code: public SparseIntArray getExpanded() {
          SparseIntArray expandedItems = new SparseIntArray();
          Item item;
          for (int i = 0, size = mFastAdapter.getItemCount(); i < size; i++) {
              item = mFastAdapter.getItem(i);
              if (item instanceof IExpandable && ((IExpandable) item).isExpanded()) {
                  expandedItems.put(i, ((IExpandable) item).getSubItems().size());
              }
          }
          return expandedItems;
      }
docs: returns the expanded items this contains position and the count of items
  which are expanded by this position
  
  @return the expanded items