Stream transform

main.dart

import 'dart:async';

void main() {
  var controller = StreamController<num>();
  
  // Create StreamTransformer with transformer closure
  var streamTransformer = StreamTransformer<num, num>.fromHandlers(
    handleData: (num data, EventSink sink) {
      sink.add(data * 2);
    }, 
    handleError: (error, stacktrace, sink) {
      sink.addError('Something went wrong: $error');
    }, 
    handleDone: (sink) {
      sink.close();
    },
  );
  
  // Call the `transform` method on the controller's stream
  // while passing in the stream transformer
  var controllerStream = controller.stream.transform(streamTransformer);

  // Just print out transformations to the console
  controllerStream.listen(print);

  // Add data to stream to see transformations in effect
  controller.sink.add(1); // 2
  controller.sink.add(2); // 4
  controller.sink.add(3); // 6
  controller.sink.add(4); // 8
  controller.sink.add(5); // 10
}