Copy and paste this code snippet in the Python interactor. Watch what happens when you change the ‘ModelToRas’ transform using e.g. the Transforms module.

ObserveTransformNode.py

# Create coordinate system models to visualize the transforms

createModelsLogic = slicer.modules.createmodels.logic()

preModelNode = createModelsLogic.CreateCoordinate(20,2)
preModelNode.SetName('PreModel')
preModelNode.GetDisplayNode().SetColor(1,1,0)

postModelNode = createModelsLogic.CreateCoordinate(20,2)
postModelNode.SetName('PostModel')
postModelNode.GetDisplayNode().SetColor(0,1,0)

# Create two transforms. ModelToRas will be the one to change manually

modelToRas = slicer.vtkMRMLLinearTransformNode()
modelToRas.SetName('ModelToRas')
slicer.mrmlScene.AddNode(modelToRas)
preModelNode.SetAndObserveTransformNodeID(modelToRas.GetID())

otherTransformNode = slicer.vtkMRMLLinearTransformNode()
otherTransformNode.SetName('OtherTransform')
slicer.mrmlScene.AddNode(otherTransformNode)
postModelNode.SetAndObserveTransformNodeID(otherTransformNode.GetID())

# Function to be called when ModelToRas changes

def transformModified(caller, eventid):
 modelToRasTransform = modelToRas.GetTransformToParent()
 otherTransform = vtk.vtkTransform()
 otherTransform.DeepCopy(modelToRasTransform)
 otherTransform.Translate(100,0,0)
 otherTransformNode.SetAndObserveTransformToParent(otherTransform)

# Adding callback function as observer of the ModelToRas transform

modelToRas.AddObserver( slicer.vtkMRMLTransformNode.TransformModifiedEvent, transformModified)

# Call modified once, so the positions get initialized by the callback function

modelToRas.TransformModified()