Migrating to App Groups

Quick Follow Up

This is a quick follow up to the previous post regarding app groups. After verifying how it works, I went ahead and looked at how I can migrate my current settings from the standard defaults to the app group container. Here is how it went down…

First, I over-complicated things, as usual. I thought I could use a cascading similar to the one we saw in the UIFont post. Basically, I would have to wrap the NSUserDefault with a subclass, and on every access to the defaults, I would check if that key is migrated or not, then migrate it from the standard defaults if it isn’t.

Something like:

var object: AnyObject? = objectForKey(defaultKey)
if object == nil {
    object = NSUserDefaults.standardUserDefaults().objectForKey(defaultName)
    setObject(object, forKey: defaultName)
return object

Then, I found an interesting method on NSUserDefaults, namely addSuiteNamed:. It seems that I can somehow compose the two defaults without worrying much about where the setting was actually read from.

Ultimately, I realized that this “lazy loading” approach is a really bad idea, since I can’t guarantee that the settings have been all migrated! I need them migrated ASAP, so the widget can make use of it.


The final solution was extremely simple, and I should’ve seen it coming. All we do is copy all the settings from the old defaults to the new container (pardon the ugly ObjC):

static NSString *const didMigrateToAppGroups = @"MCEngineSettingsMigrateToAppGroups";

if (![[NSUserDefaults MCDefaults] boolForKey:didMigrateToAppGroups])
    NSDictionary *oldDefaults = [[NSUserDefaults standardUserDefaults] dictionaryRepresentation];

    // Massive kudos to Sean for pointing this out
    for (id key in oldDefaults.allKeys)
        [[NSUserDefaults MCDefaults] setObject:oldDefaults[key] forKey:key];

    [[NSUserDefaults MCDefaults] setBool:YES forKey:didMigrateToAppGroups];
    [[NSUserDefaults MCDefaults] synchronize];

NOTE: MCDefaults, as presented in the previous post, is just a convenience method that returns the user defaults associated with the app group identifier.