How to map JSON objects to Objective C classes?

I'm mapping JSON-formatted data from a web server to Objective C classes (NSManagedObjects modeled in Xcode, handled by Core Data). For each attribute of the Objective C class, I need to:

1. Determine if the attribute's key exists in the JSON object,
2. Determine if the value for t开发者_Go百科hat key is not null, and
3. Pass the value to the modeled class instance if conditions 1 and 2 are true

Right now, I'm hard-coding this sequence for each of the attributes, so every attribute needs code like the following:

// dictObject is the JSON object converted into a NSDictionary,
// and person is the instance of the modeled class

if ([dictObject objectForKey:@"nameFirst"] &&
    [dictObject objectForKey:@"nameFirst"] != [NSNull null]) {
    person.nameFirst = [dictObject objectForKey:@"nameFirst"];
}

Besides requiring a lot of code to handle the various classes, this seems kludgy and brittle: any name change (or language localization) would cause the mapping to fail.

There has to be a better way... what am I missing?

For handling the NULL values, make use of a category on NSDictionary. I use a category that looks like this:

NSDictionary+Utility.h

#import <Foundation/Foundation.h>


@interface NSDictionary (Utility) 
- (id)objectForKeyNotNull:(id)key;
@end

NSDictionary+Utility.m

#import "NSDictionary+Utility.h"


@implementation NSDictionary (Utility)

- (id)objectForKeyNotNull:(NSString *)key {
   id object = [self objectForKey:key];
   if ((NSNull *)object == [NSNull null] || (CFNullRef)object == kCFNull)
      return nil;

   return object;
}

@end

Retrieve any values from your JSON dictionary like this:

NSString *stringObject = [jsonDictionary objectForKeyNotNull:@"SomeString"];
NSArray *arrayObject = [jsonDictionary objectForKeyNotNull:@"SomeArray"];

There is a framework called RestKit that allows you to do just that. I have not tested it but it appears to be supported by an active company and available as open source: http://restkit.org/

As a lightweight alternative to RestKit, you might try GoldenFleece, which converts between JSON and Objective-C objects using a convention-over-configuration pattern inspired by Jackson.

Disclaimer: I'm the author.