How to create a StreamController from a database Stream using StreamTransformer

Question

I am trying to transform a stream from a database with the database data model to the domain data model.

I am quite confused bringing the different pieces of information together. While in the StreamTransformer examples that I have found, the stream is always a single object an not a list, my result from the example streaming ObjectBox data, returns Stream<List<PropertyObjectBox>>

And then it appears as if a piece of the puzzle is missing, how to come from Stream to StreamController .

So how what do I have to change at the following code?

/// <PropertyObjectBox> data model of Property in ObjectBox database
/// <PropertyModel> data model in data layer (not really needed, I know)
/// <Property> data model in domain layer


@override
// -->> should this return Stream<List<Property>?> or Stream<Property>?
// -->> or something else to comply with handleError?
Stream<List<Property>?> streamOnDeviceProperties() {
  Stream<List<PropertyObjectBox>> propObStream = objectbox.propertyBox.query()
      .watch(triggerImmediately: true).map((query) => query
  // Watching the query produces a Stream<Query<Property>>
  // To get the actual data inside a List<Property>, we need to call find() on the query
      .find());

//-->> again, PropertyObjectBox or List<PropertyObjectBox>?
  var streamTransformer = StreamTransformer<PropertyObjectBox, dynamic>.fromHandlers(
    handleData: (PropertyObjectBox data, EventSink sink) {
      final propertyModel = PropertyModel.fromObjectbox(data);
      final Property property = propertyModel.toDomain();
      sink.add(property);
    },
    handleError: (Object error, StackTrace stacktrace, EventSink sink) {
      sink.addError('Something went wrong: $error');
    },
    handleDone: (EventSink sink) => sink.close(),
  );
//-->> next line causes error 'the getter 'stream' isn't defined for the type 'Stream<List<PropertyObjectBox>>'  
  var controllerStream = propObStream.stream.transform(streamTransformer);

Answer 1

This should do the trick: Updated

@override
Stream<List<Property>?> streamOnDeviceProperties() {

  Stream<List<PropertyObjectBox>?> propObStream = objectbox.propertyBox.query()
      .watch(triggerImmediately: true).map((query) => query.find());

// List<PropertyObjectBox>?
  StreamTransformer<List<PropertyObjectBox>?,List<Property>?> streamTransformer = StreamTransformer<List<PropertyObjectBox>?,List<Property>?>.fromHandlers(
    handleData: (List<PropertyObjectBox>? data, EventSink sink) {
      var newList = data!.map((value) {
        final propertyModel = PropertyModel.fromObjectbox(value);
        final Property property = propertyModel.toDomain();
        return property;
      }).toList();
      sink.add(newList);
    },
    handleError: (Object error, StackTrace stacktrace, EventSink sink) {
      sink.addError('Something went wrong: $error');
    },
    handleDone: (EventSink sink) => sink.close(),
  );
  
 // if you need a Controller although I don't know why
  Stream<List<Property>?> newStream = propObStream.transform(streamTransformer);
  
  final StreamController<List<Property>?> streamController = StreamController<List<Property>?>(
    onPause: () => print('Paused'),
    onResume: () => print('Resumed'),
    onCancel: () => print('Cancelled'),
    onListen: () => print('Listens'),
  );
  streamController.addStream(newStream);
//********************************
  
  return streamTransformer.bind(propObStream);
}

Access the stream like this:

Stream<List<Property>?> _mystream = streamOnDeviceProperties();