docs: public void startDrag(ViewHolder viewHolder) {
          if (!mCallback.hasDragFlag(mRecyclerView, viewHolder)) {
              Log.e(TAG, "Start drag has been called but dragging is not enabled");
              return;
          }
          if (viewHolder.itemView.getParent() != mRecyclerView) {
              Log.e(TAG, "Start drag has been called with a view holder which is not a child of "
                      + "the RecyclerView which is controlled by this ItemTouchHelper.");
              return;
          }
          obtainVelocityTracker();
          mDx = mDy = 0f;
          select(viewHolder, ACTION_STATE_DRAG);
      }