Can you explain what is meant by 'Subject' in the context of system design or patterns?

In the context of design systems and patterns, especially in reactive programming (for example, using RxDart in Flutter), the term Subject refers to an object that is both an observable and an observer.

This means that a Subject can receive events (data) and broadcast them to all subscribers. It acts as an intermediary for data transmission in streams.

Example of using Subject in Flutter with RxDart:

import 'package:rxdart/rxdart.dart';

void main() {
  final subject = PublishSubject<int>();

  subject.listen((value) => print('Listener 1: $value'));

  subject.add(1);
  subject.add(2);

  subject.listen((value) => print('Listener 2: $value'));

  subject.add(3);

  subject.close();
}

Here, PublishSubject accepts values and distributes them to all subscribers. Thus, a Subject combines the capabilities of a data source and a data consumer.