Why NSUserDefaults failed to save NSMutableDictionary in iPhone SDK?

Question

I'd like to save an NSMutableDictionary object in NSUserDefaults. The key type in NSMutableDictionary is NSString, the value type is NSArray, which contains a list of object which implements NSCoding. Per document, NSString and NSArray both are conform to NSCoding.

I am getting error :

-[NSUserDefaults setObject:forKey:]: Attempt to insert non-property value .... of class 'NSCFDictionary'.

Any solution for that?

Thanks.

Answer 1

Are all of your keys in the dictionary NSStrings? I think they have to be in order to save the dictionary to a property list.

Answer 2

If you're saving an object in user defaults, all objects, recursively, all the way down, must be property list objects. Conforming to NSCoding doesn't mean anything here-- NSUserDefaults won't automatically encode them into NSData, you have to do that yourself. If your "list of object which implements NSCoding" means objects that are not property list objects, then you'll have to do something with them before saving to user defaults.

FYI the property list classes are NSDictionary, NSArray, NSString, NSDate, NSData, and NSNumber. You can write mutable subclasses (like NSMutableDictionary) to user preferences but the objects you read out will always be immutable.

Answer 3

I found out one alternative, before save, I encode the root object (NSArray object) using NSKeyedArchiver, which ends with NSData. Then use UserDefaults save the NSData.

When I need the data, I read out the NSData, and use NSKeyedUnarchiver to convert NSData back to the object.

It is a little cumbersome, because i need to convert to/from NSData everytime, but it just works.

Here is one example per request:

Save:

NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];

NSMutableArray *arr = ... ; // set value

NSData *data = [NSKeyedArchiver archivedDataWithRootObject:arr];

[defaults setObject:data forKey:@"theKey"];

Load:

NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];

NSData *data = [defaults objectForKey:@"theKey"];

NSArray *arr = [NSKeyedUnarchiver unarchiveObjectWithData:data];

The element in the array implements

@interface CommentItem : NSObject {

NSString *value;

}

Then in the implementation of CommentItem, provides two methods:

-(void)encodeWithCoder:(NSCoder *)encoder

{

[encoder encodeObject:value forKey:@"Value"];

}

-(id)initWithCoder:(NSCoder *)decoder

{

self.value = [decoder decodeObjectForKey:@"Value"];

    return self;

}

Anyone has better solution?

Thanks everyone.

Answer 4

There is no better solution. Another option would be to just save the coded object to disk - but that is doing the same thing. They both end up with NSData that gets decoded when you want it back.

Answer 5

Have you considered looking at implementing the NSCoding Protocol? This will allow you encode and decode on the iPhone with two simple methods that are implemented with the NSCoding. First you would need to adding the NSCoding to your Class.

Here is an example:

This is in the .h file

@interface GameContent : NSObject <NSCoding>

Then you will need to implement two methods of the NSCoding Protocol.

    - (id) initWithCoder: (NSCoder *)coder
    {
        if (self = [super init])
        {
               [self setFoundHotSpots:[coder decodeObjectForKey:@"foundHotSpots"]];
            }
    }

    - (void) encodeWithCoder: (NSCoder *)coder
    {
           [coder encodeObject:foundHotSpots forKey:@"foundHotSpots"];
    }

Check out the documentation on NSCoder for more information. That has come in really handy for my projects where I need to save the state of the application on the iPhone if the application is closed and restore it back to it's state when its back on.

The key is to add the protocol to the interface and then implement the two methods that are part of NSCoding.

I hope this helps!