# listenTo method Null safety

1. @override

StreamSubscription<BookletModel?> listenTo(

1. String documentId,
2. BookletChanged changed,
3. {BookletErrorHandler? errorHandler}

)
override

## Implementation

    @override
    StreamSubscription<BookletModel?> listenTo(
        String documentId, BookletChanged changed,
        {BookletErrorHandler? errorHandler}) {
      return reference.listenTo(documentId, ((value) {
        if (value != null) {
          fullCache[value.documentID] = value;
        }
        changed(value);
      }), errorHandler: errorHandler);
    }