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android java层事件分发机制

我们知道在系统启动的时候，SystemServer会启动WindowManagerService，而WindowManagerService在启动的时候就会通过InputManagerService来负责监控键盘消息。
这些键盘消息一般都是分发给当前激活的Activity窗口来处理的，因此，当前激活的Activity窗口在创建的时候，会到WindowManagerService中去注册一个接收键盘消息的通道，表明它要处理键盘消息，而当InputManager监控到有键盘消息时，就会分给给它处理。

当前激活的Activity窗口不再处于激活状态时，它也会到WindowManagerService中去反注册之前的键盘消息接收通道，这样，InputManager就不会再把键盘消息分发给它来处理。

本文主要介绍当前激活的Activity窗口已经注册好了消息接受通道，这些按键或者触屏事件是如何一步一步的传递到当前的activity或者View中的。而且本文主要关注java层的处理，后续我们会介绍native层的事件读取以及分发，敬请期待。

step1：Looper.pollOnce

这个函数定义在frameworks/base/libs/utils/Looper.cpp文件中。在Looper类中，会创建一个管道，当调用Looper类的pollOnce函数时，如果管道中没有内容可读，那么当前线程就会进入到空闲等待状态；当有事件发生时，InputReader就会往这个管道中写入新的内容，这样就会唤醒前面正在等待事件发生的线程。

int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
    int result = 0;
    for (;;) {
				//do simething

        result = pollInner(timeoutMillis);
    }
}

该函数调用Looper.pollInner做进一步的处理。

step2：Looper.pollInner

该函数定义在system\core\libutils\Looper.cpp中；这个函数的实现比较长：

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;

    // We are about to idle.
    mIdling = true;

    struct epoll_event eventItems[EPOLL_MAX_EVENTS];
    int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);

    // No longer idling.
    mIdling = false;

    // 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);
            }
        }
    }
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 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 = ALOOPER_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 == ALOOPER_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
            int callbackResult = response.request.callback->handleEvent(fd, events, data);
            if (callbackResult == 0) {
                removeFd(fd);
            }
            // 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 = ALOOPER_POLL_CALLBACK;
        }
    }
    return result;
}

下面我们分段进行分析。
（1）利用epoll_wait函数来监听looper上的event，一旦有event发生，其eventCount就会回传event的数目。假若在最后一个参数指定的timeoutMillis内都没有事件发生, 其eventCount就会回传0, 若有发生错误回传-1。由 EPOLL_MAX_EVENTS 的定义值可以知道, 所监听到的event数最多为16个event。代码如下

    struct epoll_event eventItems[EPOLL_MAX_EVENTS];
    int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);

（2）回调执行looper中所有request的函数handleEvent。代码如下

            int callbackResult = response.request.callback->handleEvent(fd, events, data);
            if (callbackResult == 0) {
                removeFd(fd);
            }

（3）此时大家就会有疑问了。response成员request的callback实例到底是什么呢？ Server端的InputChannel注册完成后，把Client端的InputChannel转换成addWindow的参数outInputChannel中，然后返回到ViewRootImpl.setView函数中，继续执行Client端的InputChannel的注册过程，即为应用程序这一侧注册消息接收通道。代码如下：

                if (mInputChannel != null) {
                    if (mInputQueueCallback != null) {
                        mInputQueue = new InputQueue();
                        mInputQueueCallback.onInputQueueCreated(mInputQueue);
                    }
                    mInputEventReceiver = new WindowInputEventReceiver(mInputChannel,
                            Looper.myLooper());
                }

实例化了一个带有两个回调函数的WindowInputEventReceiver对象。这两个回调函数onInputEvent 和 onBatchedInputEventPending 与native 层的InputDispatcherThread 监听事件处理有关。

下面我们看与request的callback设置相关的代码，这些code在Looper.cpp 的函数addFd中；

       Request request;
       request.fd = fd;
       request.ident = ident;
       request.callback = callback;
       request.data = data;

       struct epoll_event eventItem;
       memset(& eventItem, 0, sizeof(epoll_event)); // zero out unused members of data field union
       eventItem.events = epollEvents;
       eventItem.data.fd = fd;

       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, errno=%d", fd, errno);
               return -1;
           }
           mRequests.add(fd, request);
       } else {
           int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_MOD, fd, & eventItem);
           if (epollResult < 0) {
               ALOGE("Error modifying epoll events for fd %d, errno=%d", fd, errno);
               return -1;
           }
           mRequests.replaceValueAt(requestIndex, request);
       }

         此处我们发现，此初始化的流程正是注册Looper上的CallBack event 所带的信息的地方；也就是说先把request的information记录下来，等到要回调reponse的时候就知道要调哪一个request。
         由此可知, response.request.callback->handleEvent 等同于 (NativeInputEventReceiver*) receiver-> handleEvent。

下面我们进一步分析NativeInputEventReceiver.handleEvnet的实现。

Step 3. NativeInputEventReceiver. handleEvent

该函数定义在android_view_InputEventReceiver.cpp中；

int NativeInputEventReceiver::handleEvent(int receiveFd, int events, void* data) {
    if (events & (ALOOPER_EVENT_ERROR | ALOOPER_EVENT_HANGUP)) {
#if DEBUG_DISPATCH_CYCLE
        // This error typically occurs when the publisher has closed the input channel
        // as part of removing a window or finishing an IME session, in which case
        // the consumer will soon be disposed as well.
        ALOGD("channel '%s' ~ Publisher closed input channel or an error occurred.  "
                "events=0x%x", getInputChannelName(), events);
#endif
        return 0; // remove the callback
    }

    if (events & ALOOPER_EVENT_INPUT) {
        JNIEnv* env = AndroidRuntime::getJNIEnv();
        status_t status = consumeEvents(env, false /*consumeBatches*/, -1, NULL);
        mMessageQueue->raiseAndClearException(env, "handleReceiveCallback");
        return status == OK || status == NO_MEMORY ? 1 : 0;
    }

    if (events & ALOOPER_EVENT_OUTPUT) {
        for (size_t i = 0; i < mFinishQueue.size(); i++) {
            const Finish& finish = mFinishQueue.itemAt(i);
            status_t status = mInputConsumer.sendFinishedSignal(finish.seq, finish.handled);
            if (status) {
                mFinishQueue.removeItemsAt(0, i);

                if (status == WOULD_BLOCK) {
#if DEBUG_DISPATCH_CYCLE
                    ALOGD("channel '%s' ~ Sent %u queued finish events; %u left.",
                            getInputChannelName(), i, mFinishQueue.size());
#endif
                    return 1; // keep the callback, try again later
                }

                ALOGW("Failed to send finished signal on channel '%s'.  status=%d",
                        getInputChannelName(), status);
                if (status != DEAD_OBJECT) {
                    JNIEnv* env = AndroidRuntime::getJNIEnv();
                    String8 message;
                    message.appendFormat("Failed to finish input event.  status=%d", status);
                    jniThrowRuntimeException(env, message.string());
                    mMessageQueue->raiseAndClearException(env, "finishInputEvent");
                }
                return 0; // remove the callback
            }
        }
#if DEBUG_DISPATCH_CYCLE
        ALOGD("channel '%s' ~ Sent %u queued finish events; none left.",
                getInputChannelName(), mFinishQueue.size());
#endif
        mFinishQueue.clear();
        setFdEvents(ALOOPER_EVENT_INPUT);
        return 1;
    }

    ALOGW("channel '%s' ~ Received spurious callback for unhandled poll event.  "
            "events=0x%x", getInputChannelName(), events);
    return 1;
}

该函数继续调用consumeEvents做进一步的处理。

Step 4. NativeInputEventReceiver.consumeEvents

该函数定义在android_view_InputEventReceiver.cpp中；

status_t NativeInputEventReceiver::consumeEvents(JNIEnv* env,
        bool consumeBatches, nsecs_t frameTime, bool* outConsumedBatch) {
#if DEBUG_DISPATCH_CYCLE
    ALOGD("channel '%s' ~ Consuming input events, consumeBatches=%s, frameTime=%lld.",
            getInputChannelName(), consumeBatches ? "true" : "false", frameTime);
#endif

    if (consumeBatches) {
        mBatchedInputEventPending = false;
    }
    if (outConsumedBatch) {
        *outConsumedBatch = false;
    }

    ScopedLocalRef receiverObj(env, NULL);
    bool skipCallbacks = false;
    for (;;) {
        uint32_t seq;
        InputEvent* inputEvent;
        status_t status = mInputConsumer.consume(&mInputEventFactory,
                consumeBatches, frameTime, &seq, &inputEvent);
        if (status) {
            if (status == WOULD_BLOCK) {
                if (!skipCallbacks && !mBatchedInputEventPending
                        && mInputConsumer.hasPendingBatch()) {
                    // There is a pending batch.  Come back later.
                    if (!receiverObj.get()) {
                        receiverObj.reset(jniGetReferent(env, mReceiverWeakGlobal));
                        if (!receiverObj.get()) {
                            ALOGW("channel '%s' ~ Receiver object was finalized "
                                    "without being disposed.", getInputChannelName());
                            return DEAD_OBJECT;
                        }
                    }

                    mBatchedInputEventPending = true;
#if DEBUG_DISPATCH_CYCLE
                    ALOGD("channel '%s' ~ Dispatching batched input event pending notification.",
                            getInputChannelName());
#endif
                    env->CallVoidMethod(receiverObj.get(),
                            gInputEventReceiverClassInfo.dispatchBatchedInputEventPending);
                    if (env->ExceptionCheck()) {
                        ALOGE("Exception dispatching batched input events.");
                        mBatchedInputEventPending = false; // try again later
                    }
                }
                return OK;
            }
            ALOGE("channel '%s' ~ Failed to consume input event.  status=%d",
                    getInputChannelName(), status);
            return status;
        }
        assert(inputEvent);

        if (!skipCallbacks) {
            if (!receiverObj.get()) {
                receiverObj.reset(jniGetReferent(env, mReceiverWeakGlobal));
                if (!receiverObj.get()) {
                    ALOGW("channel '%s' ~ Receiver object was finalized "
                            "without being disposed.", getInputChannelName());
                    return DEAD_OBJECT;
                }
            }

            jobject inputEventObj;
            switch (inputEvent->getType()) {
            case AINPUT_EVENT_TYPE_KEY:
#if DEBUG_DISPATCH_CYCLE
                ALOGD("channel '%s' ~ Received key event.", getInputChannelName());
#endif
                inputEventObj = android_view_KeyEvent_fromNative(env,
                        static_cast(inputEvent));
                break;

            case AINPUT_EVENT_TYPE_MOTION: {
#if DEBUG_DISPATCH_CYCLE
                ALOGD("channel '%s' ~ Received motion event.", getInputChannelName());
#endif
                MotionEvent* motionEvent = static_cast(inputEvent);
                if ((motionEvent->getAction() & AMOTION_EVENT_ACTION_MOVE) && outConsumedBatch) {
                    *outConsumedBatch = true;
                }
                inputEventObj = android_view_MotionEvent_obtainAsCopy(env, motionEvent);
                break;
            }

            default:
                assert(false); // InputConsumer should prevent this from ever happening
                inputEventObj = NULL;
            }

            if (inputEventObj) {
#if DEBUG_DISPATCH_CYCLE
                ALOGD("channel '%s' ~ Dispatching input event.", getInputChannelName());
#endif
                env->CallVoidMethod(receiverObj.get(),
                        gInputEventReceiverClassInfo.dispatchInputEvent, seq, inputEventObj);
                if (env->ExceptionCheck()) {
                    ALOGE("Exception dispatching input event.");
                    skipCallbacks = true;
                }
                env->DeleteLocalRef(inputEventObj);
            } else {
                ALOGW("channel '%s' ~ Failed to obtain event object.", getInputChannelName());
                skipCallbacks = true;
            }
        }

        if (skipCallbacks) {
            mInputConsumer.sendFinishedSignal(seq, false);
        }
    }
}

函数consumeEvents主要做了两部分的工作；第一部分：将input event取出来。代码如下：

        uint32_t seq;
        InputEvent* inputEvent;
        status_t status = mInputConsumer.consume(&mInputEventFactory,
                consumeBatches, frameTime, &seq, &inputEvent);

第二部分：将取出的input event传给java 层的 dispatchInputEvent 函数继续做分发处理

                    env->CallVoidMethod(receiverObj.get(),
                            gInputEventReceiverClassInfo.dispatchBatchedInputEventPending);

Step 5. InputEventReceiver.dispatchInputEvent

我们基于step4，继续分析按键消息的分发，该函数定义在文件InputEventReceiver.java中；终于消息从native层传递到了java层。

    // Called from native code.
    @SuppressWarnings("unused")
    private void dispatchInputEvent(int seq, InputEvent event) {
        mSeqMap.put(event.getSequenceNumber(), seq);
        onInputEvent(event);
    }

dispatchInputEvent函数会继续将event传递给onInputEvent函数，然而此函数所属的InputEventReceiver是一个抽像类别，抽象类是无法实体化的，所以此函数最有可能是在抽像类所衍生的类中。InputEventReceiver类衍生的类是WindowInputEventReceiver类，所以就继续从WindowInputEventReceiver的onInputEvent函数分析。