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This page explains how to create a MRML node that reference and observes other MRML nodes.

API Description

The node reference API automatically takes care of (1) read/write/copy of node references, (2) updating references on scene import, (3) adding and deleting nodes. All that functionality is implemented in the vtkMRMLNode base class. The only thing that needs to happen in the derived MRML node class (usually in the constructor of the class) is a call to vtkMRMLNode::AddNodeReferenceRole(const char *referenceRole, const char *mrmlAttributeName) that takes a unique string defining the reference role between this node and the referenced node, and a MRML attribute name for storing the reference in the .mrml file.

The only other call that is needed is either:

vtkMRMLNode* SetAndObserveNodeReferenceID(const char* referenceRole , const char* referencedNodeID, vtkIntArray *events=0);


vtkMRMLNode* AddAndObserveNodeReferenceID(const char* referenceRole , const char* referencedNodeID, vtkIntArray *events=0);

if the multiple instances of this reference are allowed.

vtkMRMLNode also provides virtual callbacks that can be extended in the derived classes:

  • OnNodeReferenceAdded(vtkMRMLNodeReference *reference)
  • OnNodeReferenceRemoved(vtkMRMLNodeReference *reference)
  • OnNodeReferenceModified(vtkMRMLNodeReference *reference)

By default those methods generate the following events:

  • vtkMRMLNode::ReferenceAddedEvent
  • vtkMRMLNode::ReferenceRemovedEvent
  • vtkMRMLNode::ReferenceModifiedEvent

In the derived classes those methods could be extended using vtkMRMLNode API that allows querying of node's references:

  • char *GetNthNodeReferenceID(const char* referenceRole, int n);
  • vtkMRMLNode* GetNthNodeReference(const char* referenceRole, int n);
  • int GetNumberOfNodeReferences(const char* referenceRole);
  • etc.

For full API see:

Currently the following MRML nodes are implemented using MRML Node Reference API:

  • vtkMRMLStorableNode
  • vtkMRMLTransformableNode
  • vtkMRMLDisplayableNode

Other references to MRML nodes such as to vtkMRMLColorTableNode, vtkMRMLDiffusionTensorDisplayPropertiesNode are currently not using this API.

Module parameter nodes that contain references to input/output Volume/Model, etc. nodes may or may not require a new API since not all of them contain logic to deal with reference changes.


  • vtkMRMLTransformableNode implementation:

  • vtkMRMLDisplayableNodeimplementation:

  • vtkMRMLStorableNodeimplementation:


In Slicer4.2 prior to March of 2013 each MRML node that contained references to other MRML nodes (for example each vtkMRMLDisplayableNode contained a reference to an instance vtkMRMLDisplayNode) was responsible for managing the references internally. This involved complicated logic and bookkeeping to maintain the references in sync when importing scenes, adding/deleting nodes, modifying referenced nodes, etc.

When creating a new node with the reference to another node the following methods needed to be created/updated to handle the references:

  • Copy
  • PrintSelf
  • ReadXMLAttributes
  • WriteXML
  • ProcessMRMLEvents
  • UpdateReferenceID
  • SetSceneReferences
  • UpdateScene
  • UpdateReferences
  • SetAndObserveNodeReferenceID