materialize method Null safety
Converts the onData, on Done, and onError events into
objects that are passed into the downstream onData listener.
Notification object contains the Kind of event (OnData, onDone, or
OnError), and the item or error that was emitted. In the case of onDone,
no data is emitted as part of the
Stream<int>.error(Exception()) .materialize() .listen((i) => print(i)); // Prints onError Notification
Stream<Notification<T>> materialize() => transform(MaterializeStreamTransformer<T>());