Android Native -- Message/Handler/Looper机制（原理篇）
⌈Android Native消息队列处理系列文章⌋
Android Native -- Message/Handler/Looper机制（原理篇）
Android Native -- Message/Handler/Looper机制（应用篇）
前言
在分析Android Framework Native层代码的时候，很多地方都用到了Android系统中重要的辅助类Looper来进行线程间通信或设计事件处理逻辑，本文将深入分析一下Message/Handler/Looper机制，方便理解其运行原理。
Tips:
先给出几篇非常不错的参考文章
https://blog.csdn.net/xiaosayidao/article/details/73992078
https://blog.csdn.net/chwan_gogogo/article/details/46953549
另外要理解Looper机制的原理，还需要额外理解Linux中的epool机制
https://blog.csdn.net/xiajun07061225/article/details/9250579
Linux中的eventfd的使用
https://blog.csdn.net/qq_28114615/article/details/97929524
基本类图
Android Native层的消息Looper机制代码，主要实现位于：
system/core/libutils/Looper.cpp
system/core/include/utils/Looper.h
基本类图如下：
基本对象说明
- Message : 消息的载体，代表了一个事件，通过一个what字段来标记是什么事件，源码定义如下：
/**
* A message that can be posted to a Looper.
*/
struct Message {
Message() : what(0) { }
Message(int w) : what(w) { }
/* The message type. (interpretation is left up to the handler) */
int what;
};- MessageHandler/WeakMessageHandler 消息处理的接口(基类), 子类通过实现handleMessage来实现特定Message的处理逻辑。WeakMessageHandler包含了一个MessageHandler的弱指针
/**
* 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;
};
/**
* A simple proxy that holds a weak reference to a message handler.
*/
class WeakMessageHandler : public MessageHandler {
protected:
virtual ~WeakMessageHandler();
public:
WeakMessageHandler(const wp<MessageHandler>& handler);
virtual void handleMessage(const Message& message);
private:
wp<MessageHandler> mHandler;
};- LooperCallback/SimpleLooperCallback ： 用于Looper回调，实际上就是保存一个Looper_callbackFunc指针的包装基类。在Looper::addFd()方法添加监测的fd时来设置回调。
/**
* 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 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;
};
/**
* Wraps a Looper_callbackFunc function pointer.
*/
class SimpleLooperCallback : public LooperCallback {
protected:
virtual ~SimpleLooperCallback();
public:
SimpleLooperCallback(Looper_callbackFunc callback);
virtual int handleEvent(int fd, int events, void* data);
private:
Looper_callbackFunc mCallback;
};- Looper核心类，它其中维护着一个消息/监测的fd队列，当用户调用pollOnce或pollAll时，就会去判断是否有消息要处理（调用对应的handler::handleMessage）或监测对的fd有事件发生（回调对应的callback函数）
关键方法分析
- Create Looper == 创建Looper的方法
Looper提供两种方式创建Looper：
1. 直接调用Looper 构造函数：
Looper::Looper(bool allowNonCallbacks) :
mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),
mPolling(false), mEpollFd(-1), mEpollRebuildRequired(false),
mNextRequestSeq(0), mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {
mWakeEventFd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
LOG_ALWAYS_FATAL_IF(mWakeEventFd < 0, "Could not make wake event fd: %s",
strerror(errno));
AutoMutex _l(mLock);
rebuildEpollLocked();
}
2. 调用Looper的静态函数 prepare() ：如果线程已经有对应的Looper，则直接返回，否则才会创建新的Looper。
sp<Looper> Looper::prepare(int opts) {
bool allowNonCallbacks = opts & PREPARE_ALLOW_NON_CALLBACKS;
sp<Looper> looper = Looper::getForThread();
if (looper == NULL) {
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;
}Looper的构造函数里主要做两件事情：
- > 调用eventfd(0, EFD_NONBLOCK)返回mWakeEventFd，用于唤醒epoll_wait()
- > 调用rebuildEpollLocked() 创建epoll 文件描述符，并将mWakeEventFd加入到epoll监听队列中
void Looper::rebuildEpollLocked() {
// Close old epoll instance if we have one.
if (mEpollFd >= 0) {
#if DEBUG_CALLBACKS
ALOGD("%p ~ rebuildEpollLocked - rebuilding epoll set", this);
#endif
close(mEpollFd);
}
// Allocate the new epoll instance and register the wake pipe.
mEpollFd = epoll_create(EPOLL_SIZE_HINT);
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;
int result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem);
LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake event fd to epoll instance: %s",
strerror(errno));
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, 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));
}
}
}注：参数allowNonCallbacks表明是否可以在Looper_addFd时不提供callback
- Send Message == 发送消息的方法
发送消息是指将消息插入到消息队列 mMessageEnvelopes。mMessageEnvelopes 里面的是根据时间顺序排列存放MessageEnvlope：下标越小，越早被处理。
发送消息的函数有如下三个，但最终都是调用sendMessageAtTime() 来实现的。
/**
* Enqueues a message to be processed by the specified handler.
*
* The handler must not be null.
* This method can be called on any thread.
*/
void sendMessage(const sp<MessageHandler>& handler, const Message& message);
/**
* Enqueues a message to be processed by the specified handler after all pending messages
* after the specified delay.
*
* The time delay is specified in uptime nanoseconds.
* The handler must not be null.
* This method can be called on any thread.
*/
void sendMessageDelayed(nsecs_t uptimeDelay, const sp<MessageHandler>& handler,
const Message& message);
/**
* Enqueues a message to be processed by the specified handler after all pending messages
* at the specified time.
*
* The time is specified in uptime nanoseconds.
* The handler must not be null.
* This method can be called on any thread.
*/
void sendMessageAtTime(nsecs_t uptime, const sp<MessageHandler>& handler,
const Message& message);来看一下sendMessageAtTime()函数的具体实现：
void Looper::sendMessageAtTime(nsecs_t uptime, const sp<MessageHandler>& handler,
const Message& message) {
#if DEBUG_CALLBACKS
ALOGD("%p ~ sendMessageAtTime - uptime=%" PRId64 ", 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();
}
}首先根据uptime在mMessageEnvelopes遍历，找到合适的位置，并将message 封装成MessageEnvlope，插入找到的位置上。
然后决定是否要唤醒Looper：
-
- 如果Looper此时正在派发message，则不需要wakeup Looper。因为这一次looper处理完消息之后，会重新估算下一次epoll_wait() 的wakeup时间。
- 如果是插在消息队列的头部，则需要立即wakeup Looper
另外还有提供移除消息的方法removeMessages：
/**
* Removes all messages for the specified handler from the queue.
*
* The handler must not be null.
* This method can be called on any thread.
*/
void removeMessages(const sp<MessageHandler>& handler);
/**
* Removes all messages of a particular type for the specified handler from the queue.
*
* The handler must not be null.
* This method can be called on any thread.
*/
void removeMessages(const sp<MessageHandler>& handler, int what);- Add fd == 添加fd的方法
添加新的文件描述符并设置回调方法，用于监测事件，提供了两种方法：
/**
* 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 pollOnce().
* The identifier must be >= 0, or POLL_CALLBACK if providing a non-NULL callback.
* "events" are the poll events to wake up on. Typically this is 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 Looper_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, 0 if the arguments were invalid.
*
* This method can be called on any thread.
* This method may block briefly if it needs to wake the poll.
*
* The callback may either be specified as a bare function pointer or as a smart
* pointer callback object. The smart pointer should be preferred because it is
* easier to avoid races when the callback is removed from a different thread.
* See removeFd() for details.
*/
int addFd(int fd, int ident, int events, Looper_callbackFunc callback, void* data);
int addFd(int fd, int ident, int events, const sp<LooperCallback>& callback, void* data);来看一下具体实现：
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) {
ALOGE("Invalid attempt to set NULL callback with ident < 0.");
return -1;
}
} else {
ident = POLL_CALLBACK;
}
{ // acquire lock
AutoMutex _l(mLock);
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);
if (requestIndex < 0) {
int epollResult = epoll_ctl(mEpollFd, 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, 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, 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;
}主要做了3件事：
1. 把要监测的文件描述符fd、事件标识events、回调函数指针callback、一些额外参数data封装成一个Request对象；
2. 调用epoll_ctl对文件描述符fd进行监测；
3. 把Request对象添加到mRequests列表（KeyedVector<int, Request> mRequests）
- Poll Looper == 轮询处理的方法
要让Looper运行起来才能处理消息。Looper提供了接口：pollOnce()
int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData);
调用poolOnce返回，返回结果如下四种情况
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, // 在超时前通过wake()唤醒，没有callback被执行，且没有文件描述符事件
/**
* Result from Looper_pollOnce() and Looper_pollAll():
* One or more callbacks were executed.
*/
POLL_CALLBACK = -2, // 一个或多个callback被执行
/**
* 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
};如果Looper中没有任何要处理的event/message,则会阻塞在epoll_wait() 等待事件到来。
调用流程：pollOnce() -> pollInner() -> epoll_wait()
struct epoll_event eventItems[EPOLL_MAX_EVENTS];
int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);epoll_wait()其有三种情况会返回，返回值eventCount为上来的epoll event数量。
- 出现错误返回， eventCount < 0;
- timeout返回，eventCount = 0，表明监听的文件描述符中都没有事件发生，将直接进行native message的处理；
- 监听的文件描述符中有事件发生导致的返回，eventCount > 0; 有eventCount 数量的epoll event 上来。
epoll_wait唤醒后，接下首先是判断是否有监测的文件描述符事件，并把事件标识events和对应封装为Response对象，加入到mResponses队列中(Vector<Response> mResponses)
for (int i = 0; i < eventCount; i++) {
int fd = eventItems[i].data.fd;
uint32_t epollEvents = eventItems[i].events;
if (fd == mWakeEventFd) {
if (epollEvents & EPOLLIN) {
awoken();
} else {
ALOGW("Ignoring unexpected epoll events 0x%x on wake event fd.", epollEvents);
}
} else {
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);
}
}
}然后 开始遍历消息队列，判断是否有消息到了处理时间了，并调用消息对应的handler->handleMessage(message);
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);
} // 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;
}
}同样会遍历mResponses队列，并回调对应的response.request.callback->handleEvent(fd, events, data)
// Invoke all response callbacks.
for (size_t i = 0; i < mResponses.size(); i++) {
Response& response = mResponses.editItemAt(i);
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.
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;
}
}注：每个消息处理完，会被从消息队列移除 mMessageEnvelopes.removeAt(0);
每个fd事件回调完，也会被从mRequests列表中移除
对于pollInner()
调整timeout：Adjust the timeout based on when the next message is due.
mNextMessageUptime 是 消息队列 mMessageEnvelopes 中最近一个即将要被处理的message的时间点。
所以需要根据mNextMessageUptime 与 调用者传下来的timeoutMillis 比较计算出一个最小的timeout，这将决定epoll_wait() 可能会阻塞多久才会返回。
epoll_wait()
处理epoll_wait() 返回的epoll events.
判断epoll event 是哪个fd上发生的事件
如果是mWakeEventFd，则执行awoken(). awoken() 只是将数据read出来，然后继续往下处理了。其目的也就是使epoll_wait() 从阻塞中返回。
如果是通过Looper.addFd() 接口加入到epoll监听队列的fd，并不是立马处理，而是先push到mResponses，后面再处理。
处理消息队列 mMessageEnvelopes 中的Message.
如果还没有到处理时间，就更新一下mNextMessageUptime
处理刚才放入mResponses中的 事件.
只处理ident 为POLL_CALLBACK的事件。其他事件在pollOnce中处理
基本运行机制
大概画一个抽象的处理概念图示，不一定准确
- 创建消息Message并指定指定MessageHandler， 调用sendMessage()把消息传递给Looper。
- Looper根据Message 和 MessageHandler创建MessageEnvelope 。然后将MessageEnvelope 添加到Looper的消息队列 mMessageEnvelopes 中。
- Native Looper除了处理Message之外，还可以监听指定的文件描述符。
- 通过addFd() 添加要监听的fd到epoll的监听队列中，并将传进来的fd，ident，callback，data 封装成Request 对象，然后加入到Looper 的mRequests 中。
- 外部逻辑（可以是某一个Thread）不断调用poolOnce-> pollInner() -> epoll_wait()阻塞，等待事件发生或超时
- 当该fd有事件发生时，epoll_wait()会返回epoll event，然后从mRequests中找到对应的request对象，并加上返回的epoll event 类型（EPOLLIN、EPOLLOUT…）封装成Response对象，加入到mResponses 中。
- 然后在需要处理Responses的时候，从mResponses遍历取出Response进行处理。
- 同样遍历消息队列 mMessageEnvelopes中的消息进行处理
- 如此不断循环
