Simple example of how Streams works in Dart

strem_example.dart

import 'dart:async';

void addLessThanFive(StreamController controller, int value) {
  if(value < 5) {
    controller.sink.add(value);
  } else {
    controller.sink.addError(StateError('$value is not less than 5'));
  }
}

void main() {
  final controller = StreamController();
  
  // in this example we are passing integer values but we can pass any value to the stream 
	addLessThanFive(controller, 1);
  addLessThanFive(controller, 2);
  addLessThanFive(controller, 3);
  addLessThanFive(controller, 4);
  addLessThanFive(controller, 5);
  
  // example of any value been passed to the stream
  controller.sink.add(null);
  controller.sink.add([10, 12.3, 'another string']);
  
  controller.close();
  
  // this will rise and unhandled exception
  // addLessThanFive(controller, 0);
  
  controller.stream.listen(print, onError: print, onDone: () {
    print('done');
  });
}