Android訊息機制不完全解析（下） .

轉自：http://blog.csdn.net/a220315410/article/details/10444171

首先，看看在/frameworks/base/core/jni/android_os_MessageQueue.cpp檔案中看看android.os.MessageQueue類中的四個原生函式的實現：

static void android_os_MessageQueue_nativeInit(JNIEnv* env, jobject obj) {
    NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();//構造NativeMessageQueue例項
    if (!nativeMessageQueue) {
        jniThrowRuntimeException(env, "Unable to allocate native queue");
        return;
    }
    nativeMessageQueue->incStrong(env);//強引用+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);//強引用-1，實際上會導致釋放NativeMessageQueue例項
    }
}

static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj,
        jint ptr, jint timeoutMillis) {
    NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);//指標強制轉換
    nativeMessageQueue->pollOnce(env, timeoutMillis);//呼叫nativeMessageQueue的pollonce函式
}

static void android_os_MessageQueue_nativeWake(JNIEnv* env, jobject obj, jint ptr) {
    NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
    return nativeMessageQueue->wake();//呼叫nativeMessageQueue的wake函式
}

    從程式碼中，可以看到這四個函式的實現都是依賴於NativeMessageQueue類。不過，在開始解析NativeMessageQueue之前，我們再看一些有意思的程式碼：

static void android_os_MessageQueue_setNativeMessageQueue(JNIEnv* env, jobject messageQueueObj,
        NativeMessageQueue* nativeMessageQueue) {
    env->SetIntField(messageQueueObj, gMessageQueueClassInfo.mPtr,
             reinterpret_cast<jint>(nativeMessageQueue));//把nativeMessageQueue的例項地址強轉為java的int型別並儲存到gMessageQueueClassInfo.mPtr中
}

    那麼gMessageQueueClassInfo.mPtr是什麼呢？

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);
//這個函式在Android啟動的時候，會被系統呼叫
int register_android_os_MessageQueue(JNIEnv* env) {
    int res = jniRegisterNativeMethods(env, "android/os/MessageQueue",
            gMessageQueueMethods, NELEM(gMessageQueueMethods));//關聯MessageQueueQueue的原生函式
    LOG_FATAL_IF(res < 0, "Unable to register native methods.");

    jclass clazz;
    FIND_CLASS(clazz, "android/os/MessageQueue");//獲取MessageQueue的class

    GET_FIELD_ID(gMessageQueueClassInfo.mPtr, clazz,
            "mPtr", "I");//獲取MessageQueue class的mPtr field的Id
    
    return 0;
}

    上面的程式碼很像java的反射有木有？

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

    到這裡，我們就明白了android_os_MessageQueue_setNativeMessageQueue函式實際上把android.os.MessageQueue例項的mPtr值設定為nativeMessageQueue例項的地址。雖然Java語言沒有指標的說法，但是，這裡的mPtr卻的的確確是作為一個指標使用的。現在，我們也就理解了，為什麼mPtr可以被強制轉換為nativeMessageQueue了。

    小結：

1. android_os_MessageQueue_nativeInit和android_os_MessageQueue_nativeDestory兩個函式做了些什麼：

• android_os_MessageQueue_nativeInit：構造NativeMessageQueue例項
• android_os_MessageQueue_nativeDestory:銷燬NativeMessageQeue例項

NativeMessageQueue

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繼承自MessageQueue（不是java中的android.os.MessageQueue哦），關於MessageQueue我們只需要瞭解，它包含了一個成員mLooper即可（有興趣的同學可以檢視/frameworks/base/core/jni/android_os_MessageQueue.h）    

class MessageQueue  {

    ......

protected:
    sp<Looper> mLooper;
};

    繼續看NativeMessageQueue的程式碼：

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

    NativeMessageQueue構造例項的時候，會例項化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();
}

    小結：

1. NativeMessageQueue的函式pollonce和wake實現相當簡單，交給mLooper的同名函式。

Looper

    先來看看Looper的宣告/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();
};

    因為程式碼有點多，所以上面的宣告，已經被我精簡了大部分，現在我們只關注我們關心的：pollonce和wake函式。

    還是從建構函式開始（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);//建立命名管道
    LOG_ALWAYS_FATAL_IF(result != 0, "Could not create wake pipe.  errno=%d", errno);
    // 儲存命名管道
    mWakeReadPipeFd = wakeFds[0];
    mWakeWritePipeFd = wakeFds[1];

    result = fcntl(mWakeReadPipeFd, F_SETFL, O_NONBLOCK);//設定為非阻塞模式
    LOG_ALWAYS_FATAL_IF(result != 0, "Could not make wake read pipe non-blocking.  errno=%d",
            errno);

    result = fcntl(mWakeWritePipeFd, F_SETFL, O_NONBLOCK);//設定為非阻塞模式
    LOG_ALWAYS_FATAL_IF(result != 0, "Could not make wake write pipe non-blocking.  errno=%d",
            errno);
    // 開始使用epoll API，實現輪詢
    // Allocate the epoll instance and register the wake pipe.
    mEpollFd = epoll_create(EPOLL_SIZE_HINT);//建立epoll檔案描述符
    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); // 把剛才建立的命名管道的讀端加入的到epoll的監聽佇列中
    LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake read pipe to epoll instance.  errno=%d",
            errno);
}

Looper::~Looper() {
    close(mWakeReadPipeFd);//釋放命名管道
    close(mWakeWritePipeFd);
    close(mEpollFd);//釋放epoll檔案描述符
}

    Looper的建構函式中，出現了命名管道和epoll相關的程式碼，這是為什麼呢？別急，接著看下去就知道了：

int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
    int result = 0;
    for (;;) {
       // 這段程式碼暫時無視
       while (mResponseIndex < mResponses.size()) {
            const Response& response = mResponses.itemAt(mResponseIndex++);
            int ident = response.request.ident;
            if (ident >= 0) {//ident > 0, 即此response為noncallback，需要返回event，data等資料給呼叫者處理
                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);//這一行才是重點！
    }
}

    接著往下看，程式碼有些長，但請仔細看：

int Looper::pollInner(int timeoutMillis) {
    
#if DEBUG_POLL_AND_WAKE
    ALOGD("%p ~ pollOnce - waiting: timeoutMillis=%d", this, timeoutMillis);
#endif

    // 設定timeoutMillis的值為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;

    // 開始輪詢
    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) { //處理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，則處理
        int fd = eventItems[i].data.fd;
        uint32_t epollEvents = eventItems[i].events;
        if (fd == mWakeReadPipeFd) {
            if (epollEvents & EPOLLIN) {
                awoken();//讀取命名管道內的資料
            } 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新增到<span style="FONT-FAMILY: Arial, Helvetica, sans-serif">mResponses中，等待後續處理</span>
            } else {
                ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "
                        "no longer registered.", epollEvents, fd);
            }
        }
    }
Done: ;
    // 處理C++層的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);//呼叫handler-><span style="FONT-FAMILY: Arial, Helvetica, sans-serif">handleMessage</span>
            } // 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<span style="FONT-FAMILY: Arial, Helvetica, sans-serif">mNextMessageUptime</span>
            break;
        }
    }

    // Release lock.
    mLock.unlock();
    // 處理<span style="FONT-FAMILY: Arial, Helvetica, sans-serif">mResponses</span>
    // 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);//呼叫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;
}

    程式碼比較長，所以分段分析：

    // 設定timeoutMillis的值為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
    }

    為了解析這段程式碼，需要先補充一些C++層的Message相關的程式碼：

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
}

    是不是覺得上面的程式碼似曾相識？和Java層的MessageQueue很像似有木有？和java層一樣，C++層存在訊息佇列和訊息處理機制，訊息被儲存到成員mMessageEvelopes中，並在pollInner函式中處理訊息（呼叫MesageHandler的handleMesage函式處理）。

    現在回到Looper：：pollonceh函式，我們就應該能夠理解，pollOnce函式到timeOutMillis引數僅僅代表了Java層下一個Message的觸發延遲，所以，我們還需要考慮C++層下一個Message的觸發延遲，所以，程式碼設定timeoutMillis為timeoutMillis和mNextMessageUpTime中的較小值。

    繼續下一段程式碼：

    int result = ALOOPER_POLL_WAKE;
    mResponses.clear();
    mResponseIndex = 0;
    // 開始輪詢
    struct epoll_event eventItems[EPOLL_MAX_EVENTS];
    int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);

    // Acquire lock.
    mLock.lock();

    呼叫epoll函式，等待event發生，epoll_wait函式的返回值有三種可能：失敗出錯、沒有event、有一個或多個event。

1. 失敗的處理：

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

2. 沒有event發生：

    // 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發生：

    // 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，則處理
        int fd = eventItems[i].data.fd;
        uint32_t epollEvents = eventItems[i].events;
        if (fd == mWakeReadPipeFd) {//說明java層或者C++層有新的Message
            if (epollEvents & EPOLLIN) {
                awoken();//讀取命名管道內的資料
            } 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新增到<span style="FONT-FAMILY: Arial, Helvetica, sans-serif">mResponses中，等待後續處理</span>
            } else {
                ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "
                        "no longer registered.", epollEvents, fd);
            }
        }
    }

    處理event的時候，需要分為兩類：fd==mWakeReadPipeFd和fd！=mWakeReadPipeFd

fd==mWakeReadPipeFd：說明C++層，或者java層有新的Message出現，需要處理。這種情況下，只需要讀mWakeReadPipeFd內的資料即可

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是哪來的：

    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) {//僅當Looper支援NonCallbacks，並且ident大於0時，允許新增callback為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 {//存在則替換
            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;
}

    從上面的程式碼，我們可知，Looper還支援新增Fd和自定義的callback，類似java層的Message.callback。 

   通過addFd函式，可以向Looper的mEpollFd新增指定的Fd，當Fd觸發指定的event .e.i  EPOLLIN or EPOLLOUT時，指定的相應的自定義callback就會得到執行。

   另外，當Looper支援noncallback時，還可以向Looper新增callback為null的Fd，因為沒有callback，所以Fd新增者需要呼叫int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) 通過outFd、outEvent、outData引數獲取資料並進行處理。

    所以當Fd觸發訊息時，需要生成對應到reponse並新增到meResponses中，等待後續的處理。

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

    到這裡為止，epoll_wait函式返回的三種結果的不同處理已經解析完畢，接下來程式碼進入共同的Done環節。

處理C++層的Message：

Done: ;
    // 處理C++層的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);//呼叫handler-><span style="FONT-FAMILY: Arial, Helvetica, sans-serif">handleMessage</span>
            } // 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<span style="FONT-FAMILY: Arial, Helvetica, sans-serif">mNextMessageUptime</span>
            break;
        }
    }

處理Response：

    // 處理<span style="FONT-FAMILY: Arial, Helvetica, sans-serif">mResponses</span>
    // 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);//呼叫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;
        }
    }

    搞懂了上面的程式碼，我們就很容易明白wake函式做了些什麼：

void Looper::wake() {
#if DEBUG_POLL_AND_WAKE
    ALOGD("%p ~ wake", this);
#endif

    ssize_t nWrite;
    do {
        nWrite = write(mWakeWritePipeFd, "W", 1);//向<span style="font-family: Arial, Helvetica, sans-serif;">mWakeWritePipeFd</span><span style="font-family: Arial, Helvetica, sans-serif;">中寫入一個字元，所以mWakeReadPipeFd就會觸發</span><span style="font-family: Arial, Helvetica, sans-serif;">EPOLLIN event</span><span style="font-family: Arial, Helvetica, sans-serif;">
</span>    } while (nWrite == -1 && errno == EINTR);

    if (nWrite != 1) {
        if (errno != EAGAIN) {
            ALOGW("Could not write wake signal, errno=%d", errno);
        }
    }
}

    小結：

1. Looper通過epoll函式組實現了一個可以支援隨時喚醒的阻塞機制
2. Looper支援兩種不同的方式處理訊息：Message + MessageHandler 和 LooperCallback。
3. Looper的阻塞在如下四種條件下會被喚醒：
   • 發生錯誤
   • 等待超時
   • 出現需要處理的新Message（包括C++層和Java層）
   • 由addFd函式新增的Fd觸發event

 

總結：

1. 在哪個執行緒呼叫JAVA層的Looper.loop()，Mesage和callback（包括Java層和C++層）就在哪個執行緒被處理，上圖為Looper.loop函式的時序圖。
2. C++層的NativeMesasgeQueue不應該是Java層的MesageQueue的內部實現，而更接近於“欒生兄弟”的關係。MessageQueue負責處理java層上到訊息，NativeMessageQueue負責處理C++層上的訊息。其中Java層是在android.os.Looper.looper函式中呼叫android.os.Handler.dispatchMessage處理，而C++層是在android::Looper::pollInner函式中呼叫android::MessageHandler::handleMessage & android:LooperCallback::handleEvent函式處理。
3. NativeMessageQueue利用Looper類實現了一個基於epoll函式和檔案描述符（Fd）的可喚醒的阻塞機制。