我在Flutter上的流控制器上遇到错误

Question

当我打开应用程序时，它会打开，当我单击图像时它会打开，但是当我再次按n时，它会抛出一个异常，说

“流已经听过”。

这是我的代码：

    StreamSubscription<T> _subscribe(void onData(T data), Function onError,
      void onDone(), bool cancelOnError) {
    if (!_isInitialState) {
      throw new StateError("Stream has already been listened to."); //here I get the exception
    }
    _ControllerSubscription<T> subscription = new _ControllerSubscription<T>(
        this, onData, onError, onDone, cancelOnError);

    _PendingEvents<T> pendingEvents = _pendingEvents;
    _state |= _STATE_SUBSCRIBED;