需求

    最近公司来了一个需求，需要将touch事件转成key事件，只针对滑动事件与触摸事件。

需求分析

    首先这个需求是可以在kernel里面做的，由于我们没有kernel代码，因此这个方案就被pass掉了。第二个想到的是在java层做，想找一个拦截touch事件的方法，类似PhoneWindowMananger中拦截key事件的方法，可是没找到，找到的地方都已经到app层了，这样就可能有点晚了（也可能java层有，只是我还没找到合适的地方）。最后没办法将android事件分发机制重新看了一遍，决定直接在native层做，也就是读取event的地方----InputReader.cpp中。

具体实现

    这个需求里面最重要的就是找到合适的地方拦截touch然后转成key。

touch事件分发的函数：

void TouchInputMapper::dispatchMotion(nsecs_t when, uint32_t policyFlags, uint32_t source,
        int32_t action, int32_t actionButton, int32_t flags,
        int32_t metaState, int32_t buttonState, int32_t edgeFlags,
        const PointerProperties* properties, const PointerCoords* coords,
        const uint32_t* idToIndex, BitSet32 idBits, int32_t changedId,
        float xPrecision, float yPrecision, nsecs_t downTime) {
    PointerCoords pointerCoords[MAX_POINTERS];
    PointerProperties pointerProperties[MAX_POINTERS];
    uint32_t pointerCount = 0;
    while (!idBits.isEmpty()) {
        uint32_t id = idBits.clearFirstMarkedBit();
        uint32_t index = idToIndex[id];
        pointerProperties[pointerCount].copyFrom(properties[index]);
        pointerCoords[pointerCount].copyFrom(coords[index]);

        if (changedId >= 0 && id == uint32_t(changedId)) {
            action |= pointerCount << AMOTION_EVENT_ACTION_POINTER_INDEX_SHIFT;
        }

        pointerCount += 1;
    }

    ALOG_ASSERT(pointerCount != 0);

    if (changedId >= 0 && pointerCount == 1) {
        // Replace initial down and final up action.
        // We can compare the action without masking off the changed pointer index
        // because we know the index is 0.
        if (action == AMOTION_EVENT_ACTION_POINTER_DOWN) {
            action = AMOTION_EVENT_ACTION_DOWN;
        } else if (action == AMOTION_EVENT_ACTION_POINTER_UP) {
            action = AMOTION_EVENT_ACTION_UP;
        } else {
            // Can't happen.
            ALOG_ASSERT(false);
        }
    }

    NotifyMotionArgs args(when, getDeviceId(), source, policyFlags,
            action, actionButton, flags, metaState, buttonState, edgeFlags,
            mViewport.displayId, pointerCount, pointerProperties, pointerCoords,
            xPrecision, yPrecision, downTime);
    getListener()->notifyMotion(&args);
}

可以看到dispatchMotion这个函数的最后，构建了一个NotifyMotionArgs，然后通过getListener()->notifyMotion(&args)函数分发，NotifyMotionArgs这个类构造函数中的参数就是touch事件的参数，比如eventTime（也就是when）、action、xy坐标等等都是后面我们要用的。

InputReader.cpp中也有key事件的分发函数，我们可以利用这个函数来转key事件：

void KeyboardInputMapper::processKey(nsecs_t when, bool down, int32_t scanCode,
        int32_t usageCode) {
    int32_t keyCode;
    int32_t keyMetaState;
    uint32_t policyFlags;

    if (getEventHub()->mapKey(getDeviceId(), scanCode, usageCode, mMetaState,
                              &keyCode, &keyMetaState, &policyFlags)) {
        keyCode = AKEYCODE_UNKNOWN;
        keyMetaState = mMetaState;
        policyFlags = 0;
    }

    if (down) {
        // Rotate key codes according to orientation if needed.
        if (mParameters.orientationAware && mParameters.hasAssociatedDisplay) {
            keyCode = rotateKeyCode(keyCode, mOrientation);
        }

        // Add key down.
        ssize_t keyDownIndex = findKeyDown(scanCode);
        if (keyDownIndex >= 0) {
            // key repeat, be sure to use same keycode as before in case of rotation
            keyCode = mKeyDowns.itemAt(keyDownIndex).keyCode;
        } else {
            // key down
            if ((policyFlags & POLICY_FLAG_VIRTUAL)
                    && mContext->shouldDropVirtualKey(when,
                            getDevice(), keyCode, scanCode)) {
                return;
            }
            if (policyFlags & POLICY_FLAG_GESTURE) {
                mDevice->cancelTouch(when);
            }

            mKeyDowns.push();
            KeyDown& keyDown = mKeyDowns.editTop();
            keyDown.keyCode = keyCode;
            keyDown.scanCode = scanCode;
        }

        mDownTime = when;
    } else {
        // Remove key down.
        ssize_t keyDownIndex = findKeyDown(scanCode);
        if (keyDownIndex >= 0) {
            // key up, be sure to use same keycode as before in case of rotation
            keyCode = mKeyDowns.itemAt(keyDownIndex).keyCode;
            mKeyDowns.removeAt(size_t(keyDownIndex));
        } else {
            // key was not actually down
            ALOGI("Dropping key up from device %s because the key was not down.  "
                    "keyCode=%d, scanCode=%d",
                    getDeviceName().string(), keyCode, scanCode);
            return;
        }
    }

    if (updateMetaStateIfNeeded(keyCode, down)) {
        // If global meta state changed send it along with the key.
        // If it has not changed then we'll use what keymap gave us,
        // since key replacement logic might temporarily reset a few
        // meta bits for given key.
        keyMetaState = mMetaState;
    }

    nsecs_t downTime = mDownTime;

    // Key down on external an keyboard should wake the device.
    // We don't do this for internal keyboards to prevent them from waking up in your pocket.
    // For internal keyboards, the key layout file should specify the policy flags for
    // each wake key individually.
    // TODO: Use the input device configuration to control this behavior more finely.
    if (down && getDevice()->isExternal()) {
        policyFlags |= POLICY_FLAG_WAKE;
    }

    if (mParameters.handlesKeyRepeat) {
        policyFlags |= POLICY_FLAG_DISABLE_KEY_REPEAT;
    }

    if (down && !isMetaKey(keyCode)) {
        getContext()->fadePointer();
    }

    NotifyKeyArgs args(when, getDeviceId(), mSource, policyFlags,
            down ? AKEY_EVENT_ACTION_DOWN : AKEY_EVENT_ACTION_UP,
            AKEY_EVENT_FLAG_FROM_SYSTEM, keyCode, scanCode, keyMetaState, downTime);
    getListener()->notifyKey(&args);
}

函数的最后我们可以看到一个与touch事件类似的分发调用，getListener()->notifyKey(&args)。

补充：其实这个getListener就是InputDispatcher对象。

总结

    找到这两个地方，转换的需求也就简单了，就是在TouchInputMapper::dispatchMotion中拦截touch事件，然后构建一个NotifyKeyArgs对象，通过调用getListener()->notifyKey(&args)来转发key事件。代码涉及到公司私密的逻辑我就不贴出来了，大概就是根据滑动的坐标转成相应的键值再转发key事件。