convert Core Data NSManagedObject in to JSON on iPhone?

Question

I was to post some of my COre Data objects back to a web service and would like to send them as JSON. I am receiving objects from the server a JSON using this library:

http://code.google.com/p/json-framework/

But I cannot figure out how to change my objects back to JSON?

Answer 1

To create json from you r objects, you have to build an NSDictionary from your object, and then convert to string with the SBJsonWriter class.

NSDictionary *jsonDictionary = [NSDictionary dictionaryWithObject:(NSArray *)YourArrayOfElements forKey:@"objects"];
SBJsonWriter *jsonWriter = [SBJsonWriter new];
//Just for error tracing
jsonWriter.humanReadable = YES;
NSString *json = [jsonWriter stringWithObject:jsonDictionary];
if (!json){
    NSLog(@"-JSONRepresentation failed. Error trace is: %@", [jsonWriter errorTrace]);
}
[jsonWriter release];
NSData *data = [json dataUsingEncoding:NSUTF8StringEncoding];

And then you can set as your post request's body.

Answer 2

If you would like a more full-featured solution that what is offered by a standalone parsing library, you may want to take a look at RestKit: http://restkit.org/

The framework wraps the operations of fetching, parsing, and mapping JSON payloads into objects. It also allows you to update remote representations by POST/PUT'ing the objects back with a request. By default, outbound requests are form-encoded but the library ships with a class for using JSON as the wire format for posting back to the server.

At a high level, here's what your fetch & post operations would feel like in RestKit:

- (void)loadObjects {
  [[RKObjectManager sharedManager] loadObjectsAtResourcePath:[@"/path/to/stuff.json" delegate:self];
}

- (void)objectLoader:(RKObjectLoader*)loader didLoadObjects:(NSArray*)objects {
  NSLog(@"These are my JSON decoded, mapped objects: %@", objects);

  // Mutate and PUT the changes back to the server
  MyObject* anObject = [objects objectAtIndex:0];
  anObject.name = @"This is the new name!";
  [[RKObjectManager sharedManager] putObject:anObject delegate:self];
}

The framework takes care of the JSON parsing/encoding on a background thread and let's you declare how attributes in the JSON map to properties on your object. Mapping to Core Data backed classes is fully supported.