Question

I have the code like below. I am retrieving JSON from a web server.

AFJSONRequestOperation *operation = [AFJSONRequestOperation JSONRequestOperationWithRequest:request success:^(NSURLRequest *request, NSHTTPURLResponse *response, id JSON)
                                     {    

                                         for (NSMutableDictionary * dataDict in JSON) {


                                             if ([dataDict objectForKey:@"userPicture"])
                                             {

                                                objectForKey:@"userPicture"]);
                                                 NSURL *url = [NSURL URLWithString:[dataDict objectForKey:@"userPicture"]];
                                                 UIImage *image = [UIImage imageWithData: [NSData dataWithContentsOfURL:url]];     


                                                 [dataDict setValue:@"userPicture"
                                                             forKey:@"userPicture"]; 


                                             } 


                                             [messageList addObject: dataDict];
                                         }                                             

                                         [self processComplete];

                                     } 
                                    failure:^(NSURLRequest *request , NSURLResponse *response , NSError *error , id JSON)
                                     {
                                         NSLog(@"Failed: %@", error);        

                                     }];

However it crashes with the following error:

*** Terminating app due to uncaught exception ‘NSInternalInconsistencyException’, reason: ‘-[__NSCFDictionary
setObject:forKey:]: mutating method sent to immutable object’

Now, what I don’t get is that, I am trying to setValue:forKey in dataDict, which is instantiated NSMutableDictionary, so I should be able to setValue.

Answer 1

You have declared your dataDict as an NSMutableDictionary but that doesn’t mean that you get a mutable dictionary from your JSON. What you could do is either instruct your JSON serializer to fetch you mutable copies (if it supports such option) or manually do this:

for (NSDictionary *dict in JSON) {

    NSMutableDictionary * dataDict = [dict mutableCopy];
    // Rest of the code here...

}