code: public CheckableField<T> addCheckableField(int viewResId,
                                                 BooleanExtractor<T> isCheckedExtractor) {
  
          CheckableField<T> field = new CheckableField<T>(viewResId, isCheckedExtractor);
  
          mCheckableFields.add(field);
  
          return field;
      }
docs: Checkable field methods