handler发送延迟处理消息的原理

handler实现消息延迟发送的原理

handler每次发送消息，不管是否发送空消息，最终都会调用到如下方法中：

    public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        //时间为当前毫秒时间+需要延迟的毫秒时间
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }
    
    public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
        MessageQueue queue = mQueue;
        if (queue == null) {
            RuntimeException e = new RuntimeException(
                    this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
            return false;
        }
        return enqueueMessage(queue, msg, uptimeMillis);
    }
    
    private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        //mAsynchronous是在handler初始化的时候就确定了的，所以同一个handler发送的消息不会出现同步与异步消息混合发送的情况
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    

MessageQueue中enqueueMessage方法主要负责将从handler发送过来的message根据when的大小来添加到单向链表中，when的数据越大在链表中的位置越靠后，

    boolean enqueueMessage(Message msg, long when) {
        ......

        synchronized (this) {
            .....处理正在退出的情况
            msg.markInUse();
            msg.when = when;
            Message p = mMessages;
            boolean needWake;
            if (p == null || when == 0 || when < p.when) {
                // 当前链表为空，则添加为新的表头，如果当前阻塞，则唤醒队列
                msg.next = p;
                mMessages = msg;
                needWake = mBlocked;
            } else {
                // 将消息插入到链表中，when越大越靠后，通常我们不需要唤醒队列，除非该消息是最早的消息
                needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
                for (;;) {
                    prev = p;
                    p = p.next;
                    if (p == null || when < p.when) {
                        break;
                    }
                    if (needWake && p.isAsynchronous()) {
                        needWake = false;
                    }
                }
                msg.next = p; // invariant: p == prev.next
                prev.next = msg;
            }

            // We can assume mPtr != 0 because mQuitting is false.
            if (needWake) {
                nativeWake(mPtr);
            }
        }
        return true;
    }

该方法是将消息加入到链表中，是关键代码块加入了同步处理，这里跟从队列中获取消息想对应，都采用同步，防止出现添加与获取之间时间差导致异常问题。
消息获取到之后会检测当前时间与msg执行时间when，如果小于when，则通过nativePollOnce设置一个阻塞延迟唤醒，如果不是，则将该异步消息返回给handler去执行消费掉
消息获取如下：

    Message next() {
       //初始化成功，ptr就不可能为0
        final long ptr = mPtr;
        if (ptr == 0) {
            return null;
        }

        int pendingIdleHandlerCount = -1; 
        int nextPollTimeoutMillis = 0;
        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();
            }
            //阻塞延迟，该方法最终通过linux的epoll机制实现，调用方法为epoll_wait
            nativePollOnce(ptr, nextPollTimeoutMillis);
            //同步从链表中获取异步msg
            synchronized (this) {
                //一般情况下不会一直卡这里，往往都是异步消息
                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) {
                        // 消息执行时间未到，这里设置一个唤醒时间
                        nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                    } else {
                        // 得到可执行消息，将其返回
                        mBlocked = false;
                        if (prevMsg != null) {
                            prevMsg.next = msg.next;
                        } else {
                            mMessages = msg.next;
                        }
                        msg.next = null;
                        if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                        msg.markInUse();
                        return msg;
                    }
                } else {
                    // No more messages.
                    nextPollTimeoutMillis = -1;
                }

                ......
        }
    }

延时功能主要在nativePollOnce中实现，代码如下：
\frameworks\base\core\jni\android_os_MessageQueue.cpp

static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj,
        jlong ptr, jint timeoutMillis) {
    NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
    nativeMessageQueue->pollOnce(env, obj, timeoutMillis);
}

void NativeMessageQueue::pollOnce(JNIEnv* env, jobject pollObj, int timeoutMillis) {
    mPollEnv = env;
    mPollObj = pollObj;
    mLooper->pollOnce(timeoutMillis);
    mPollObj = NULL;
    mPollEnv = NULL;

    if (mExceptionObj) {
        env->Throw(mExceptionObj);
        env->DeleteLocalRef(mExceptionObj);
        mExceptionObj = NULL;
    }
}

这里调用到了looper.cpp中的方法pollOnce
\system\core\libutils\Looper.cpp

int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
    int result = 0;
    for (;;) {
        while (mResponseIndex < mResponses.size()) {
            const Response& response = mResponses.itemAt(mResponseIndex++);
            int ident = response.request.ident;
            if (ident >= 0) {
                int fd = response.request.fd;
                int events = response.events;
                void* data = response.request.data;
#if DEBUG_POLL_AND_WAKE
                ALOGD("%p ~ pollOnce - returning signalled identifier %d: "
                        "fd=%d, events=0x%x, data=%p",
                        this, ident, fd, events, data);
#endif
                if (outFd != NULL) *outFd = fd;
                if (outEvents != NULL) *outEvents = events;
                if (outData != NULL) *outData = data;
                return ident;
            }
        }

        if (result != 0) {
#if DEBUG_POLL_AND_WAKE
            ALOGD("%p ~ pollOnce - returning result %d", this, result);
#endif
            if (outFd != NULL) *outFd = 0;
            if (outEvents != NULL) *outEvents = 0;
            if (outData != NULL) *outData = NULL;
            return result;
        }

        result = pollInner(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 %" 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];
    //在指定文件描述符上等待IO时间，直到timeoutMillis超时时间到
    int eventCount = epoll_wait(mEpollFd, 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.
    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.
    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

    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);
            }
        }
    }
Done: ;

    // Invoke pending message callbacks.
    mNextMessageUptime = LLONG_MAX;
    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;
        }
    }

    // Release lock.
    mLock.unlock();

    // 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;
        }
    }
    return result;
}

至此，handler发送延时消息就通过向messagequeue中按照消息执行时间when添加链表后，通过next方法不断获取出来，对比当前时间与执行时间的大小，设置超时阻塞等待时间nextPollTimeoutMillis，然后调用本地方法通过looper.cpp中使用epoll_wait实现阻塞等待超时时间到达。
但是，如果正在等待超时时间过程中，又有新的消息发送过来呢 ？如何处理，
next方法中如果在阻塞等待后，mBlocked为true，在添加消息到链表之后，如果是添加在链表前面，则会调用唤醒方法：

needWake = mBlocked && p.target == null && msg.isAsynchronous();
......
if (needWake) {
                nativeWake(mPtr);
            }

\frameworks\base\core\jni\android_os_MessageQueue.cpp

static void android_os_MessageQueue_nativeWake(JNIEnv* env, jclass clazz, jlong ptr) {
    NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
    nativeMessageQueue->wake();
}

void NativeMessageQueue::wake() {
    mLooper->wake();
}

\system\core\libutils\Looper.cpp

void Looper::wake() {
#if DEBUG_POLL_AND_WAKE
    ALOGD("%p ~ wake", this);
#endif

    uint64_t inc = 1;
    ssize_t nWrite = TEMP_FAILURE_RETRY(write(mWakeEventFd, &inc, sizeof(uint64_t)));
    if (nWrite != sizeof(uint64_t)) {
        if (errno != EAGAIN) {
            LOG_ALWAYS_FATAL("Could not write wake signal to fd %d: %s",
                    mWakeEventFd, strerror(errno));
        }
    }
}

这里通过向mWakeEventFd文件描述符写入数据，将epoll_wait唤醒，停止阻塞，正常的去执行发送不需要延迟的消息。