VMJsonableObject helps developer to easily fill an NSObject with NSDictionary (probably parsed from a JSON string).

VMJsonableObject

// For example, let's define a VMUser class that saves user login info.

// VMUser.h
@interface VMUser : VMJsonableObject

@property (nonatomic)NSString *userId;
@property (nonatomic)NSString *userName;
@property (nonatomic)NSString *email;

@end

// VMUser.m
// Note the implementation part of `VMUser.m` file is empty.
// You only needs to write logic-related code now. YAY.
@implementation VMUser

@end

// And in the method you want to use it
id object = [NSJSONSerialization
                JSONObjectWithData:serverResponseData
                options:0
                error:nil];
if([object isKindOfClass:[NSDictionary class]]) {
    NSDictionary *results = (NSDictionary *)object;
    VMUser *user = [[VMUser alloc] initWithDictionary:results];
    NSLog(@"Login: %@", user.userName);
}

VMJsonableObject.h

#import <Foundation/Foundation.h>
#import <objc/runtime.h>

@interface VMJsonableObject : NSObject

- (id)initWithDictionary:(NSDictionary *)dictionary;

@end

VMJsonableObject.m

#import "VMJsonableObject.h"

@implementation VMJsonableObject

- (id)initWithDictionary:(NSDictionary *)dictionary
{
    if (self = [super init]) {
        [self fillValues:dictionary];
    }

    return self;
}

// This is problem oriented method. Please change to match your own situation.
// In my case, the API property is `user_id` while the property name is `userId`
// If your json property name is exactly as your property name, simply return it.
// Otherwise, do whatever it takes to make them match.
- (NSString *)convertToJSONName:(NSString *)propertyName
{
    // Step 1: find all (unique) uppercase characters
    NSArray *occurrences = [propertyName componentsSeparatedByCharactersInSet:[[NSCharacterSet uppercaseLetterCharacterSet] invertedSet]];
    NSSet *uniqueUppercaseCharacters = [NSSet setWithArray:occurrences];

    // Step 2: replace the uppercase letters with format of `_[lowercase]`
    for (NSString *uppercaseCharacter in uniqueUppercaseCharacters) {
        propertyName = [propertyName stringByReplacingOccurrencesOfString:uppercaseCharacter
                                                               withString:[NSString stringWithFormat:@"_%@", [uppercaseCharacter lowercaseString]]];
    }
    return propertyName;
}

// Fill object with parsed dictionary from JSON string.
// This method uses runtime support provided by Objective-C.
// The advantage is quite obvious. It literally makes the function smaller.
- (void)fillValues:(NSDictionary *)dictionary
{
    NSString *className = NSStringFromClass([self class]);
    id curClass = objc_getClass([className UTF8String]);
    unsigned int outCount, i;
    objc_property_t *properties = class_copyPropertyList(curClass, &outCount);
    for (i = 0; i < outCount; i++) {
        objc_property_t property = properties[i];
        NSString *propertyName = [NSString stringWithCString:property_getName(property) encoding:NSUTF8StringEncoding];
        id value = [dictionary objectForKey:[self convertToJSONName:propertyName]];
        if (value) {
            [self setValue:value forKey:propertyName];
        }
    }
}

@end

f**king cool!!!