afterData method Null safety
- AsyncSnapshot<T> current,
- T data
override
    Returns an updated version of the current summary following a data event.
Sub-classes must override this method to specify how the current summary is combined with the new data item in the fold computation.
Implementation
@override
AsyncSnapshot<T> afterData(AsyncSnapshot<T> current, T data) {
  return AsyncSnapshot<T>.withData(ConnectionState.active, data);
}