ES7 Observables + WHATWG Streams

observable-from-stream.js

// ES7 Observables + WHATWG Streams
//
//   https://github.com/jhusain/asyncgenerator
//   https://github.com/whatwg/streams
//
// Continuation from file:
//   https://github.com/jhusain/asyncgenerator/blob/master/src/observable.js

Observable.fromStream = function(readable) {
  return new Observable(function(generator) {
    var done = false;

    var decoratedGenerator = decorate(generator, function() {
      if (!done) {
        done = true;
        readable.cancel();
      }
    });

    function pump() {
      var writableReady;

      // Check done before each read, observer may have canceled on us
      if (done) {
        readable.cancel();
        return;
      }
      
      if (readable.state === "readable") {
        // observer may signal backpressure by returning a promise
        writableReady = decoratedGenerator.next(readable.read());
      }

      if (readable.state === "closed") {
        done = true;
        decoratedGenerator.return();
      } else {
        // Wait until both reader and writer are ready
        Promise.all([readable.ready, writableReady]).then(pump, function(e) {
          done = true;
          readable.cancel();
          decoratedGenerator.throw(e);
        });
      }
    }
    pump();

    return decoratedGenerator;
  });
}