Why NSUserDefaults failed to save NSMutableDictionary in iOS?

BlueDolphin picture BlueDolphin · Jan 23, 2009 · Viewed 76.7k times · Source

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 this error:

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

Answer

BlueDolphin picture BlueDolphin · Jan 23, 2009

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"];
[defaults synchronize];

Load:

NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
NSData *data = [defaults objectForKey:@"theKey"];
NSArray *arr = [NSKeyedUnarchiver unarchiveObjectWithData:data];

The element in the array implements

@interface CommentItem : NSObject<NSCoding> {
    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.