一般实现滑动监听操作的View会保存一个mActivePointerId变量来记录活动手指。
这个变量一般在MotionEvent.ACTION_DOWN中获取：

mActivePointerId = MotionEventCompat.getPointerId(ev, 0);

而在MotionEventCompat.ACTION_POINTER_UP时，判断抬起手指是否是活动手指，如果是，则重新选择活动手指：

int pointerIndex = MotionEventCompat.getActionIndex(ev);
int pointerId = MotionEventCompat.getPointerId(ev, pointerIndex);
if (pointerId == mActivePointerId) {
    int newPointerIndex = pointerIndex == 0 ? 1 : 0;
    mActivePointerId = MotionEventCompat.getPointerId(ev, newPointerIndex);
}