使用ObjC RestKit库如何将对象映射到JSON NSString*

#import <RestKit/RestKit.h>

@interface User : NSObject
@property (nonatomic, strong) NSString *name;
@property (nonatomic, assign) NSInteger age;
@end

RKObjectMapping *mapping = [RKObjectMapping mappingForClass:[User class]];
[mapping addAttributeMappingsFromDictionary:@{
    @"name": @"name",
    @"age": @"age"
}];
[[RKObjectManager sharedManager].mappingProvider setMapping:mapping forKeyPath:@"user"];

User *user = [[User alloc] init];
user.name = @"John";
user.age = 25;

NSError *error;
NSDictionary *JSONDictionary = [RKMIMETypeSerialization objectFromData:[RKMIMETypeSerialization dataFromObject:user MIMEType:@"application/json" error:&error] MIMEType:@"application/json" error:&error];
NSString *JSONString = [RKMIMETypeSerialization stringFromObject:JSONDictionary MIMEType:@"application/json" error:&error];

NSError *error;
NSDictionary *JSONDictionary = [RKMIMETypeSerialization objectFromData:[JSONString dataUsingEncoding:NSUTF8StringEncoding] MIMEType:@"application/json" error:&error];
User *user = [RKMIMETypeSerialization objectFromData:[RKMIMETypeSerialization dataFromObject:JSONDictionary MIMEType:@"application/json" error:&error] MIMEType:@"application/json" error:&error];