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 (value)
    _positionController.forward();
  else
    _positionController.reverse();
  if (isInteractive) {
    switch (_reactionController.status) {
      case AnimationStatus.forward:
        _reactionController.forward();
        break;
      case AnimationStatus.reverse:
        _reactionController.reverse();
        break;
      case AnimationStatus.dismissed:
      case AnimationStatus.completed:
        // nothing to do
        break;
    }
  }
}