NSUserDefaults won't save NSDictionary

Question

I'm writing an application which uses NSUserDefaults as the data storage mechanism, and am hitting a problem when trying to save data (that conforms to the Property List protocols):

+ (BOOL)storeAlbum:(Album *)album
{
    NSUserDefaults *prefs = [NSUserDefaults standardUserDefaults];
    NSMutableDictionary *albums = (NSMutableDictionary *)[prefs objectForKey:@"my_adventure_book_albums"];
    NSLog(@"Existing albums: %@",albums);
    if (!albums)
        albums = [NSMutableDictionary dictionaryWithObject:album forKey:@"album"];
    else
        [albums setObject:album forKey:@"album"];
    NSLog(@"%@",album);
    [prefs setObject:albums forKey:@"my_adventure_book_albums"];
    return [prefs synchronize];
}

I get this output:

2010-06-29 17:17:09.929 MyAdventureBook[39892:207] Existing albums: (null)
2010-06-29 17:17:09.930 MyAdventureBook[39892:207] test
2010-06-29 17:17:09.931 MyAdventureBook[39892:207] *** -[NSUserDefaults setObject:forKey:]: Attempt to insert non-property value '{
    album = test;
}' of class 'NSCFDictionary'.

The description method of Album looks like:

- (NSString *)description
{
    // Convert to a NSDictionary for serializing
    if (!title) title = @"";
    if (!date) date = [NSDate dateWithTimeIntervalSinceNow:0];
    if (!coverImage) coverImage = @"";
    if (!images) images = [[NSArray alloc] initWithObjects:@"",nil];

    //NSDictionary *dict = [NSDictionary dictionaryWithObjects:[NSArray arrayWithObjects:title,date,coverImage,images,nil] forKeys:[NSArray arrayWithObjects:@"title",@"date",@"coverImage",@"images",nil]];
    //NSDictionary *dict = [NSDictionary dictionaryWithObjects:[NSArray arrayWithObjects:title,nil] forKeys:[NSArray arrayWithObjects:@"title",nil]];
    //return [dict description];
    return @"test";
}

All of the commented-out lines have the same result, so I just decided to see if the NSString "test" would work, which it (of course) doesn't.

Answer 1

NSUserDefaults always returns immutable objects, so you can't just cast them to mutable. Do [prefs objectForKey:@"my_adventure_book_albums"] mutableCopy] (and remember to release it when finished).

Answer 2

But the object you put inside the dictionary, an Album* is most likely not a property list object, is it? Every object, all the way down, needs to be a property list object for this to work. A description method isn't good enough to make this happen.

As a workaround, you can use NSCoding and an NSKeyedArchiver to write out your dictionary to an NSData, which you can store among the preferences.

Answer 3

You can only put basic foundation types into a property list. NSUserDefaults writes preferences out as a property list. See here for property list allowed types. In a nutshell, it is numbers, strings, data, dates, and arrays and dictionaries of those. Dictionaries must have string keys.