兴趣的同学可以查看/frameworks/base/core/jni/android_os_MessageQueue.h）

class MessageQueue  {    ......protected:    sp<Looper> mLooper;};

继续看NativeMessageQueue的代码：

NativeMessageQueue::NativeMessageQueue() : mInCallback(false), mExceptionObj(NULL) {    mLooper = Looper::getForThread();    if (mLooper == NULL) {        mLooper = new Looper(false);//实例化mLooper        Looper::setForThread(mLooper);    }}

NativeMessageQueue构造实例的时候，会实例化mLooper。

void NativeMessageQueue::pollOnce(JNIEnv* env, int timeoutMillis) {    mInCallback = true;    mLooper->pollOnce(timeoutMillis);    mInCallback = false;    if (mExceptionObj) {        env->Throw(mExceptionObj);        env->DeleteLocalRef(mExceptionObj);        mExceptionObj = NULL;    }}void NativeMessageQueue::wake() {    mLooper->wake();}

小结：

NativeMessageQueue的函数pollonce和wake实现相当简单，交给mLooper的同名函数。

Looper

先来看看Looper的声明/frameworks/native/include/utils/Looper.h：

class Looper : public ALooper, public RefBase {protected:    virtual ~Looper();public:    Looper(bool allowNonCallbacks);    bool getAllowNonCallbacks() const;    int pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData);    inline int pollOnce(int timeoutMillis) {        return pollOnce(timeoutMillis, NULL, NULL, NULL);    }    void wake();private:    const bool mAllowNonCallbacks; // immutable    int mWakeReadPipeFd;  // immutable    int mWakeWritePipeFd; // immutable    Mutex mLock;    int mEpollFd; // immutable    int pollInner(int timeoutMillis);    void awoken();};

因为代码有点多，所以上面的声明，已经被我精简了大部分，现在我们只关注我们关心的：pollonce和wake函数。

还是从构造函数开始（frameworks/native/utils/Looper.cpp）：

Looper::Looper(bool allowNonCallbacks) :        mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),        mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {    int wakeFds[2];    int result = pipe(wakeFds);//创建命名管道    LOG_ALWAYS_FATAL_IF(result != 0, "Could not create wake pipe.  errno=%d", errno);    // 保存命名管道    mWakeReadPipeFd = wakeFds[0];    mWakeWritePipeFd = wakeFds[1];    result = fcntl(mWakeReadPipeFd, F_SETFL, O_NONBLOCK);//设置为非阻塞模式    LOG_ALWAYS_FATAL_IF(result != 0, "Could not make wake read pipe non-blocking.  errno=%d",            errno);    result = fcntl(mWakeWritePipeFd, F_SETFL, O_NONBLOCK);//设置为非阻塞模式    LOG_ALWAYS_FATAL_IF(result != 0, "Could not make wake write pipe non-blocking.  errno=%d",            errno);    // 开始使用epoll API，实现轮询    // Allocate the epoll instance and register the wake pipe.    mEpollFd = epoll_create(EPOLL_SIZE_HINT);//创建epoll文件描述符    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 = mWakeReadPipeFd;    result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeReadPipeFd, & eventItem); // 把刚才创建的命名管道的读端加入的到epoll的监听队列中    LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake read pipe to epoll instance.  errno=%d",            errno);}Looper::~Looper() {    close(mWakeReadPipeFd);//释放命名管道    close(mWakeWritePipeFd);    close(mEpollFd);//释放epoll文件描述符}

Looper的构造函数中，出现了命名管道和epoll相关的代码，这是为什么呢？别急，接着看下去就知道了：

int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {    int result = 0;    for (;;) {       // 这段代码暂时无视       while (mResponseIndex < mResponses.size()) {      const Response& response = mResponses.itemAt(mResponseIndex++);      int ident = response.request.ident;      if (ident >= 0) {//ident > 0, 即此response为noncallback，需要返回event，data等数据给调用者处理        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);//这一行才是重点！    }}

接着往下看，代码有些长，但请仔细看：

int Looper::pollInner(int timeoutMillis) {    #if DEBUG_POLL_AND_WAKE  ALOGD("%p ~ pollOnce - waiting: timeoutMillis=%d", this, timeoutMillis);#endif    // 设置timeoutMillis的值为Math.min(timeoutMills, mNextMessageUptime)  // 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 %lldns, adjusted timeout: timeoutMillis=%d",        this, mNextMessageUptime - now, timeoutMillis);#endif  }  // Poll.  int result = ALOOPER_POLL_WAKE;  mResponses.clear();  mResponseIndex = 0;    // 开始轮询    struct epoll_event eventItems[EPOLL_MAX_EVENTS];    int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);    // Acquire lock.    mLock.lock();    // Check for poll error.    if (eventCount < 0) { //处理error        if (errno == EINTR) {            goto Done;        }        ALOGW("Poll failed with an unexpected error, errno=%d", errno);        result = ALOOPER_POLL_ERROR;        goto Done;    }    // Check for poll timeout.    if (eventCount == 0) { // 未能等到event，故timeout#if DEBUG_POLL_AND_WAKE        ALOGD("%p ~ pollOnce - timeout", this);#endif        result = ALOOPER_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++) {//有event，则处理        int fd = eventItems[i].data.fd;        uint32_t epollEvents = eventItems[i].events;        if (fd == mWakeReadPipeFd) {            if (epollEvents & EPOLLIN) {                awoken();//读取命名管道内的数据            } else {                ALOGW("Ignoring unexpected epoll events 0x%x on wake read pipe.", epollEvents);            }        } else {            ssize_t requestIndex = mRequests.indexOfKey(fd);            if (requestIndex >= 0) {                int events = 0;                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;                pushResponse(events, mRequests.valueAt(requestIndex));//把event添加到mResponses中，等待后续处理            } else {                ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "                        "no longer registered.", epollEvents, fd);            }        }    }Done: ;    // 处理C++层的Message    // Invoke pending message callbacks.    mNextMessageUptime = LLONG_MAX;    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);//调用handler->handleMessage            } // release handler            mLock.lock();            mSendingMessage = false;            result = ALOOPER_POLL_CALLBACK;        } else {            // The last message left at the head of the queue determines the next wakeup time.            mNextMessageUptime = messageEnvelope.uptime;//更新mNextMessageUptime            break;        }    }    // Release lock.    mLock.unlock();    // 处理mResponses    // Invoke all response callbacks.    for (size_t i = 0; i < mResponses.size(); i++) {        Response& response = mResponses.editItemAt(i);        if (response.request.ident == ALOOPER_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            int callbackResult = response.request.callback->handleEvent(fd, events, data);//调用callback->handleEvent            if (callbackResult == 0) {                removeFd(fd);            }            // 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 = ALOOPER_POLL_CALLBACK;        }    }    return result;}

代码比较长，所以分段分析：

    // 设置timeoutMillis的值为Math.min(timeoutMillis, mNextMessageUptime)    // 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 %lldns, adjusted timeout: timeoutMillis=%d",                this, mNextMessageUptime - now, timeoutMillis);#endif    }

为了解析这段代码，需要先补充一些C++层的Message相关的代码：

struct Message {    Message() : what(0) { }    Message(int what) : what(what) { }    /* The message type. (interpretation is left up to the handler) */    int what;};class MessageHandler : public virtual RefBase {    protected:        virtual ~MessageHandler() { }    public:        /**         * Handles a message.         */        virtual void handleMessage(const Message& message) = 0;}struct MessageEnvelope {    MessageEnvelope() : uptime(0) { }    MessageEnvelope(nsecs_t uptime, const sp<MessageHandler> handler,const Message& message) : uptime(uptime), handler(handler), message(message) {}    nsecs_t uptime;    MessageHandler> handler;    Message message;};void Looper::sendMessage(const sp<MessageHandler>& handler, const Message& message) {    nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);    sendMessageAtTime(now, handler, message);}void Looper::sendMessageDelayed(nsecs_t uptimeDelay, const sp<MessageHandler>& handler,        const Message& message) {    nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);    sendMessageAtTime(now + uptimeDelay, handler, message);}void Looper::sendMessageAtTime(nsecs_t uptime, const sp<MessageHandler>& handler,        const Message& message) {#if DEBUG_CALLBACKS    ALOGD("%p ~ sendMessageAtTime - uptime=%lld, 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();    }}void Looper::removeMessages(const sp<MessageHandler>& handler) {#if DEBUG_CALLBACKS    ALOGD("%p ~ removeMessages - handler=%p", this, handler.get());#endif    { // acquire lock        AutoMutex _l(mLock);        for (size_t i = mMessageEnvelopes.size(); i != 0; ) {            const MessageEnvelope& messageEnvelope = mMessageEnvelopes.itemAt(--i);            if (messageEnvelope.handler == handler) {                mMessageEnvelopes.removeAt(i);            }        }    } // release lock}void Looper::removeMessages(const sp<MessageHandler>& handler, int what) {#if DEBUG_CALLBACKS    ALOGD("%p ~ removeMessages - handler=%p, what=%d", this, handler.get(), what);#endif    { // acquire lock        AutoMutex _l(mLock);        for (size_t i = mMessageEnvelopes.size(); i != 0; ) {            const MessageEnvelope& messageEnvelope = mMessageEnvelopes.itemAt(--i);            if (messageEnvelope.handler == handler                    && messageEnvelope.message.what == what) {                mMessageEnvelopes.removeAt(i);            }        }    } // release lock}

是不是觉得上面的代码似曾相识？和Java层的MessageQueue很像似有木有？和java层一样，C++层存在消息队列和消息处理机制，消息被保存到成员mMessageEvelopes中，并在pollInner函数中处理消息（调用MesageHandler的handleMesage函数处理）。

现在回到Looper：：pollonceh函数，我们就应该能够理解，pollOnce函数到timeOutMillis参数仅仅代表了Java层下一个Message的触发延迟，所以，我们还需要考虑C++层下一个Message的触发延迟，所以，代码设置timeoutMillis为timeoutMillis和mNextMessageUpTime中的较小值。

继续下一段代码：

    int result = ALOOPER_POLL_WAKE;    mResponses.clear();    mResponseIndex = 0;    // 开始轮询    struct epoll_event eventItems[EPOLL_MAX_EVENTS];    int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);    // Acquire lock.    mLock.lock();

调用epoll函数，等待event发生，epoll_wait函数的返回值有三种可能：失败出错、没有event、有一个或多个event。

1. 失败的处理：

    // Check for poll error.    if (eventCount < 0) { //处理error        if (errno == EINTR) {            goto Done;        }        ALOGW("Poll failed with an unexpected error, errno=%d", errno);        result = ALOOPER_POLL_ERROR;        goto Done;    }

2. 没有event发生：

    // Check for poll timeout.    if (eventCount == 0) { // 未能等到event，故timeout#if DEBUG_POLL_AND_WAKE        ALOGD("%p ~ pollOnce - timeout", this);#endif        result = ALOOPER_POLL_TIMEOUT;        goto Done;    }

3. 有event发生：

    // 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++) {//有event，则处理        int fd = eventItems[i].data.fd;        uint32_t epollEvents = eventItems[i].events;        if (fd == mWakeReadPipeFd) {//说明java层或者C++层有新的Message            if (epollEvents & EPOLLIN) {                awoken();//读取命名管道内的数据            } else {                ALOGW("Ignoring unexpected epoll events 0x%x on wake read pipe.", epollEvents);            }        } else {            ssize_t requestIndex = mRequests.indexOfKey(fd);            if (requestIndex >= 0) {                int events = 0;                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;                pushResponse(events, mRequests.valueAt(requestIndex));//把event添加到mResponses中，等待后续处理            } else {                ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "                        "no longer registered.", epollEvents, fd);            }        }    }

处理event的时候，需要分为两类：fd==mWakeReadPipeFd和fd！=mWakeReadPipeFd

fd==mWakeReadPipeFd：说明C++层，或者java层有新的Message出现，需要处理。这种情况下，只需要读mWakeReadPipeFd内的数据即可

void Looper::awoken() {#if DEBUG_POLL_AND_WAKE    ALOGD("%p ~ awoken", this);#endif    char buffer[16];    ssize_t nRead;    do {        nRead = read(mWakeReadPipeFd, buffer, sizeof(buffer));    } while ((nRead == -1 && errno == EINTR) || nRead == sizeof(buffer));}

fd！=mWakeReadPipeFd:首先需要搞明白fd是哪来的：

  struct Request {    int fd;    int ident;    sp<LooperCallback> callback;    void* data;  };  struct Response {    int events;    Request request;  };

/*** 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 ALOOPER_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;};

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) {//仅当Looper支持NonCallbacks，并且ident大于0时，允许添加callback为null的Fd            ALOGE("Invalid attempt to set NULL callback with ident < 0.");            return -1;        }    } else {        ident = ALOOPER_POLL_CALLBACK;    }    int epollEvents = 0;    if (events & ALOOPER_EVENT_INPUT) epollEvents |= EPOLLIN;    if (events & ALOOPER_EVENT_OUTPUT) epollEvents |= EPOLLOUT;    { // acquire lock        AutoMutex _l(mLock);        Request request;        request.fd = fd;        request.ident = ident;        request.callback = callback;        request.data = data;        struct epoll_event eventItem;        memset(& eventItem, 0, sizeof(epoll_event)); // zero out unused members of data field union        eventItem.events = epollEvents;        eventItem.data.fd = fd;        ssize_t requestIndex = mRequests.indexOfKey(fd);        if (requestIndex < 0) {            int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, fd, & eventItem);            if (epollResult < 0) {                ALOGE("Error adding epoll events for fd %d, errno=%d", fd, errno);                return -1;            }            mRequests.add(fd, request);        } else {//存在则替换            int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_MOD, fd, & eventItem);            if (epollResult < 0) {                ALOGE("Error modifying epoll events for fd %d, errno=%d", fd, errno);                return -1;            }            mRequests.replaceValueAt(requestIndex, request);        }    } // release lock    return 1;}

从上面的代码，我们可知，Looper还支持添加Fd和自定义的callback，类似java层的Message.callback。

通过addFd函数，可以向Looper的mEpollFd添加指定的Fd，当Fd触发指定的event .e.i EPOLLIN or EPOLLOUT时，指定的相应的自定义callback就会得到执行。

另外，当Looper支持noncallback时，还可以向Looper添加callback为null的Fd，因为没有callback，所以Fd添加者需要调用int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) 通过outFd、outEvent、outData参数获取数据并进行处理。

所以当Fd触发消息时，需要生成对应到reponse并添加到meResponses中，等待后续的处理。

void Looper::pushResponse(int events, const Request& request) {    Response response;    response.events = events;    response.request = request;    mResponses.push(response);}

到这里为止，epoll_wait函数返回的三种结果的不同处理已经解析完毕，接下来代码进入共同的Done环节。

处理C++层的Message：

Done: ;    // 处理C++层的Message    // Invoke pending message callbacks.    mNextMessageUptime = LLONG_MAX;    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);//调用handler->handleMessage            } // release handler            mLock.lock();            mSendingMessage = false;            result = ALOOPER_POLL_CALLBACK;        } else {            // The last message left at the head of the queue determines the next wakeup time.            mNextMessageUptime = messageEnvelope.uptime;//更新mNextMessageUptime            break;        }    }

处理Response：

    // 处理mResponses    // Invoke all response callbacks.    for (size_t i = 0; i < mResponses.size(); i++) {        Response& response = mResponses.editItemAt(i);        if (response.request.ident == ALOOPER_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            int callbackResult = response.request.callback->handleEvent(fd, events, data);//调用callback->handleEvent            if (callbackResult == 0) {                removeFd(fd);            }            // 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 = ALOOPER_POLL_CALLBACK;        }    }

搞懂了上面的代码，我们就很容易明白wake函数做了些什么：

void Looper::wake() {#if DEBUG_POLL_AND_WAKE    ALOGD("%p ~ wake", this);#endif    ssize_t nWrite;    do {        nWrite = write(mWakeWritePipeFd, "W", 1);//向mWakeWritePipeFd中写入一个字符，所以mWakeReadPipeFd就会触发EPOLLIN event    } while (nWrite == -1 && errno == EINTR);    if (nWrite != 1) {        if (errno != EAGAIN) {            ALOGW("Could not write wake signal, errno=%d", errno);        }    }}

小结：

Looper通过epoll函数组实现了一个可以支持随时唤醒的阻塞机制
Looper支持两种不同的方式处理消息：Message + MessageHandler 和 LooperCallback。
Looper的阻塞在如下四种条件下会被唤醒：
- 发生错误
- 等待超时
- 出现需要处理的新Message（包括C++层和Java层）
- 由addFd函数添加的Fd触发event

总结：

在哪个线程调用JAVA层的Looper.loop()，Mesage和callback（包括Java层和C++层）就在哪个线程被处理，上图为Looper.loop函数的时序图。
C++层的NativeMesasgeQueue不应该是Java层的MesageQueue的内部实现，而更接近于“栾生兄弟”的关系。MessageQueue负责处理java层上到消息，NativeMessageQueue负责处理C++层上的消息。其中Java层是在android.os.Looper.looper函数中调用android.os.Handler.dispatchMessage处理，而C++层是在android::Looper::pollInner函数中调用android::MessageHandler::handleMessage & android:LooperCallback::handleEvent函数处理。
NativeMessageQueue利用Looper类实现了一个基于epoll函数和文件描述符（Fd）的可唤醒的阻塞机制。

NativeMessageQueue

Looper

总结：

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