Question

What is the best way to implement arbitrary row reordering in a tableview that uses core data? The approach that seems obvious to me is to add a rowOrder attribute of type Int16 to the entity that is being reordered in the tableview and manually iterate through the entity updating the rowOrder attributes of all the rows whenever the user finishes reordering.

That is an incredibly inelegant solution though. I’m hoping there is a better approach that doesn’t require possibly hundreds of updates whenever the user reorders things.

Answer 1

If the ordering is something that the data model should modal and store, then the ordering should be part of the entity graph anyway.

A good, lightweight solution is to create an Order entity that has a one-to-one relationship to the actual entity being ordered. To make updating easy, create a linked-list like structure of the objects. Something like this:

Order{
    order:int;
    orderedObject<--(required,nullify)-->OrderObject.order
    previous<--(optional,nullify)-->Order.next;
    next<--(optional,nullify)-->Order.previous;
}

If you create a custom subclass, you can provide an insert method that inserts a new object in the chain and then sends a message down the next relationships and tells each object to increment its order by one then the message to its next. A delete method does the opposite. That makes the ordering integral to the model and nicely encapsulated. It’s easy to make a base class for this so you can reuse it as needed.

The big advantage is that it only requires the small Order objects to be in alive in memory.

Edit:

Of course, you can extend this with another linked object to provide section information. Just relate that entity to the Order entity then provide the order number as the one in the section.