public delegate ListModel? TreeListModelCreateModelFunc (Object item)
Prototype of the function called to create new child models when set_expanded is called.
This function can return null to indicate that
item is guaranteed to be a leaf node and will
never have children. If it does not have children but may get children later, it should return an empty model that is filled once children
The item that is being expanded
User data passed when registering the function
The model tracking the children of