I have an app getting close to release date, but it occurred to me that wherever I have core data save and/or fetch requests I’m not really handling the errors other than to check if they exist and @throw them, which I’m sure will seem almost like nails on a chalkboard to more experienced programmers, and surely there’s some kind of disaster waiting to happen.
So to be specific, what kinds of errors can I expect from A) Fetches, and B) Saves, and also C) in general terms, how should I deal with these?
You can see the Core Data Constants Reference to get an idea about what kind of errors you can expect to see in general.
For fetches, the most common issue is that the fetch returns an empty array. Make sure that your view controllers, datasources and delegates can handle an empty fetch. If you dynamically construct complex predicates, make sure catch exceptions from an invalid predicate.
Most save errors results from validation errors. You should have a error recovery for every validation you supply. One common and somewhat hidden validation error is not providing a required relationship.
One thing that trips people up with Objective-c is that errors and exceptions are slightly different critters than they are in other languages. In Objective-C an error is something that the programmer should anticipate and plan for in the normal operation of the application e.g. a missing file. By contrast an exception is something exceptional that the programmer wouldn’t expect the app to have to routinely handle e.g. a corrupted file.
Therefore, in Core Data a validation failure would be an common expected and unexceptional error whereas as corrupted persistent store would be a rare, unexpected and highly exceptional exception.
See the Exceptions Programming Guide and the Error Handling Programming Guide for details.