class RuntimeAsyncGate {
  RuntimeAsyncGate({bool initiallyClosed = false}) : _closed = initiallyClosed;

  int _generation = 0;
  bool _closed;

  int get generation => _generation;

  bool get isOpen => !_closed;

  bool get isClosed => _closed;

  RuntimeAsyncToken get token => RuntimeAsyncToken._(this, _generation);

  RuntimeAsyncToken activate() {
    _closed = false;
    _generation++;
    return token;
  }

  RuntimeAsyncToken advance() {
    _generation++;
    return token;
  }

  RuntimeAsyncToken close() {
    _closed = true;
    _generation++;
    return token;
  }

  bool accepts(RuntimeAsyncToken token) {
    return !_closed &&
        identical(token._gate, this) &&
        token.generation == _generation;
  }

  bool acceptsGeneration(int generation) {
    return !_closed && generation == _generation;
  }
}

class RuntimeAsyncToken {
  const RuntimeAsyncToken._(this._gate, this.generation);

  final RuntimeAsyncGate _gate;
  final int generation;

  bool get isAccepted => _gate.accepts(this);
}