在Android的消息机制中。不仅提供了供Application 开发使用的java的消息循环。事实上java的机制终于还是靠native来实现的。在native不仅提供一套消息传递和处理的机制，还提供了自己定义文件描写叙述符的I/O时间的监听机制。以下我们从详细代码中分析一下。
Native层的关键类：
Looper.cpp.该类中提供了pollOnce 和wake的休眠和唤醒机制。
同一时候在构造函数中也创建 管道 并增加epoll的机制中。来监听其状态变化。
Looper::Looper(bool allowNonCallbacks) :
mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),
mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {
int wakeFds[2];
int result = pipe(wakeFds);
LOG_ALWAYS_FATAL_IF(result != 0, "Could not create wake pipe. errno=%d", errno);
mWakeReadPipeFd = wakeFds[0];
mWakeWritePipeFd = wakeFds[1];
result = fcntl(mWakeReadPipeFd, F_SETFL, O_NONBLOCK);
LOG_ALWAYS_FATAL_IF(result != 0, "Could not make wake read pipe non-blocking. errno=%d",
errno);
result = fcntl(mWakeWritePipeFd, F_SETFL, O_NONBLOCK);
LOG_ALWAYS_FATAL_IF(result != 0, "Could not make wake write pipe non-blocking. errno=%d",
errno);
// Allocate the epoll instance and register the wake pipe.
mEpollFd = epoll_create(EPOLL_SIZE_HINT);
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);
LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake read pipe to epoll instance. errno=%d",
errno);
}
详细epoll机制，请參看我还有一篇。转载的epoll机制。一种高效改良的poll机制。
事实上looper在整个消息循环中，主要是实现MessageQueue中休眠和唤醒机制。
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) {
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) {
#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++) {
int fd = eventItems[i].data.fd;
uint32_t epollEvents = eventItems[i].events;
if (fd == mWakeReadPipeFd) {
if (epollEvents & EPOLLIN) {
awoken();
} 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));
} else {
ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "
"no longer registered.", epollEvents, fd);
}
}
从以上代码中能够看出，looper的pollonce主要在监听管道的read端是否有事件到来，同一时候在else以下能够实现对 自己定义文件描写叙述符的监听作用。
详细获取下个消息，还是在MessageQueue.java 的next（）实现并返回一个msg。
那什么时候应该唤醒和怎么唤醒呢？
參看MessageQueue的代码。在相关队列中加入一个msg时。调用用nativeWake方法
static void android_os_MessageQueue_nativeWake(JNIEnv* env, jobject obj, jint ptr) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
return nativeMessageQueue->wake();
}
void Looper::wake() {
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ wake", this);
#endif
ssize_t nWrite;
do {
nWrite = write(mWakeWritePipeFd, "W", 1);
} while (nWrite == -1 && errno == EINTR);
if (nWrite != 1) {
if (errno != EAGAIN) {
ALOGW("Could not write wake signal, errno=%d", errno);
}
}
}
唤醒的方法就是向mWakeWirtePipeFd写一个‘w'，触发epoll的mWakeReadPipFdd唤醒进程，进而从MessageQueue的next方法，获取下一个msg。
final Message next() {
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(mPtr, nextPollTimeoutMillis);
synchronized (this) {
if (mQuiting) {
return null;
}
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
// Next message is not ready. Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (false) Log.v("MessageQueue", "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
Looper对文件描写叙述符监控和处理：
looper能够对普通文件，设备文件或套接字都能够监听。android本身已经提供了一套机制。
/**
* Adds a new file descriptor to be polled by the looper.
* If the same file descriptor was previously added, it is replaced.
*
* "fd" is the file descriptor to be added.
* "ident" is an identifier for this event, which is returned from ALooper_pollOnce().
* The identifier must be >= 0, or ALOOPER_POLL_CALLBACK if providing a non-NULL callback.
* "events" are the poll events to wake up on. Typically this is ALOOPER_EVENT_INPUT.
* "callback" is the function to call when there is an event on the file descriptor.
* "data" is a private data pointer to supply to the callback.
*
* There are two main uses of this function:
*
* (1) If "callback" is non-NULL, then this function will be called when there is
* data on the file descriptor. It should execute any events it has pending,
* appropriately reading from the file descriptor. The 'ident' is ignored in this case.
*
* (2) If "callback" is NULL, the 'ident' will be returned by ALooper_pollOnce
* when its file descriptor has data available, requiring the caller to take
* care of processing it.
*
* Returns 1 if the file descriptor was added or -1 if an error occurred.
*
* This method can be called on any thread.
* This method may block briefly if it needs to wake the poll.
*/
int ALooper_addFd(ALooper* looper, int fd, int ident, int events,
ALooper_callbackFunc callback, void* data);
通过以上接口，大家能够在native层实现各种文件的监听和处理。另外android 还提供Native层的message和MessageHandler来处理消息机制。
这些都在Looper.h文件里。大家有时间能够研究一下。
/**
* 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;
};感觉和java的非常类似吧。
