description

# asyncExpand<E> method Null safety

Stream<E> asyncExpand <E>(

1. Stream<E>? convert(
   1. T event
   )

)

Transforms each element into a sequence of asynchronous events.

Returns a new stream and for each event of this stream, do the following:

- If the event is an error event or a done event, it is emitted directly by the returned stream.
- Otherwise it is an element. Then the `convert` function is called with the element as argument to produce a convert-stream for the element.
- If that call throws, the error is emitted on the returned stream.
- If the call returns `null`, no further action is taken for the elements.
- Otherwise, this stream is paused and convert-stream is listened to. Every data and error event of the convert-stream is emitted on the returned stream in the order it is produced. When the convert-stream ends, this stream is resumed.

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

## Implementation

    Stream<E> asyncExpand<E>(Stream<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);
        subscription.onData((T event) {
          Stream<E>? newStream;
          try {
            newStream = convert(event);
          } catch (e, s) {
            controller.addError(e, s);
            return;
          }
          if (newStream != null) {
            subscription.pause();
            controller.addStream(newStream).whenComplete(subscription.resume);
          }
        });
        controller.onCancel = subscription.cancel;
        if (!isBroadcast) {
          controller
            ..onPause = subscription.pause
            ..onResume = subscription.resume;
        }
      };
      return controller.stream;
    }