code: @Deprecated
      public static Set<IItem> getSelectedItems(FastAdapter adapter) {
          Set<IItem> selections = new HashSet<>();
          int length = adapter.getItemCount();
          List<IItem> items = new ArrayList<>();
          for (int i = 0; i < length; i++) {
              items.add(adapter.getItem(i));
          }
          updateSelectedItemsWithCollapsed(selections, items);
          return selections;
      }
docs: returns a set of selected items, regardless of their visibility
  
  @param adapter the adapter instance
  @return a set of all selected items and subitems
  @deprecated See {@link FastAdapter#getSelectedItems()} ()} ()}