NSMutableArray to JSON object

Question

I have a method where I want to return either an NSData object or an 'NSString' that must be a JSON object in format.

At the moment this is what I have;

-(NSData *)JSONData{

     NSMutableArray* arr = [NSMutableArray array];
     for (int j = 0; j < self.sales.subArray.count; j++)
     {
        SalesObject* subCategory = [self.sales.subArray objectAtIndex:j];


        NSDictionary * dict =[NSDictionary dictionaryWithObjectsAndKeys:
                                 @"category_id",subCategory.category_id,
                                 @"discounted",@"0",
                                 @"price",subCategory.price,
                                 @"active",subCategory.isActive, nil];
        NSLog(@"Dict %@",dict);

       [arr addObject:dict];

    }

    NSLog(@"Arr %@",arr);

   NSLog(@"Arr %@",arr);

   NSString *string = [arr description];
   NSData * jsonData = [NSJSONSerialization dataWithJSONObject:string options:kNilOptions error:nil];
   NSLog(@"JSON Data %@",jsonData);

   return jsonData;
}

As you can see I tried to convert an NSMutableArray to an NSData object but it didnt work. I get;

 Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Invalid (non-string) key in JSON dictionary'

I now get the following error;

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** +[NSJSONSerialization dataWithJSONObject:options:error:]: Invalid top-level type in JSON write'

Answer 1

You are on the right track, but you have your key/value pairs reversed. Instead use the new Objective-C dictionary literal syntax, which is shorter and easier to read, and therefore easier to spot mistakes:

NSDictionary *dict = @{
    @"category_id" : subCategory.category_id,
    @"discounted"  : @"0",       // Should that be @(0) ???
    @"price"       : subCategory.price,
    @"active"      : subCategory.isActive
};

EDIT The additional problem relates to the use the description of the array (ie a string), rather than the array itself, being used to create the JSON data. It should be:

NSError *error = nil;
NSData *jsonData = [NSJSONSerialization dataWithJSONObject:arr
                                                   options:kNilOptions
                                                     error:&error];
if (!jsonData) {
    NSLog(@"Achtung!  Failed to create JSON data: %@", [error localizedDescription]);
}

Answer 2

Change the dictionary definition from

 NSDictionary * dict =[NSDictionary dictionaryWithObjectsAndKeys:
                             @"category_id",subCategory.category_id,
                             @"discounted",@"0",
                             @"price",subCategory.price,
                             @"active",subCategory.isActive, nil];

to

NSDictionary * dict =[NSDictionary dictionaryWithObjectsAndKeys:
                             subCategory.category_id,@"category_id",
                             @"0", @"discounted",
                             subCategory.price,@"price",
                             subCategory.isActive, @"active", nil];