code: public ViewSelectionAttributeAssert attribute(String attributeName) {
          isNotEmpty();
          ViewSelectionAttribute attributeValues = new ViewSelectionAttribute(actual, attributeName);
          return new ViewSelectionAttributeAssert(attributeValues);
      }
docs: Creates an object for making assertions about an attribute set extracted from
  each view in the selection. This always fails for an empty selection.
  @param attributeName name of the attribute to check (e.g., {@code "text"}. The
  implementation will call a getter on each view based on this attribute name
  (e.g., {@code getText()}.
  @return a new assert over a set of attributes