dart:async

asyncMap<E> method

Stream<E> asyncMap <E>(

1. FutureOr<E> convert(
   1. T event
   )

)

Creates a new stream with each data event of this stream asynchronously mapped to a new event.

This acts like map, except that convert may return a Future, and in that case, this stream waits for that future to complete before continuing with its result.

The returned stream is a broadcast stream if this stream is.

Implementation

Stream<E> asyncMap<E>(FutureOr<E> convert(T event)) {
  _StreamControllerBase<E> controller;
  if (isBroadcast) {
    controller = _SyncBroadcastStreamController<E>(null, null);
  } else {
    controller = _SyncStreamController<E>(null, null, null, null);
  }

  controller.onListen = () {
    StreamSubscription<T> subscription = this.listen(null,
        onError: controller._addError, // Avoid Zone error replacement.
        onDone: controller.close);
    FutureOr<Null> add(E value) {
      controller.add(value);
    }

    final addError = controller._addError;
    final resume = subscription.resume;
    subscription.onData((T event) {
      FutureOr<E> newValue;
      try {
        newValue = convert(event);
      } catch (e, s) {
        controller.addError(e, s);
        return;
      }
      if (newValue is Future<E>) {
        subscription.pause();
        newValue.then(add, onError: addError).whenComplete(resume);
      } else {
        // TODO(40014): Remove cast when type promotion works.
        controller.add(newValue as dynamic);
      }
    });
    controller.onCancel = subscription.cancel;
    if (!isBroadcast) {
      controller
        ..onPause = subscription.pause
        ..onResume = resume;
    }
  };
  return controller.stream;
}