I have my app saving some objects into .sav files using NSKeyedArchiver archiveRootObject: toFile:; however, I realized that if a user were to open up one of the .sav files in textedit and change it at all, the app would fail to unarchive the objects next time it opens, and would stop working.
Is there any way I can archive the root objects to read-only file or otherwise stop users from editing them? They're buried in application support, so not super accessible, but I'd like to play it safe.
I don't think there is a way for you to avoid potentially losing the saved state (in the end the user could simply delete the file), however if you are worried about the user manipulating the data, you should look at NSSecureCoding.
I believe that is a way to avoid unarchiving "corrupt" data and guaranteeing the integrity, I have not explored the topic further so I can't say for sure whether that would allow a scenario in which the contents of specific fields are changed (i.e object type is the same, but value is different).
In conclusion, I think it is better/safer to build your system with the idea of someone trying to circumvent your security in mind and instead of trying to stop the user from manipulating/deleting the data, just making sure invalid data is not loaded in. For example in case of invalid/corrupt/missing data just reverting back to the default values (i.e as if the app is launching for the first time).