_stream_waits_for_future.md

Proof of concept for holding off a stream.listen callback until a trigger Future completes.

main.dart

import 'dart:async';

void main() {
  Future trigger = new Future.delayed(new Duration(seconds: 2));
  Stream stream = new Stream.fromIterable([1,2,3]);
  stream.transform(new WaitFor(trigger)).listen((v) {print(v);});
  trigger.then((_) {print('Future fired');});
}

class WaitFor<T> implements StreamTransformer<T,T> {
  final _resultStreamController = new StreamController<T>();
  final Future _trigger;
  bool _isTriggered = false;
  
  WaitFor(this._trigger);
  
  Stream<T> bind(Stream<T> original) {
    original.listen((T value) {
      if (_isTriggered) {
        _resultStreamController.add(value);
      } else {
        _trigger.then((_) {
          _isTriggered = true;
          _resultStreamController.add(value);
        });
      }
    });
    return _resultStreamController.stream;
  }
}