NSUserDefaults flexibility for version updates

Question

Hi,

I am using the NSUserDefaults' api -registerDefaults api to register the factory defaults of my application. All is fine for the first version of the application. But when I provide an update for the iPhone application, I have 3 criteria to make sure:

1. The user preferences of older version app should be intact.
2. Addition to the factory defaults specific to the new version should be applied.
3. Providing flexibility in the design to the future version updates so that the user defaults modifies itself based on the new version.

The api -registerDefaults does not register factory defaults in new version updates because a plist file containing user preferences already exists in the /Library/Preferences of sandbox. And in case if we reset the preferences with new factory defaults, user preferences of previous version will be lost.

I think there should be a design pattern to handle such cases, so in general how do we cater with such situations? Doesnt Apple provide any inherent support for this? Or am I missing some links, I tried googling around without any success.

Thanks, Raj

Answer 1

What is the problem? If the user never changes (or more exactly: sets) a preference, the factory default will be used. If an update of your app changes the factory defaults, those will be used instead. If the user has set his own preference, that will continue to be used.

A new factory default doesn't (and shouldn't) overwrite an user set preference. NSUserDefaults does exactly what you seem to be after.

Answer 2

I agree with Johan Kool. User defaults work as they should. If there's a preference that you want changed (for whatever reason) in an updated app version, just slightly rename the preference. For example, if someValue was a preference in version 1, then in version 2 name it someValue_v2 and your new factory default will take effect.

Answer 3

If this list of items you're talking about is an array registered under a single key, then yes, a larger array passed to -registerDefaults: will be ignored in favor of the smaller array stored in the persistent plist. NSUserDefaults tracks stuff on a per-key basis, and doesn't do any interpretation of the contents you store in there, so it's not going to try to automagically merge previous array values with new array values, or anything daft like that.

If you want new items to be added to this array by default when the user upgrades, I would recommend adding the items explicitly to the key yourself by fetching the existing array, adding the new items, and then saving the larger array back to the user defaults again. You can use a separate boolean flag to indicate that the upgrade has been done, to ensure that the addition doesn't get performed multiple times.