User function that is called to map an @item of the original model to an item expected by the map model.
The returned items must conform to the item type of the model they are used with.
The item to map to. This function may not return %NULL
See Implementation
User function that is called to map an @item of the original model to an item expected by the map model.
The returned items must conform to the item type of the model they are used with.