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
{
    NSDictionary *JSONDictionary = [NSJSONSerialization JSONObjectWithData:data options:0 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