Question

I have a core data ‘ShoppingList’ which contains ‘Item’ objects. I store a display order as an attribute of each item.

I would like to update the display order of all other items in the shopping list whenever an item is deleted. The code to do this is working fine when I use it in my view controller (from where the item is deleted), but since it is really related to the business objects and not the view, it would be better placed in either ShoppingList or Item.

Ideally, I would like it incorporated into the deletion of the item. So far I have tried the following:

1) Customize the standard Core Data generated ShoppingList.RemoveItemsObject (making sure to observe KVO before.after). What’s strange about this way is that the item passed is stripped of its relationships to other core data entities before it gets to my code, which I need to process display orders correctly.
2) Customize Item.didTurnIntoFault. Same applies – but even attributes of the item are gone by this stage.

One answer would be to simply define a new method on ShoppingList that does my processing and then calls the original removeItemsObject. But I would prefer to know that whenever an item is removed, from anywhere, this is taken care of. This works nicely when I customize awakeFromInsert, for example – I know that whenever an item is created certain things are setup for me. But I’m surprised there’s no equivalent for deletion.

Answer 1

Did you try to implement prepareForDeletion? Sounds like it’s exactly what you’re looking for.

The doc says:

You can implement this method to perform any operations required before the object is deleted, such as custom propagation before relationships are torn down, or reconfiguration of objects using key-value observing.