Android Looper原理ソース分析

概要
ずいぶん前に「Android Message Queue、Message、Looper、Handler」という小さな記事を転載し、Android Message Queue、Looper、Handlerという概念の関係を紹介しました.その中でLooperはこのいくつかの概念の核心に位置し、1つのLooperの中にMessageQueueがあります.複数のHandlerは1つのLooperを共有することができ、1つのスレッドには1つのLooperしかありません.今日はLooperの原理ソースコードを詳しく分析します
ソース位置
frameworks/native/lib/utils/Looper.cpp
Looper.h

class Looper : public ALooper, public RefBase {
protected:
    virtual ~Looper();
public:
    /**
     * Creates a looper.
     *
     * If allowNonCallbaks is true, the looper will allow file descriptors to be
     * registered without associated callbacks.  This assumes that the caller of
     * pollOnce() is prepared to handle callback-less events itself.
     */
    Looper(bool allowNonCallbacks);
    /**
     * Returns whether this looper instance allows the registration of file descriptors
     * using identifiers instead of callbacks.
     */
    bool getAllowNonCallbacks() const;
    /**
     * Waits for events to be available, with optional timeout in milliseconds.
     * Invokes callbacks for all file descriptors on which an event occurred.
     *
     * If the timeout is zero, returns immediately without blocking.
     * If the timeout is negative, waits indefinitely until an event appears.
     *
     * Returns ALOOPER_POLL_WAKE if the poll was awoken using wake() before
     * the timeout expired and no callbacks were invoked and no other file
     * descriptors were ready.
     *
     * Returns ALOOPER_POLL_CALLBACK if one or more callbacks were invoked.
     *
     * Returns ALOOPER_POLL_TIMEOUT if there was no data before the given
     * timeout expired.
     *
     * Returns ALOOPER_POLL_ERROR if an error occurred.
     *
     * Returns a value >= 0 containing an identifier if its file descriptor has data
     * and it has no callback function (requiring the caller here to handle it).
     * In this (and only this) case outFd, outEvents and outData will contain the poll
     * events and data associated with the fd, otherwise they will be set to NULL.
     *
     * This method does not return until it has finished invoking the appropriate callbacks
     * for all file descriptors that were signalled.
     */
    int pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData);
    inline int pollOnce(int timeoutMillis) {
        return pollOnce(timeoutMillis, NULL, NULL, NULL);
    }
    /**
     * Like pollOnce(), but performs all pending callbacks until all
     * data has been consumed or a file descriptor is available with no callback.
     * This function will never return ALOOPER_POLL_CALLBACK.
     */
    int pollAll(int timeoutMillis, int* outFd, int* outEvents, void** outData);
    inline int pollAll(int timeoutMillis) {
        return pollAll(timeoutMillis, NULL, NULL, NULL);
    }
    /**
     * Wakes the poll asynchronously.
     *
     * This method can be called on any thread.
     * This method returns immediately.
     */
    void wake();
    /**
     * 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 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, 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, ALooper_callbackFunc callback, void* data);
    int addFd(int fd, int ident, int events, const sp& callback, void* data);
    /**
     * Removes a previously added file descriptor from the looper.
     *
     * When this method returns, it is safe to close the file descriptor since the looper
     * will no longer have a reference to it.  However, it is possible for the callback to
     * already be running or for it to run one last time if the file descriptor was already
     * signalled.  Calling code is responsible for ensuring that this case is safely handled.
     * For example, if the callback takes care of removing itself during its own execution either
     * by returning 0 or by calling this method, then it can be guaranteed to not be invoked
     * again at any later time unless registered anew.
     *
     * A simple way to avoid this problem is to use the version of addFd() that takes
     * a sp instead of a bare function pointer.  The LooperCallback will
     * be released at the appropriate time by the Looper.
     *
     * Returns 1 if the file descriptor was removed, 0 if none was previously registered.
     *
     * This method can be called on any thread.
     * This method may block briefly if it needs to wake the poll.
     */
    int removeFd(int fd);
    /**
     * 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& 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& 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& handler,
            const Message& message);
    /**
     * 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& 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& handler, int what);
    /**
     * Prepares a looper associated with the calling thread, and returns it.
     * If the thread already has a looper, it is returned.  Otherwise, a new
     * one is created, associated with the thread, and returned.
     *
     * The opts may be ALOOPER_PREPARE_ALLOW_NON_CALLBACKS or 0.
     */
    static sp prepare(int opts);
    /**
     * Sets the given looper to be associated with the calling thread.
     * If another looper is already associated with the thread, it is replaced.
     *
     * If "looper" is NULL, removes the currently associated looper.
     */
    static void setForThread(const sp& looper);
    /**
     * Returns the looper associated with the calling thread, or NULL if
     * there is not one.
     */
    static sp getForThread();
private:
    struct Request {
        int fd;
        int ident;
        sp callback;
        void* data;
    };
    struct Response {
        int events;
        Request request;
    };
    struct MessageEnvelope {
        MessageEnvelope() : uptime(0) { }
        MessageEnvelope(nsecs_t uptime, const sp handler,
                const Message& message) : uptime(uptime), handler(handler), message(message) {
        }
        nsecs_t uptime;
        sp handler;
        Message message;
    };
    const bool mAllowNonCallbacks; // immutable
    int mWakeReadPipeFd;  // immutable
    int mWakeWritePipeFd; // immutable
    Mutex mLock;
    Vector mMessageEnvelopes; // guarded by mLock
    bool mSendingMessage; // guarded by mLock
    int mEpollFd; // immutable
    // Locked list of file descriptor monitoring requests.
    KeyedVector mRequests;  // guarded by mLock
    // This state is only used privately by pollOnce and does not require a lock since
    // it runs on a single thread.
    Vector mResponses;
    size_t mResponseIndex;
    nsecs_t mNextMessageUptime; // set to LLONG_MAX when none
    int pollInner(int timeoutMillis);
    void awoken();
    void pushResponse(int events, const Request& request);
    static void initTLSKey();
    static void threadDestructor(void *st);
};

Lopperの原理
Looperイベントメカニズムは,実際にはシステム呼び出しepollに依存して実現される.複数のI/Oイベントを同時に監視できるI/O多重モデルです.Looperにとって、I/Oイベントとは、監視されているファイル記述子にデータが到着していないことです.メッセージが到着しないとスレッドはスリープ状態にあり,メッセージが到着したらメッセージを処理する.
Looper構造と解析
コンストラクション関数では、mWakeReadPipeFd、mWakeWritePipeFd、およびmEpollFdを構築することが最も重要です.解析関数では3つを解析しています.ここで、mWakeReadPipeFdとmWakeWritePipeFdは双方向パイプの両端であり、次にepollインスタンスを作成し、読み取り専用パイプファイル記述子をepollの監視リストに追加する.

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);
}
Looper::~Looper() {
    close(mWakeReadPipeFd);
    close(mWakeWritePipeFd);
    close(mEpollFd);
}

Looperコアメソッド
Androidアプリケーション層のLooperループがMessageを取得して処理する場合(message)queue.next()は、最終的にnativeレイヤのLooperに呼び出されます.PollInnterメソッド.最終的にはこの行のコードint eventCount=epoll_に実行されます.wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis); epoll_waitは、コンストラクション関数で初期化されたepollインスタンスに登録されたファイル記述子を傍受するためのIO読み書きイベントである.このファイル記述子にIO読み書きイベントが発生しない場合、現在のスレッドはepoll_になります.waitでは睡眠待ち状態に入り、待ち時間が長いtimeoutMillis.

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);
        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;
    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);
            }
        }
    }

epoll_wait目覚ましタイミング
上記の現在のスレッドを除いてepoll_waitでは睡眠待ち状態に入り、待ち時間が長くなるとtimeoutMillisが目を覚ます.もう一つの目覚めのタイミングは、新しいMessageを送信することです.このメッセージがdelayを必要としない場合は、直接起動します.delayがあり、メッセージキューのヘッダ(すなわち、最新の起動時間が更新された)が変化した場合にも、起動が行われる.

void Looper::sendMessageAtTime(nsecs_t uptime, const sp& 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::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);
        }
    }
}

に感謝
http://blog.ifjy.me/android/2016/07/16/Looper類分析html https://android.googlesource.com/platform/frameworks/native/+/jb-dev/libs/utils/Looper.cpp https://baike.baidu.com/item/epoll/10738144