frameworks 之Lopper
frameworks 之Lopper
- prepare
- Looper()
- addFd
- pollAll
Cpp的消息循环也是按Lopper 进行的。
涉及到的类如下
- system/core/libutils/Looper.cpp
- system/core/libutils/include/utils/Looper.h
prepare
Looper 通过 prepare 方法获取 prepare 对象， 方法需要传入参数 allowNonCallbacks 如果为 false 则表示 addFd 时候call不能为 null 。该方法里面会调用 getForThread 获取唯一的对象，如果为空，则创建 Lopoper对象 并且放到唯一的线程里面。
sp<Looper> Looper::prepare(int opts) {
// 该参数用来判断调用addFd 添加文件描述符的时候参数callback是否可以为空
bool allowNonCallbacks = opts & PREPARE_ALLOW_NON_CALLBACKS;
// 去当前线程池找 没有就创建
sp<Looper> looper = Looper::getForThread();
if (looper == nullptr) {
looper = new Looper(allowNonCallbacks);
Looper::setForThread(looper);
}
if (looper->getAllowNonCallbacks() != allowNonCallbacks) {
ALOGW("Looper already prepared for this thread with a different value for the "
"LOOPER_PREPARE_ALLOW_NON_CALLBACKS option.");
}
return looper;
}
getForThread 也是从 pthread_getspecific 缓存中获取，跟 IPCThreadState 一样。 setForThread 则通过 pthread_setspecific 放到缓存中。其中 decStrong， incStrong 增加减少引用。
sp<Looper> Looper::getForThread() {
int result = pthread_once(& gTLSOnce, initTLSKey);
LOG_ALWAYS_FATAL_IF(result != 0, "pthread_once failed");
return (Looper*)pthread_getspecific(gTLSKey);
}
void Looper::setForThread(const sp<Looper>& looper) {
sp<Looper> old = getForThread(); // also has side-effect of initializing TLS
if (looper != nullptr) {
looper->incStrong((void*)threadDestructor);
}
pthread_setspecific(gTLSKey, looper.get());
if (old != nullptr) {
old->decStrong((void*)threadDestructor);
}
}
Looper()
当不存在的时候会初始化 该对象 初始化时候传入参数 allowNonCallbacks， 并且会初始化
mWakeEventFd 文件描述符。该文件描述符不是平常的socket 只是用于唤醒epoll。
Looper::Looper(bool allowNonCallbacks)
: mAllowNonCallbacks(allowNonCallbacks),
mSendingMessage(false),
mPolling(false),
mEpollRebuildRequired(false),
mNextRequestSeq(0),
mResponseIndex(0),
mNextMessageUptime(LLONG_MAX) {
// 创建文件描述符，不是平常见到的socket fd那种 用于唤醒epoll
mWakeEventFd.reset(eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC));
LOG_ALWAYS_FATAL_IF(mWakeEventFd.get() < 0, "Could not make wake event fd: %s", strerror(errno));
AutoMutex _l(mLock);
rebuildEpollLocked();
}
调用 rebuildEpollLocked 则用来重置 epoll 。里面通过 reset 方法重置，并添加 mWakeEventFd 到 eploll 中。因为重置了，所以还需要将通过addFd 方法的文件ID遍历添加到 epoll 中。
void Looper::rebuildEpollLocked() {
// Close old epoll instance if we have one.
// 判断是否有epoll了 有就调用reset
if (mEpollFd >= 0) {
#if DEBUG_CALLBACKS
ALOGD("%p ~ rebuildEpollLocked - rebuilding epoll set", this);
#endif
mEpollFd.reset();
}
// Allocate the new epoll instance and register the wake pipe.
mEpollFd.reset(epoll_create1(EPOLL_CLOEXEC));
LOG_ALWAYS_FATAL_IF(mEpollFd < 0, "Could not create epoll instance: %s", strerror(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 = mWakeEventFd.get();
// 添加到epoll中
int result = epoll_ctl(mEpollFd.get(), EPOLL_CTL_ADD, mWakeEventFd.get(), &eventItem);
LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake event fd to epoll instance: %s",
strerror(errno));
// 因为 epoll reset了，所以要遍历所有监听的文件描述符 添加进去
for (size_t i = 0; i < mRequests.size(); i++) {
const Request& request = mRequests.valueAt(i);
struct epoll_event eventItem;
request.initEventItem(&eventItem);
int epollResult = epoll_ctl(mEpollFd.get(), EPOLL_CTL_ADD, request.fd, &eventItem);
if (epollResult < 0) {
ALOGE("Error adding epoll events for fd %d while rebuilding epoll set: %s",
request.fd, strerror(errno));
}
}
}
addFd
addFd 用于添加监听，添加时候会根据初始化传入的参数，判断是否允许 callBack 为空。生成对应 request 对象。将fd,callback 等对象放到 request 中， 并添加到 mRequests 数组中。添加时候并会通过调用 epoll_ctl 方法 添加到epoll 监听。
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()) {
// 如果为空，初始化传入的参数又没false 不允许 callback 为空 直接返回
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);
// 将文件描述符 callback 封装到 request 对象，并添加到epoll
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);
// 如果里面已经存在 调用 EPOLL_CTL_MOD, 不然调用 EPOLL_CTL_ADD
if (requestIndex < 0) {
int epollResult = epoll_ctl(mEpollFd.get(), 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 {
int epollResult = epoll_ctl(mEpollFd.get(), EPOLL_CTL_MOD, fd, &eventItem);
if (epollResult < 0) {
if (errno == ENOENT) {
// Tolerate ENOENT because it means that an older file descriptor was
// closed before its callback was unregistered and meanwhile a new
// file descriptor with the same number has been created and is now
// being registered for the first time. This error may occur naturally
// when a callback has the side-effect of closing the file descriptor
// before returning and unregistering itself. Callback sequence number
// checks further ensure that the race is benign.
//
// Unfortunately due to kernel limitations we need to rebuild the epoll
// set from scratch because it may contain an old file handle that we are
// now unable to remove since its file descriptor is no longer valid.
// No such problem would have occurred if we were using the poll system
// call instead, but that approach carries others disadvantages.
#if DEBUG_CALLBACKS
ALOGD("%p ~ addFd - EPOLL_CTL_MOD failed due to file descriptor "
"being recycled, falling back on EPOLL_CTL_ADD: %s",
this, strerror(errno));
#endif
epollResult = epoll_ctl(mEpollFd.get(), 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;
}
}
mRequests.replaceValueAt(requestIndex, request);
}
} // release lock
return 1;
}
pollAll
调用 pollAll 进行监听 ，其中第一个参数 timeoutMillis 代表超时时间，-1代表阻塞。如果是-1则会持续的监听，如果不是则会 在 pollOnce 返回 POLL_CALLBACK 判断是否超时。其中变量定义 在 Looper.h 头文件中。
enum {
/**
* Result from Looper_pollOnce() and Looper_pollAll():
* The poll was awoken using wake() before the timeout expired
* and no callbacks were executed and no other file descriptors were ready.
*/
POLL_WAKE = -1, // 在唤醒的时候，没有 callback执行
/**
* Result from Looper_pollOnce() and Looper_pollAll():
* One or more callbacks were executed.
*/
POLL_CALLBACK = -2, // 一个或多个callback执行
/**
* Result from Looper_pollOnce() and Looper_pollAll():
* The timeout expired.
*/
POLL_TIMEOUT = -3, // 超时
/**
* Result from Looper_pollOnce() and Looper_pollAll():
* An error occurred.
*/
POLL_ERROR = -4, // 发生错误
};
int Looper::pollAll(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
// timeoutMillis 传入小于0，则阻塞等待消息
if (timeoutMillis <= 0) {
int result;
do {
result = pollOnce(timeoutMillis, outFd, outEvents, outData);
} while (result == POLL_CALLBACK);
return result;
} else {
nsecs_t endTime = systemTime(SYSTEM_TIME_MONOTONIC)
+ milliseconds_to_nanoseconds(timeoutMillis);
for (;;) {
int result = pollOnce(timeoutMillis, outFd, outEvents, outData);
// 结果不是有callback执行返回则return，因为epoll wait 到时间了也没执行
if (result != POLL_CALLBACK) {
return result;
}
// 下面代表wait 执行了 跳出了pollOnce方法，判断时间是否到指定的时间，没到就计算差值继续执行
nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
// 将时间间隔
timeoutMillis = toMillisecondTimeoutDelay(now, endTime);
if (timeoutMillis == 0) {
return POLL_TIMEOUT;
}
}
}
}
查看 pollOnce 方法，该方法会调用 pollInner 方法。pollinner 方法会监听对应的 epoll_wait 方法进行等待消息返回。一开始会计算 超时时间，如果传入超时时间 大于剩余的时间，则最小的时间设置。然后进行等待状态，返回events小于则返回POLL_ERROR，的等于则返回超时 POLL_TIMEOUT。
如果消息大于0， 则遍历 mRequests 数组，将fd相等的数据 通过 pushResponse 放到 mResponses 数组。
接下来处理对应的消息队列 mMessageEnvelopes，如果时间小于当前时间，则移除队列，并调用 handleMessage 方法。
处理完 message 后，将继续遍历 mResponses 数组。并调用对应callback里面的 handleEvent 方法，如果 handleEvent 方法返回0，则会将对象从 mResponses 移除。否则则不移除。
int Looper::pollInner(int timeoutMillis) {
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - waiting: timeoutMillis=%d", this, timeoutMillis);
#endif
// 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);
// 超时时间，如果时间差已经小于设置的时间差，按最小时间差为准
if (messageTimeoutMillis >= 0
&& (timeoutMillis < 0 || messageTimeoutMillis < timeoutMillis)) {
timeoutMillis = messageTimeoutMillis;
}
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - next message in %" PRId64 "ns, adjusted timeout: timeoutMillis=%d",
this, mNextMessageUptime - now, timeoutMillis);
#endif
}
// Poll.
int result = POLL_WAKE;
mResponses.clear();
mResponseIndex = 0;
// We are about to idle.
mPolling = true;
struct epoll_event eventItems[EPOLL_MAX_EVENTS];
int eventCount = epoll_wait(mEpollFd.get(), eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
// No longer idling.
mPolling = false;
// Acquire lock.
mLock.lock();
// Rebuild epoll set if needed.
if (mEpollRebuildRequired) {
mEpollRebuildRequired = false;
rebuildEpollLocked();
goto Done;
}
// Check for poll error.
// 小于报错
if (eventCount < 0) {
if (errno == EINTR) {
goto Done;
}
ALOGW("Poll failed with an unexpected error: %s", strerror(errno));
result = POLL_ERROR;
goto Done;
}
// Check for poll timeout.
// 超时了 没消失s
if (eventCount == 0) {
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - timeout", this);
#endif
result = POLL_TIMEOUT;
goto Done;
}
// Handle all events.
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - handling events from %d fds", this, eventCount);
#endif
// 收到消息开始遍历
for (int i = 0; i < eventCount; i++) {
int fd = eventItems[i].data.fd;
uint32_t epollEvents = eventItems[i].events;
if (fd == mWakeEventFd.get()) {
if (epollEvents & EPOLLIN) {
awoken();
} else {
ALOGW("Ignoring unexpected epoll events 0x%x on wake event fd.", epollEvents);
}
} else {
// 查找是监听的fd, 通过 pushResponse 放进response 队列
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));
} else {
ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "
"no longer registered.", epollEvents, fd);
}
}
}
Done: ;
// Invoke pending message callbacks.
//处理消息队列
mNextMessageUptime = LLONG_MAX;
while (mMessageEnvelopes.size() != 0) {
nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
const MessageEnvelope& messageEnvelope = mMessageEnvelopes.itemAt(0);
// 判断消息时间是否小于当前时间是的话 移除并调用 handleMessage 方法
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);
} // 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();
// Invoke all response callbacks.
for (size_t i = 0; i < mResponses.size(); i++) {
Response& response = mResponses.editItemAt(i);
// 如果是 回调类型 调用 handleEvent 方法
if (response.request.ident == 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
// Invoke the callback. Note that the file descriptor may be closed by
// the callback (and potentially even reused) before the function returns so
// we need to be a little careful when removing the file descriptor afterwards.
// handleEvent 返回0 则移除
int callbackResult = response.request.callback->handleEvent(fd, events, data);
if (callbackResult == 0) {
removeFd(fd, response.request.seq);
}
// 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 = POLL_CALLBACK;
}
}
return result;
}