Episode #116 | 16 minutes | published on April 24, 2014 | Uses mantle-1.4.1
In this episode we go over a handy little model framework called Mantle. With Mantle we can easily get support for NSCoding, NSCopying, and JSON serialization for our model objects.

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Implementing Mantle for JSON Serialization

We need to inherit from MTLModel for equals:, hashCode, NSCoding, and NSCopying. To get JSON serialization support, we also need to conform to the MTLJSONSerializing protocol:

#import "Mantle.h"

typedef NS_ENUM(NSInteger, EpisodeType) {

@interface Episode : MTLModel <MTLJSONSerializing>

@property (nonatomic, assign) NSInteger episodeNumber;
@property (nonatomic, copy) NSString *title;
@property (nonatomic, copy) NSString *episodeDescription;
@property (nonatomic, copy) NSURL *thumbnailImageUrl;
@property (nonatomic, assign) EpisodeType episodeType;
@property (nonatomic, strong) NSDate *publishedAt;


Next we need to provide a mapping for our keys. Names that match up directly are copied automatically, but we have to specify the deviations:

+ (NSDictionary *)JSONKeyPathsByPropertyKey {
    return @{
             @"episodeDescription": @"description",
             @"episodeNumber" : @"episode_number",
             @"episodeType" : @"episode_type",
             @"publishedAt" : @"published_at",
             @"thumbnailImageUrl" : @"thumbnail_url"

For the type conversions, such as enums, urls, or dates, you must specify a value transformer. Keep in mind that we can serialize in both directions, so some of these transformers can be specified as reversible.

+ (NSValueTransformer *)episodeTypeJSONTransformer {
    return [NSValueTransformer mtl_valueMappingTransformerWithDictionary:@{
                                                                           @"paid": @(EpisodeTypePaid),
                                                                           @"free": @(EpisodeTypeFree)

+ (NSDateFormatter *)dateFormatter {
    static NSDateFormatter *__dateFormatter = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        __dateFormatter = [[NSDateFormatter alloc] init];
        __dateFormatter.locale = [NSLocale localeWithLocaleIdentifier:@"en_US_POSIX"];
        __dateFormatter.dateFormat = @"yyyy-MM-dd'T'HH:mm:ssZ";
    return __dateFormatter;

+ (NSValueTransformer *)publishedAtJSONTransformer {
    return [MTLValueTransformer transformerWithBlock:^id(NSString *dateString) {
        return [self.dateFormatter dateFromString:dateString];

+ (NSValueTransformer *)thumbnailImageUrlJSONTransformer {
    return [NSValueTransformer valueTransformerForName:MTLURLValueTransformerName];

The thing to notice here is that the name of these methods is dynamic, and is the combination of your property key and the value "JSONTransformer". If you don't specify a transformer, it will try to copy the value over directly, which could end in a crash if the types don't match up.

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