AbstractNode rootNode

The unique object managed by this pipeline that has no parent.

This object does not have to be a RenderObject.

Source 

AbstractNode get rootNode => _rootNode;
void rootNode=(AbstractNode value)

Source 

set rootNode(AbstractNode value) {
  if (_rootNode == value)
    return;
  _rootNode?.detach();
  _rootNode = value;
  _rootNode?.attach(this);
}