How to create a StreamTransformer in Dart?

Question

Trying to build a custom StreamTransformer class, however a lot of the examples out there seem to be out of date, and the one found in the documentation isn't (what some typed languages might consider anyway) as a class (found here: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/dart:async.StreamTransformer). This doesn't seem like a very Dart-like way of approaching it and rather more of a Javascript-like way (which I'm using Dart to avoid).

Many online sources say this is how you create a StreamTransformer, however there errors when extending it.

class exampleStreamTransformer extends StreamTransformer
{
  //... (This won't work)
}

'Implements' seems to be the way to go, along with implementing the bind function needed:

class exampleStreamTransformer implements StreamTransformer
{
  Stream bind(Stream stream)
  {
    //... (Go on to return new stream, etc)
  }
}

I can't seem to find any examples of this way, but have thrown something together myself (which is accepted in my IDE, but isn't accepted at runtime, I get a null object error when it tries to use pause getter):

class exampleStreamTransformer implements StreamTransformer
{
  StreamController<String> _controller;
  StreamSubscription<String> _subscription;

  Stream bind(Stream stream)
  {
    _controller = new StreamController<String>(
        onListen: ()
        {
          _subscription = stream.listen((data)
          {
            // Transform the data.
            _controller.add(data);
          },
          onError: _controller.addError,
          onDone: _controller.close,
          cancelOnError: true); // Unsure how I'd pass this in?????
        },
        onPause: _subscription.pause,
        onResume: _subscription.resume,
        onCancel: _subscription.cancel,
        sync: true
    );

    return _controller.stream;
  }
}

Would like to achieve it this way, as in the 'typed' way of producing the class, any help is much appreciated, thank you.

Answer 1

Why don't you use StreamTransformer.fromHandler():

import 'dart:async';

void handleData(data, EventSink sink) {
  sink.add(data*2);
}

void main() {
  StreamTransformer doubleTransformer = new StreamTransformer.fromHandlers(handleData: handleData);

  StreamController controller = new StreamController();
  controller.stream.transform(doubleTransformer).listen((data) {
    print('data: $data');
  });

  controller.add(1);
  controller.add(2);
  controller.add(3);
}

Output:

data: 2
data: 4
data: 6