Android Native -- Message/Handler/Looper机制（原理篇）

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⌈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）

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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。

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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

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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：

1. 1. 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。
   2. 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