【input系统】MotionEvent的分解

在打开多窗口的时候，比如A，B两个应用窗口，我们通过onTouchEvent查看log可以发现，在A窗口上的点击事件不会传给B窗口，当有多个点击事件时，A，B接收到的事件是不一样的，也就是说，在InputDispatcher的notifyMotion中的一个MotionEvent，在传送给app的时候，会根据窗口进行拆分。

这个处理是splitMotionEvent方法，

InputDispatcher::MotionEntry*
InputDispatcher::splitMotionEvent(const MotionEntry* originalMotionEntry, BitSet32 pointerIds) {
    ALOG_ASSERT(pointerIds.value != 0);

    uint32_t splitPointerIndexMap[MAX_POINTERS];
    PointerProperties splitPointerProperties[MAX_POINTERS];
    PointerCoords splitPointerCoords[MAX_POINTERS];

    uint32_t originalPointerCount = originalMotionEntry->pointerCount;
    uint32_t splitPointerCount = 0;

    for (uint32_t originalPointerIndex = 0; originalPointerIndex < originalPointerCount;
            originalPointerIndex++) {
        const PointerProperties& pointerProperties =
                originalMotionEntry->pointerProperties[originalPointerIndex];
        uint32_t pointerId = uint32_t(pointerProperties.id);
        if (pointerIds.hasBit(pointerId)) {
            splitPointerIndexMap[splitPointerCount] = originalPointerIndex;
            splitPointerProperties[splitPointerCount].copyFrom(pointerProperties);
            splitPointerCoords[splitPointerCount].copyFrom(
                    originalMotionEntry->pointerCoords[originalPointerIndex]);
            splitPointerCount += 1;
        }
    }

    if (splitPointerCount != pointerIds.count()) {
        // This is bad.  We are missing some of the pointers that we expected to deliver.
        // Most likely this indicates that we received an ACTION_MOVE events that has
        // different pointer ids than we expected based on the previous ACTION_DOWN
        // or ACTION_POINTER_DOWN events that caused us to decide to split the pointers
        // in this way.
        ALOGW("Dropping split motion event because the pointer count is %d but "
                "we expected there to be %d pointers.  This probably means we received "
                "a broken sequence of pointer ids from the input device.",
                splitPointerCount, pointerIds.count());
        return NULL;
    }

    int32_t action = originalMotionEntry->action;
    int32_t maskedAction = action & AMOTION_EVENT_ACTION_MASK;
    if (maskedAction == AMOTION_EVENT_ACTION_POINTER_DOWN
            || maskedAction == AMOTION_EVENT_ACTION_POINTER_UP) {
        int32_t originalPointerIndex = getMotionEventActionPointerIndex(action);
        const PointerProperties& pointerProperties =
                originalMotionEntry->pointerProperties[originalPointerIndex];
        uint32_t pointerId = uint32_t(pointerProperties.id);
        if (pointerIds.hasBit(pointerId)) {
            if (pointerIds.count() == 1) {
                // The first/last pointer went down/up.
                action = maskedAction == AMOTION_EVENT_ACTION_POINTER_DOWN
                        ? AMOTION_EVENT_ACTION_DOWN : AMOTION_EVENT_ACTION_UP;
            } else {
                // A secondary pointer went down/up.
                uint32_t splitPointerIndex = 0;
                while (pointerId != uint32_t(splitPointerProperties[splitPointerIndex].id)) {
                    splitPointerIndex += 1;
                }
                action = maskedAction | (splitPointerIndex
                        << AMOTION_EVENT_ACTION_POINTER_INDEX_SHIFT);
            }
        } else {
            // An unrelated pointer changed.
            action = AMOTION_EVENT_ACTION_MOVE;
        }
    }

    MotionEntry* splitMotionEntry = new MotionEntry(
            originalMotionEntry->eventTime,
            originalMotionEntry->deviceId,
            originalMotionEntry->source,
            originalMotionEntry->policyFlags,
            action,
            originalMotionEntry->actionButton,
            originalMotionEntry->flags,
            originalMotionEntry->metaState,
            originalMotionEntry->buttonState,
            originalMotionEntry->edgeFlags,
            originalMotionEntry->xPrecision,
            originalMotionEntry->yPrecision,
            originalMotionEntry->downTime,
            originalMotionEntry->displayId,
            splitPointerCount, splitPointerProperties, splitPointerCoords, 0, 0);

    if (originalMotionEntry->injectionState) {
        splitMotionEntry->injectionState = originalMotionEntry->injectionState;
        splitMotionEntry->injectionState->refCount += 1;
    }

    return splitMotionEntry;

其调用流程是

dispatchMotionLocked

dispatchEventLocked

prepareDispatchCycleLocked

splitMotionEvent

主要是区别出各窗口上的点，根据pointID来进行拆分，具体的分析可以参考
ViewGroup事件分发总结-多点触摸事件拆分 - 掘金
在应用中也有类似的拆分处理，

可以看出，touch事件（MotionEvent）的传送多么的不容易，那么多的操作，多次加工，千辛万苦才传给了接收者。这一路，太辛苦。