FlowScene(std::shared_ptr<DataModelRegistry> registry =
              std::make_shared<DataModelRegistry>(),
            QWidget* parent = nullptr);

FlowScene::
FlowScene(std::shared_ptr<DataModelRegistry> registry, QWidget* parent)
  : _registry(registry)
{
  setItemIndexMethod(QGraphicsScene::NoIndex);
  setParent(parent);
}