Using AFNetworking 2.0 and Mantle for some hot RESTful JSON action

NPClientAPI.h

@interface NPClientAPI : AFHTTPSessionManager

- (instancetype)initWithBaseURL:(NSString *)baseURL;

/**
 Singleton object.
 */
+ (NPClientAPI *)sharedAPI;

/**
 Fetches topics. Array of @c NPTopic objects.
 */
- (NSURLSessionDataTask *)fetchTopicsOnSuccess:(void (^)(NSURLSessionDataTask *task, NSArray *topics))success failure:(void (^)(NSURLSessionDataTask *task, NSError *error))failure;

@end

NPClientAPI.m

@implementation NPClientAPI

NSString *const NPClientAPITopicsIndexURI = @"mobile/topics.json";

#pragma mark - Singleton

+ (NPClientAPI *)sharedAPI
{
    static NPClientAPI *sharedClientAPI;
    
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedClientAPI = [[self alloc] initWithBaseURL:[NSURL URLWithString:NP_BASE_URL]];
    });
    
    return sharedClientAPI;
}

#pragma mark - Lifecycle

- (instancetype)initWithBaseURL:(NSURL *)baseURL
{
    if (self = [super initWithBaseURL:baseURL]) {
        // Note: probably not necessary, since each API call will set the request/response serializers to custom subclasses
        self.requestSerializer = [AFJSONRequestSerializer serializer];
        self.responseSerializer = [AFJSONResponseSerializer serializer];
    }
    return self;
}

#pragma mark - Public methods

- (NSURLSessionDataTask *)fetchTopicsOnSuccess:(void (^)(NSURLSessionDataTask *task, NSArray *topics))success failure:(void (^)(NSURLSessionDataTask *task, NSError *error))failure
{
    // Set up the response serializer to our custom serializer
    self.responseSerializer = [NPTopicsSerializer serializer];
    
    NSURL *url = [NSURL URLWithString:NPClientAPITopicsIndexURI relativeToURL:self.baseURL];
    
    NSURLSessionDataTask *task = [self GET:[url absoluteString] parameters:nil success:^(NSURLSessionDataTask *task, id responseObject) {
        if (success) success(task, responseObject);
    } failure:^(NSURLSessionDataTask *task, NSError *error) {
        // Just a random note: it's a good practice to wrap NSError objects to provide more context
        // about the type of failure
        NSError *wrappedError = [NSError errorWithDomain:NPClientAPIErrorDomain code:NPClientAPIErrorTypeNetworkIssue userInfo:@{ NSUnderlyingErrorKey : error, NSLocalizedDescriptionKey : @"Failed to fetch topics" }];
        if (failure) failure(task, wrappedError);
    }];
    
    return task;
}

@end

NPTopic.h

#import <Mantle/Mantle.h>

@interface NPTopic : MTLModel <MTLJSONSerializing>

@property (strong, nonatomic) NSNumber *identifier;
@property (strong, nonatomic) NSString *name;

@end

NPTopic.m

@implementation NPTopic

#pragma mark - Protocol conformance

#pragma mark MTLJSONSerializing

// This is a Mantle method override. It's a declarative method that maps the NPTopic object's
// property names to their equivalent JSON key name.
+ (NSDictionary *)JSONKeyPathsByPropertyKey
{
    return @{@"identifier" : @"id",
             @"name" : @"name"
             };
}

@end

NPTopicsSerializer.h

#import "AFURLResponseSerialization.h"

@interface NPTopicsSerializer : AFJSONResponseSerializer

@end

NPTopicsSerializer.m

#import "NPTopicsSerializer.h"
#import "NPTopic.h"

@implementation NPTopicsSerializer

// Returns array of @c NPTopic objects
- (id)responseObjectForResponse:(NSURLResponse *)response data:(NSData *)data error:(NSError *__autoreleasing *)error
{
    if (![self validateResponse:(NSHTTPURLResponse *)response data:data error:error]) {
        return nil;
    }
    
    NSDictionary *JSONDictionary = [super responseObjectForResponse:response data:data error:error];
    if (!JSONDictionary) return nil;
    
    // Note: the expected JSON format of this response is { data: [ { <a topic> }, { <another topic>} ], metadata: { ...} }
    NSArray *JSONTopics = JSONDictionary[@"data"];
    
    NSMutableArray *topics = [NSMutableArray array];
    for (NSDictionary *JSONTopic in JSONTopics) {
        // For each topic in JSON format, we can deserialize it from JSON to our desired model class using Mantle
        NPTopic *topic = [MTLJSONAdapter modelOfClass:[NPTopic class] fromJSONDictionary:JSONTopic error:error];
        if (!topic) return nil;
        [topics addObject:topic];
    }
    
    *error = nil;
    return [topics copy];
}

@end

FWIW, if anyone copied this before 10/5, please take a close look at NPTopicsSerializer. I've made a few critical fixes that help it handle errors properly.

FWIW, this gist uses a custom response serializer to deserialize the server response data into an array of NPTopic objects. If your custom AFHTTPSessionManager subclass will use different types of response serializers for different API calls, I highly recommend reading up on AFCompoundResponseSerializer. It prevents a lot of headaches that result from juggling response serializers for different methods -- i.e. calling two different methods back-to-back and having one overwrite the response serializer of the other.