How to combine multi data steams in one BLOC? - dart

Question

I have a widget to represent list of stores sorted by nearest to the user current locations also filtering should be applied.
Data in:
Stores data coming from stream of Firestore collection
Current user location from geolacator.
Filtering options from shared preferences
(can be changed any time)
List sorting mode selected by user
Data out: Filtered, sorted, list of stores.
What pattern is best practice in this case?

Answer 1

rxdart : https://pub.dartlang.org/packages/rxdart
if you wanna combine data together you can use
var myObservable = Observable.combineLatest3(
myFirstStream,
mySecondStream,
myThirdStream,
(firstData, secondData, thirdData) => print("$firstData $secondData $thirdData"));
you can combine from ( combineLatest2, combineLatest... combineLatest9 )
or
CombineLatestStream
like this example
CombineLatestStream.list<String>([
Stream.fromIterable(["a"]),
Stream.fromIterable(["b"]),
Stream.fromIterable(["C", "D"])])
.listen(print);

Answer 2

Numbers 2, 3 and 4 are inputs to the bloc that you'd send in through sinks. The bloc listens on those sinks and updates the Firestore query accordingly. This alone might be enough to make Firestore send the appropriate snapshots to the output stream the widget is listening to.
If you can't sort or filter how you want directly with Firestore's APIs, you can use stream.map or apply a StreamTransformer on it. The transformer gives you a lot of flexibility to listen to a stream and change or ignore events on the fly by implementing its bind method.
So you can do something like:
Stream<Store> get stores => _firestoreStream
.transform(filter)
.transform(sort);
Have a look at this page for streams in dart in general, and look into rxdart for more complex stream manipulations.

Answer 3

From personal experience I found having multiple inputs to a block leads to hard to test code. The implicit concurrency concerns inside the block lead to confusing scenarios.
The way I built it out in my Adding testing to a Flutter app post was to create a single input stream, but add markers to the messages notating which data stream the message was a part of. It made testing sane.

Answer 4

In this situation, I think there are multiple asynchronous processing. This implementation can be complicated. And there is a possibility of race condition.
I will implement as follows.
Separate streams of Model from Firestore and user-visible ViewModel in Bloc. Widgets listen to only ViewModel.(eg. with StreamBuilder)
Limit Business logic processing only in Bloc. First, relocate processing with SharedPreferences into Bloc.
Create UserControl class just for user input.
Branch processing depends on user input type of extended UserControl
I hope you this will help you.
For example:
import 'dart:async';
import 'package:rxdart/rxdart.dart';
class ViewModel {}
class DataFromFirestoreModel {}
abstract class UserControl {}
class UserRequest extends UserControl {}
class UserFilter extends UserControl {
final String keyWord;
UserFilter(this.keyWord);
}
enum SortType { ascending, descending }
class UserSort extends UserControl {
final SortType sortType;
UserSort(this.sortType);
}
class Bloc {
final controller = StreamController<UserControl>();
final viewModel = BehaviorSubject<ViewModel>();
final collection = StreamController<DataFromFirestoreModel>();
Bloc() {
controller.stream.listen(_handleControl);
}
_handleControl(UserControl control) {
if (control is UserRequest) {
_handleRequest();
} else if (control is UserFilter) {
handleFilter(control.keyWord);
} else if (control is UserSort) {
handleSort(control.sortType);
}
}
_handleRequest() {
//get location
//get data from sharedPreferences
//get data from firestore
ViewModel modifiedViewModel; // input modifiedViewModel
viewModel.add(modifiedViewModel);
}
handleSort(SortType sortType) {
final oldViewModel = viewModel.value;
//sorting oldViewModel
ViewModel newViewModel; // input sorted oldViewModel
viewModel.add(newViewModel);
}
handleFilter(String keyWord) {
//store data to sharedPreferences
//get data from Firestore
ViewModel modifiedViewModel; // input modifiedViewModel
viewModel.add(modifiedViewModel);
}
}