一个类簇的实现示例（来自mantle的github主页）

@interface XYMessage : MTLModel@end@interface XYTextMessage: XYMessage@property (readonly, nonatomic, copy) NSString *body;@end@interface XYPictureMessage : XYMessage@property (readonly, nonatomic, strong) NSURL *imageURL;@end@implementation XYMessage+ (Class)classForParsingJSONDictionary:(NSDictionary *)JSONDictionary {    if (JSONDictionary[@"image_url"] != nil) {        return XYPictureMessage.class;    }    if (JSONDictionary[@"body"] != nil) {        return XYTextMessage.class;    }    NSAssert(NO, @"No matching class for the JSON dictionary '%@'.", JSONDictionary);    return self;}@end