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。