Android輸入子系統之應用程序註冊消息監聽過程分析

應用程序註冊消息監聽過程分析

CPP層InputManagerService啓動後就需要監聽按鍵輸入了，當InputManagerService監聽到鍵盤輸入的事件後就需要分發鍵盤事件，但是分發給誰呢？這裏首先應該是分發給當前激活的Window窗口，但是當前激活的window窗口怎麼才能接受到消息呢，window窗口需要註冊一個鍵盤消息接收通道到InputManagerService中，那麼如何註冊的呢？我們知道一個Activity對應一個ViewRootImpl對象，在Activity啓動時會創建一個ViewRootImpl對象，並調用其setView函數把Activity的DecorView設置到ViewRootImpl中，而Activity正是在setView函數中註冊鍵盤消息的接收通道的。

Step 1. ViewRootImpl.setView

該函數定義在frameworks/base/core/java/android/view/ViewRootImpl.java中

public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) { 
    //調用requestLayout來通知InputManagerService當前的窗口是激活的窗口
    requestLayout();
    if ((mWindowAttributes.inputFeatures
            & WindowManager.LayoutParams.INPUT_FEATURE_NO_INPUT_CHANNEL) == 0) {
        //如果該窗口沒有指定INPUT_FEATURE_NO_INPUT_CHANNEL屬性，則創建消息接收通道InputChannel
        mInputChannel = new InputChannel();
    }
    try {
        //通過binder調用，調用server端的Session對象來跟WindowManagerService通信，該函數最後會調      
        //用到WindowManagerService的addWindow函數，函數中會創建一對InputChannel(server/client)，
        //這樣在函數調用結束後，mInputChannel就變成了client端的對象。在
        //frameworks/base/core/java/android/view/IWindowSession.aidl的
        //addToDisplay函數的聲明中，InputChannel指定的數據流的流向是out，因此
        //WindowManagerService修改了mInputChannel,客戶端就能拿到這個對象的數據了。
        res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
             getHostVisibility(), mDisplay.getDisplayId(),
             mAttachInfo.mContentInsets, mAttachInfo.mStableInsets,
             mAttachInfo.mOutsets, mInputChannel);
    } catch (Exception e) {
        ...
    }
    if (mInputChannel != null) {
        if (mInputQueueCallback != null) {
            mInputQueue = new InputQueue();
            mInputQueueCallback.onInputQueueCreated(mInputQueue);
            //初始化WindowInputEventReceiver，按鍵消息會從native層傳到該對象的onInputEvent函數
            //中，onInputEvent函數是按鍵在應用端java層分發的起始端。
            mInputEventReceiver = new WindowInputEventReceiver(mInputChannel,
                    Looper.myLooper());
        }
    }
}

這個函數與註冊鍵盤消息通道的相關主要有三個功能：
一是調用requestLayout函數來通知InputManagerService，這個Activity窗口是當前被激活的窗口,同時將所有的窗口註冊到InputDispatcher中
二是調用mWindowSession的add成員函數來把鍵盤消息接收通道的server端註冊端註冊到CPP層的InputManagerService中，client端註冊到本應用程序的消息循環Looper中，這樣當InputManagerService監控到有鍵盤消息的時候，就會找到當前被激活的窗口，然後找到其在InputManagerService中對應的鍵盤消息接收通道(InputChannel)，通過這個通道在InputManagerService的server端來通知應用程序消息循環的client端，這樣就把鍵盤消息分發給當前激活的Activity窗口了
三是應用程序這一側註冊消息接收通道

Step 2. ViewRootImpl.requestLayout

該函數定義在frameworks/base/core/java/android/view/ViewRootImpl.java中

@Override
public void requestLayout() {
    if (!mHandlingLayoutInLayoutRequest) {
        checkThread();
        mLayoutRequested = true;
        scheduleTraversals();
    }    
} 

這裏調用了scheduleTraversals函數來做進一步的操作，該函數調用mChoreographer來post一個Runnable到Looper中，之後會執行mTraversalRunnable中的run方法，即調用doTraversal函數

Step 3. ViewRootImpl.doTraversal

該函數定義在frameworks/base/core/java/android/view/ViewRootImpl.java中

void doTraversal() {
    if (mTraversalScheduled) {
        mTraversalScheduled = false;
        mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
        performTraversals();
    }
}

該函數主要是執行performTraversals()函數，進而調用relayoutWindow函數，在該函數中又會調用mWindowSession的relayout進入到java層的WindowManagerService的relayoutWindow函數，該函數會調用mInputMonitor.updateInputWindowsLw(true /force/);mInputMonitor是InputMonitor對象。

Step 4. InputMonitor.updateInputWindowsLw

該函數定義在frameworks/base/core/java/android/view/ViewRootImpl.java中

public void updateInputWindowsLw(boolean force) {

    boolean addInputConsumerHandle = mService.mInputConsumer != null;

    // Add all windows on the default display.
    final int numDisplays = mService.mDisplayContents.size();
    for (int displayNdx = 0; displayNdx < numDisplays; ++displayNdx) {
        WindowList windows = mService.mDisplayContents.valueAt(displayNdx).getWindowList();
        for (int winNdx = windows.size() - 1; winNdx >= 0; --winNdx) {
            final WindowState child = windows.get(winNdx);
            final InputChannel inputChannel = child.mInputChannel;
            final InputWindowHandle inputWindowHandle = child.mInputWindowHandle;
            if (inputChannel == null || inputWindowHandle == null || child.mRemoved) {
                // Skip this window because it cannot possibly receive input.
                continue;
            }
            if (addInputConsumerHandle
                    && inputWindowHandle.layer <= mService.mInputConsumer.mWindowHandle.layer) {
                addInputWindowHandleLw(mService.mInputConsumer.mWindowHandle);
                addInputConsumerHandle = false;
            }

            final int flags = child.mAttrs.flags;
            final int privateFlags = child.mAttrs.privateFlags;
            final int type = child.mAttrs.type;

            final boolean hasFocus = (child == mInputFocus);
            final boolean isVisible = child.isVisibleLw();
            if ((privateFlags
                    & WindowManager.LayoutParams.PRIVATE_FLAG_DISABLE_WALLPAPER_TOUCH_EVENTS)
                        != 0) {
                disableWallpaperTouchEvents = true;
            }
            final boolean hasWallpaper = (child == mService.mWallpaperTarget)
                    && (privateFlags & WindowManager.LayoutParams.PRIVATE_FLAG_KEYGUARD) == 0
                    && !disableWallpaperTouchEvents;
            final boolean onDefaultDisplay = (child.getDisplayId() == Display.DEFAULT_DISPLAY);

            // If there's a drag in progress and 'child' is a potential drop target,
            // make sure it's been told about the drag
            if (inDrag && isVisible && onDefaultDisplay) {
                mService.mDragState.sendDragStartedIfNeededLw(child);
            }

            addInputWindowHandleLw(inputWindowHandle, child, flags, type, isVisible, hasFocus,
                    hasWallpaper);
        }
    }
    // Send windows to native code.
    mService.mInputManager.setInputWindows(mInputWindowHandles);
}

這個函數將當前系統中帶有InputChannel的Activity窗口都設置爲InputManagerService的輸入窗口，但是後面我們會看到，只有當前激活的窗口才會響應鍵盤消息。

Step 5. InputManagerService.setInputWindows

該函數定義在frameworks/base/services/core/java/com/android/server/input/InputManagerService.java

public void setInputWindows(InputWindowHandle[] windowHandles) {
    nativeSetInputWindows(mPtr, windowHandles);
}

這個函數調用了本地方法nativeSetInputWindows來進一步執行操作,mPtr是native層NativeInputManager實例，在調用InputManagerService.nativeInit函數時會在native層構造NativeInputManager對象並將其保存在mPtr中。nativeSetInputWindows會調用NativeInputManager的setInputWindows函數

Step 6. NativeInputManager.setInputWindows

這個函數定義在frameworks/base/services/core/jni/com_android_server_input_InputManagerService.cpp

void NativeInputManager::setInputWindows(JNIEnv* env, jobjectArray windowHandleObjArray) {
    Vector<sp<InputWindowHandle> > windowHandles;

    if (windowHandleObjArray) {
        jsize length = env->GetArrayLength(windowHandleObjArray);
        for (jsize i = 0; i < length; i++) {
            jobject windowHandleObj = env->GetObjectArrayElement(windowHandleObjArray, i);
            if (! windowHandleObj) {
                break; // found null element indicating end of used portion of the array
            }

            sp<InputWindowHandle> windowHandle =
                    android_server_InputWindowHandle_getHandle(env, windowHandleObj);
            if (windowHandle != NULL) {
                windowHandles.push(windowHandle);
            }
            env->DeleteLocalRef(windowHandleObj);
        }
    }

    mInputManager->getDispatcher()->setInputWindows(windowHandles);

    // Do this after the dispatcher has updated the window handle state.
    bool newPointerGesturesEnabled = true;
    size_t numWindows = windowHandles.size();
    for (size_t i = 0; i < numWindows; i++) {
        const sp<InputWindowHandle>& windowHandle = windowHandles.itemAt(i);
        const InputWindowInfo* windowInfo = windowHandle->getInfo();
        if (windowInfo && windowInfo->hasFocus && (windowInfo->inputFeatures
                & InputWindowInfo::INPUT_FEATURE_DISABLE_TOUCH_PAD_GESTURES)) {
            newPointerGesturesEnabled = false;
        }
    }
}

這個函數首先將Java層的InputWindowHandle轉換成C++層的NativeInputWindowHandle，然後放在windowHandles向量中，最後將這些輸入窗口設置到InputDispatcher中去。

Step 7. InputDispatcher.setInputWindows

這個函數定義在frameworks/native/services/inputflinger/InputDispatcher.cpp 文件中：

void InputDispatcher::setInputWindows(const Vector<sp<InputWindowHandle> >& inputWindowHandles) {
    { // acquire lock
        AutoMutex _l(mLock);

        Vector<sp<InputWindowHandle> > oldWindowHandles = mWindowHandles;
        mWindowHandles = inputWindowHandles;

        sp<InputWindowHandle> newFocusedWindowHandle;
        bool foundHoveredWindow = false;
        for (size_t i = 0; i < mWindowHandles.size(); i++) {
            const sp<InputWindowHandle>& windowHandle = mWindowHandles.itemAt(i);
            if (!windowHandle->updateInfo() || windowHandle->getInputChannel() == NULL) {
                mWindowHandles.removeAt(i--);
                continue;
            }
            if (windowHandle->getInfo()->hasFocus) {
                newFocusedWindowHandle = windowHandle;
            }
        }

        if (mFocusedWindowHandle != newFocusedWindowHandle) {
            mFocusedWindowHandle = newFocusedWindowHandle;
        }
    } // release lock

    // Wake up poll loop since it may need to make new input dispatching choices.
    mLooper->wake();
}

這裏InputDispatcher的成員變量mFocusedWindowHandle 就代表當前激活的窗口的。這個函數遍歷inputWindowHandles，獲取獲得焦點的窗口，並賦值給mFocusedWindowHandle 這樣，InputManagerService就把當前激活的窗口保存在InputDispatcher中了，後面就可以把鍵盤消息分發給它來處理。

Step 8. 在前面的ViewRootImpl的setView中，下一步是server端註冊接收通道

if ((mWindowAttributes.inputFeatures
            & WindowManager.LayoutParams.INPUT_FEATURE_NO_INPUT_CHANNEL) == 0) {
        mInputChannel = new InputChannel();
    }
    try {
        res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
             getHostVisibility(), mDisplay.getDisplayId(),
             mAttachInfo.mContentInsets, mAttachInfo.mStableInsets,
             mAttachInfo.mOutsets, mInputChannel);
    } catch (Exception e) {
        ...
    }

這裏會調用到WindowManagerService的addWindow接口

Step 9. WindowManagerService.addWindow

該函數定義在frameworks/base/services/core/java/com/android/server/wm/WindowManagerService.java

public int addWindow(Session session, IWindow client, int seq,
    WindowManager.LayoutParams attrs, int viewVisibility, int displayId,
    Rect outContentInsets, Rect outStableInsets, Rect outOutsets,
    InputChannel outInputChannel) {
        if (outInputChannel != null && (attrs.inputFeatures
                & WindowManager.LayoutParams.INPUT_FEATURE_NO_INPUT_CHANNEL) == 0) {
            String name = win.makeInputChannelName();
            InputChannel[] inputChannels = InputChannel.openInputChannelPair(name);
            win.setInputChannel(inputChannels[0]);
            inputChannels[1].transferTo(outInputChannel);

            mInputManager.registerInputChannel(win.mInputChannel, win.mInputWindowHandle);
        }
}

這裏的outInputChannel即爲前面創建的InputChannel，它不爲NULL，因此，這裏會通過InputChannel.openInputChannelPair函數來創建一對輸入通道，其中一個位於WindowManagerService中，另外一個通過outInputChannel參數返回到應用程序中。WindowManagerService會爲每個窗口創建一個WindowState對象，然後將該InputChannel對的service端保存到WindowState中

Step 10. 下面看下InputChannel.openInputChannelPair的實現

函數定義在frameworks/base/core/java/android/view/InputChannel.java

public static InputChannel[] openInputChannelPair(String name) {
    if (name == null) {
        throw new IllegalArgumentException("name must not be null");
    }   

    if (DEBUG) {
        Slog.d(TAG, "Opening input channel pair '" + name + "'");
    }   
    return nativeOpenInputChannelPair(name);
} 

調用了nativeOpenInputChannelPair函數，在native創建一個InputChannel對

Step 11. InputChannel.nativeOpenInputChannelPair

函數定義在frameworks/base/core/jni/android_view_InputChannel.cpp

static jobjectArray android_view_InputChannel_nativeOpenInputChannelPair(JNIEnv* env,
        jclass clazz, jstring nameObj) {
    const char* nameChars = env->GetStringUTFChars(nameObj, NULL);
    String8 name(nameChars);
    env->ReleaseStringUTFChars(nameObj, nameChars);

    sp<InputChannel> serverChannel;
    sp<InputChannel> clientChannel;
    status_t result = InputChannel::openInputChannelPair(name, serverChannel, clientChannel);

    ...
    return channelPair;
}

nativeOpenInputChannelPair函數調用InputChannel的openInputChannelPair函數創建一對InputChannel,該對象是Native層的InputChannel,跟java層是一一對應的。

Step 12. InputChannel.openInputChannelPair

該函數定義在frameworks/native/libs/input/InputTransport.cpp中

status_t InputChannel::openInputChannelPair(const String8& name,
        sp<InputChannel>& outServerChannel, sp<InputChannel>& outClientChannel) {
    int sockets[2];
    if (socketpair(AF_UNIX, SOCK_SEQPACKET, 0, sockets)) {
        status_t result = -errno;
        ALOGE("channel '%s' ~ Could not create socket pair.  errno=%d",
                name.string(), errno);
        outServerChannel.clear();
        outClientChannel.clear();
        return result;
    }   

    int bufferSize = SOCKET_BUFFER_SIZE;
    //設置server端和client端的接收緩衝區和發送緩衝區
    setsockopt(sockets[0], SOL_SOCKET, SO_SNDBUF, &bufferSize, sizeof(bufferSize));
    setsockopt(sockets[0], SOL_SOCKET, SO_RCVBUF, &bufferSize, sizeof(bufferSize));
    setsockopt(sockets[1], SOL_SOCKET, SO_SNDBUF, &bufferSize, sizeof(bufferSize));
    setsockopt(sockets[1], SOL_SOCKET, SO_RCVBUF, &bufferSize, sizeof(bufferSize));

    String8 serverChannelName = name;
    serverChannelName.append(" (server)");
    outServerChannel = new InputChannel(serverChannelName, sockets[0]);

    String8 clientChannelName = name;
    clientChannelName.append(" (client)");
    outClientChannel = new InputChannel(clientChannelName, sockets[1]);
    return OK; 
}

這裏調用了socketpair系統調用創建了一對已經連接的UNIX租socket,這裏可以把這一對socket當成pipe返回的文件描述符一樣使用，pipe返回的管道是單向管道，即只能從一端寫入，一端讀出，但是socketpair是創建的管道是全雙工的，可讀可寫。

創建好了server端和client端通道後，在WindowManagerService.addWindow函數中，一方面它把剛纔創建的Client端的輸入通道通過outInputChannel參數返回到應用程序中：

inputChannels[1].transferTo(outInputChannel);

另外還需要把server端的InputChannel註冊到InputManagerService中：

mInputManager.registerInputChannel(win.mInputChannel, win.mInputWindowHandle);

Step 13. InputManagerService.registerInputChannel

函數定義在frameworks/base/services/core/java/com/android/server/input/InputManagerService.java

public void registerInputChannel(InputChannel inputChannel,
        InputWindowHandle inputWindowHandle) {
    if (inputChannel == null) {
        throw new IllegalArgumentException("inputChannel must not be null.");
    }    

    nativeRegisterInputChannel(mPtr, inputChannel, inputWindowHandle, false);
}

通過調用nativeRegisterInputChannel來將InputChannel註冊到native層

Step 14. InputManagerService.nativeRegisterInputChannel

這個函數定義在frameworks/base/services/core/jni/com_android_server_input_InputManagerService.cpp

static void nativeRegisterInputChannel(JNIEnv* env, jclass /* clazz */,
        jlong ptr, jobject inputChannelObj, jobject inputWindowHandleObj, jboolean monitor) {
    NativeInputManager* im = reinterpret_cast<NativeInputManager*>(ptr);

    sp<InputChannel> inputChannel = android_view_InputChannel_getInputChannel(env,
            inputChannelObj); 

    sp<InputWindowHandle> inputWindowHandle =
            android_server_InputWindowHandle_getHandle(env, inputWindowHandleObj);

    status_t status = im->registerInputChannel(
            env, inputChannel, inputWindowHandle, monitor);
}

根據java層的InputWindowHandle獲得native層的InputWindowHandle對象，根據java層的InputChannel獲得native層的InputChannel對象，然後調用NativeInputManager的resgiterInputChannel，該函數又調用了InputDispatcher的registerInputChannel

Step 15. InputDispatcher.registerInputChannel

該函數定義在frameworks/native/services/inputflinger/InputDispatcher.cpp

status_t InputDispatcher::registerInputChannel(const sp<InputChannel>& inputChannel,
        const sp<InputWindowHandle>& inputWindowHandle, bool monitor) {
    { // acquire lock
        AutoMutex _l(mLock);

        sp<Connection> connection = new Connection(inputChannel, inputWindowHandle, monitor);

        int fd = inputChannel->getFd();
        mConnectionsByFd.add(fd, connection);

        if (monitor) {
            mMonitoringChannels.push(inputChannel);
        }    

        mLooper->addFd(fd, 0, ALOOPER_EVENT_INPUT, handleReceiveCallback, this);
    } // release lock

    // Wake the looper because some connections have changed.
    mLooper->wake();
    return OK;
}

將InputWindowHandle, InputChanel封裝成Connection對象，然後fd作爲key，Connection作爲Value，保存在mConnectionsByFd中，如果傳入的monitor是true，則需要將InputChannel放到mMonitoringChannels中,從上面的InputManagerService的registerInputChannel函數裏傳入的monitor是false，所以這裏不加入到mMonitoringChannels。同時把fd加入到mLooper的監聽中，並指定當該fd有內容可讀時，Looper就會調用handleReceiveCallback函數。至此server端的InputChannel註冊完成，InputDispatcher睡眠在監聽的fds上，當有按鍵事件發生時，InputDispatcher就會往這些fd寫入InputMessage對象，進而回調handleReceiveCallback函數。//TODO這裏當客戶端的進程讀取完事件後會往InputChannel的client端寫入？？？？，而server端可以收到，這時InputDispatcher就會被喚醒，然後調用handleReceiveCallback來表明按鍵接收結束。

至此，server端的InputChannel就註冊完成了，再回到前面的WindowManagerService.addWindow上的第二步inputChannels[1].transferTo(outInputChannel);，這個是將創建的一對InputChannel的client端複製到傳入的參數InputChannel上，當addWindow返回時，就回到ViewRootImpl.setView函數中，執行應用程序這一側的鍵盤消息接收通道。

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

WindowInputEventReceiver繼承自InputEventReceiver類。

Step 16. InputEventReceiver.init

構造函數定義在frameworks/base/core/java/android/view/InputEventReceiver.java

public InputEventReceiver(InputChannel inputChannel, Looper looper) {

    mInputChannel = inputChannel;
    mMessageQueue = looper.getQueue();
    mReceiverPtr = nativeInit(new WeakReference<InputEventReceiver>(this),
            inputChannel, mMessageQueue);

    mCloseGuard.open("dispose");
} 

調用了nativeInit執行native層的初始化

Step 17. InputEventReceiver.nativeInit

定義在frameworks/base/core/jni/android_view_InputEventReceiver.cpp

static jlong nativeInit(JNIEnv* env, jclass clazz, jobject receiverWeak,
        jobject inputChannelObj, jobject messageQueueObj) {
    sp<InputChannel> inputChannel = android_view_InputChannel_getInputChannel(env,
            inputChannelObj);

    sp<MessageQueue> messageQueue = android_os_MessageQueue_getMessageQueue(env, messageQueueObj);

    sp<NativeInputEventReceiver> receiver = new NativeInputEventReceiver(env,
            receiverWeak, inputChannel, messageQueue);
    status_t status = receiver->initialize();

    receiver->incStrong(gInputEventReceiverClassInfo.clazz); // retain a reference for the object
    return reinterpret_cast<jlong>(receiver.get());
}

函數創建了一個NativeInputEventReceiver對象，並調用其initialize函數

Step 18. NativeInputEventReceiver.initialize

該函數定義在frameworks/base/core/jni/android_view_InputEventReceiver.cpp

status_t NativeInputEventReceiver::initialize() {
    setFdEvents(ALOOPER_EVENT_INPUT);
    return OK;
}

調用setFdEvents函數

Step 19. NativeInputEventReceiver.setFdEvents

該函數定義在frameworks/base/core/jni/android_view_InputEventReceiver.cpp

void NativeInputEventReceiver::setFdEvents(int events) {
    if (mFdEvents != events) {
        mFdEvents = events;
        int fd = mInputConsumer.getChannel()->getFd();
        if (events) {
            mMessageQueue->getLooper()->addFd(fd, 0, events, this, NULL);
        } else {
            mMessageQueue->getLooper()->removeFd(fd);
        }
    }
}

這裏調用傳入的MessageQueue獲取Looper對象，如果events是0，則表示要移除監聽fd，如果events不爲0，表示要監聽fd，這個fd是前面WindowManagerService創建的一對InputChannel的client端，這樣當Server端寫入事件時，client端的looper就能被喚醒，並調用handleEvent函數（Looper::addFd函數可以指定LooperCallback對象，當fd可讀時，會調用LooperCallback的handleEvent，而NativeInputEventReceiver繼承自LooperCallback，所以這裏會調用NativeInputEventReceiver的handleEvent函數）

Step 20. NativeInputEventReceiver.handleEvent

該函數定義在frameworks/base/core/jni/android_view_InputEventReceiver.cpp

int NativeInputEventReceiver::handleEvent(int receiveFd, int events, void* data) {
    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;
    } 
}

該函數調用consumeEvents函數來處理接收一個按鍵事件

Step 21. NativeInputEventReceiver.consumeEvents

該函數定義在frameworks/base/core/jni/android_view_InputEventReceiver.cpp

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

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

    ScopedLocalRef<jobject> receiverObj(env, NULL);
    bool skipCallbacks = false;
    for (;;) {
        uint32_t seq;
        InputEvent* inputEvent;
        status_t status = mInputConsumer.consume(&mInputEventFactory,
                consumeBatches, frameTime, &seq, &inputEvent);
        if (!skipCallbacks) {
            jobject inputEventObj;
            switch (inputEvent->getType()) {
            case AINPUT_EVENT_TYPE_KEY:
                inputEventObj = android_view_KeyEvent_fromNative(env,
                        static_cast<KeyEvent*>(inputEvent));
                break;
            }

            if (inputEventObj) {
                env->CallVoidMethod(receiverObj.get(),
                        gInputEventReceiverClassInfo.dispatchInputEvent, seq, inputEventObj);
                env->DeleteLocalRef(inputEventObj);
            }
        }

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

函數首先調用mInputConsumer.consume接收一個InputEvent對象,mInputConsumer在NativeInputEventReceiver構造函數中初始化

Step 22. InputConsumer.consume

該函數定義在frameworks/native/libs/input/InputTransport.cpp

status_t InputConsumer::consume(InputEventFactoryInterface* factory,
        bool consumeBatches, nsecs_t frameTime, uint32_t* outSeq, InputEvent** outEvent) {

    *outSeq = 0;
    *outEvent = NULL;

    // Fetch the next input message.
    // Loop until an event can be returned or no additional events are received.
    while (!*outEvent) {
        if (mMsgDeferred) {
            // mMsg contains a valid input message from the previous call to consume
            // that has not yet been processed.
            mMsgDeferred = false;
        } else {
            // Receive a fresh message.
            status_t result = mChannel->receiveMessage(&mMsg);
            if (result) {
                // Consume the next batched event unless batches are being held for later.
                if (consumeBatches || result != WOULD_BLOCK) {
                    result = consumeBatch(factory, frameTime, outSeq, outEvent);
                    if (*outEvent) {
                        break;
                    }   
                }   
                return result;
            }   
        }   

        switch (mMsg.header.type) {
        case InputMessage::TYPE_KEY: {
            KeyEvent* keyEvent = factory->createKeyEvent();
            if (!keyEvent) return NO_MEMORY;

            initializeKeyEvent(keyEvent, &mMsg);
            *outSeq = mMsg.body.key.seq;
            *outEvent = keyEvent;
            break;
        }   
   }
   return OK;
}

函數首先調用InputChannel的receiveMessage函數接收InputMessage對象，然後根據InputMessage對象調用initializeKeyEvent來構造KeyEvent對象。拿到可KeyEvent對象後，再對到consumeEvents中調用java層的InputEventReceiver.java的dispatchInputEvent函數

env->CallVoidMethod(receiverObj.get(),
                        gInputEventReceiverClassInfo.dispatchInputEvent, seq, inputEventObj);

Step 23. InputEventReceiver.dispatchInputEvent

該函數定義在frameworks/base/core/java/android/view/InputEventReceiver.java

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

進而調用onInputEvent函數。至此按鍵就開始了java層的分發。

參考

http://blog.csdn.net/luoshengyang/article/details/6882903