How can I know which control point is moved?

I am developing a C++ extension and my code goes like:

qvtkConnect(fidNode, vtkMRMLMarkupsFiducialNode::PointEndInteractionEvent, this, SLOT(onPointMoved()));

Since I have to do some operation according to control point’s position, how can I know which control point of fidNode is moved in slot function onPointMoved()?

This works during the interaction, not sure what happens right after
markupIndex = int(caller.GetAttribute('Markups.MovingMarkupIndex'))