Can UITableViewDiffableDataSource detect an item changed?

Question

(The question was rewritten after discussing with @AndreasOetjen below. Thanks for his comments.)

I ran into an issue with using UITableView with diffable d

Answer 1

After almost one day's clueless experiments, I believe I figured out how diffable data source worked and solved my issue based on that understanding (it turned out my original thought was almost correct).

Diffable data source uses item hash to identify item. For the same item that exists in both old and new snapshots, diffable data source checks if the item changes by doing an "==" operation with its old and new values.

Once figured out, it looks like quite obvious and simple approach. But it's so fundamental that I can't understand why it isn't mentioned explicitly anywhere.

So, to answer my original question, yes, diffable data source can detect item value change. That said, it becomes tricky when item value is of reference type and/or the text shown in row is, say, properties of objects referenced by that object (e.g., relationship in Core Data), etc.

Another note. Whether using entire item struct or just part of it to generate item hash doesn't matter, as long as it identifies the item. I prefer to using only the essential part of the item which really identifies it.

Answer 2

I'm a little confused about your last sentence: You write my item is an enum with associated values of reference type, but in your example above you use struct Book, which is a value type. Regardless of that, the following has to be kept in mind for any case:

Hashing is all about "object" identity. It's just a kind of shortcut to improve identity comparisons, folding etc.

If you provide a custom hash implementation, two objects a and b must behave in a way that a == b implies that also hash(a) == hash(b) (The other way round is almost always also true, but there may be collisions - esp. with weak hash algorithms - when this is not the case).

So if you only hash the title and author, then you have to implement the comparison operator in a way that it also only compares title and author. Then, if notes change, neither the data source nor any body will not detect identity changes at all.

UITableViewDiffableDataSource is a means to facilitate the synchronization of insert/delete statements between view and data source. If you ever got this

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Invalid update: invalid number of sections. The number of sections contained in the table view after the update (3) must be equal to the number of sections contained in the table view before the update (3), plus or minus the number of sections inserted or deleted (0 inserted, 2 deleted).'

then a diffable data source is your friend.

I hope this helps a little.