1. override
void attach(PipelineOwner owner)

Mark this node as attached to the given owner.

Typically called only from the parent's attach method, and by the owner to mark the root of a tree as attached.

Subclasses with children should attach all their children to the same owner whenever this method is called.

Source 

@override
void attach(PipelineOwner owner) {
  super.attach(owner);
  if (_child != null)
    _child.attach(owner);
}