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.


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);
      if (newStream != null) {
    controller.onCancel = subscription.cancel;
    if (!isBroadcast) {
        ..onPause = subscription.pause
        ..onResume = subscription.resume;