Jackson equivalent for iPhone?

Question

I have used Jackson extensively on the Server side to convert from POJOs to JSON and was wondering if there is a similar library for Objective C/iPhone SDK and vice versa. Objective C does provide reflection so it should be possible to make something similar to Jackson.

Answer 1

You might try GoldenFleece, which converts between JSON and Objective-C objects using a convention-over-configuration pattern inspired by Jackson.

Answer 2

The new iOS 5 APIs provide a great facility in reading/writing JSON. These are essentially a rehash of the TouchJSON library which you can use in iOS 4. While I haven't seen much out there that will generate POCO objects from an example payload, you can create classes that are just a facade for an NSDictionary instance that the aforementioned libraries will return.

For example:

@interface PBPhoto : NSObject {
    NSDictionary* data_;
}

@property (nonatomic, retain, readonly) NSDictionary *data;

- (NSString*) photoId;
- (NSString*) userId;
- (NSString*) user;
- (NSString*) title;
- (id) initWithData:(NSDictionary*)data;


@end

Implementation:

#import "PBPhoto.h"

#define PHOTO_ID @"id"
#define USER_ID @"user_id"
#define USER @"user"
#define TITLE @"title"

@implementation PBPhoto
@synthesize data = data_;

- (id) initWithData:(NSDictionary*)data {
    if ((self = [super init])) {
        self.data = data;
    }

    return self;
}

- (NSString*) photoId {
    return [super.data objectForKey:PHOTO_ID];
}

- (NSString*) userId {
    return [self.data objectForKey:USER_ID];
}

- (NSString*) user {
    return [self.data objectForKey:USER];
}

- (NSString*) title {
    return [self.data objectForKey:TITLE];
}

- (void) dealloc {
    [data_ release];
    [super dealloc];
}

@end

Answer 3

That Objective-C provides reflection may be the understatement of the year, but a lot of stuff is exposed only by the low-level C runtime and therefore is a little obtuse.

Assuming you want to take an arbitrary object and turn it into JSON, probably the smart thing is to create an NSDictionary as an intermediary, then pass it off to NSJSONSerialization (or else construct the string yourself because all of the third party libraries are quite heavyweight owing to the burden of being able to deserialise).

So, for example:

- (NSDictionary *)dictionaryOfPropertiesForObject:(id)object
{
    // somewhere to store the results
    NSMutableDictionary *result = [NSMutableDictionary dictionary];

    // we'll grab properties for this class and every superclass
    // other than NSObject 
    Class classOfObject = [object class];
    while(![classOfObject isEqual:[NSObject class]])
    {
        // ask the runtime to give us a C array of the properties defined
        // for this class (which doesn't include those for the superclass)
        unsigned int numberOfProperties;
        objc_property_t  *properties = 
                     class_copyPropertyList(classOfObject, &numberOfProperties);

        // go through each property in turn...
        for(
              int propertyNumber = 0;
              propertyNumber < numberOfProperties;
              propertyNumber++)
        {
            // get the name and convert it to an NSString
            NSString *nameOfProperty = [NSString stringWithUTF8String:
                                  property_getName(properties[propertyNumber])];

            // use key-value coding to get the property value
            id propertyValue = [object valueForKey:nameOfProperty];

            // add the value to the dictionary —
            // we'll want to transmit NULLs, even though an NSDictionary
            // can't store nils
            [result
                  setObject:propertyValue ? propertyValue : [NSNull null]
                  forKey:nameOfProperty];
        }

        // we took a copy of the property list, so...
        free(properties);

        // we'll want to consider the superclass too
        classOfObject = [classOfObject superclass];
    }

    // return the dictionary
    return result;
}

Then you can use + dataWithJSONObject:options:error: on NSJSONSerialization with the returned dictionary.

To go the other way, I guess you'd use the key-value coding setValue:forKey: method, getting keys and values from a dictionary via allKeys and valueForKey:.