AFNetworkingをViewControllerに書かないでModelに書きたいとき ref: http://qiita.com/kazuph/items/2cc92622a298079aad5e

Hoge.h

#import "AFHTTPSessionManager.h"
@interface Hoge : NSObject
+ (Hoge*)sharedClient;
- (void)hogeWithId:(NSString *)hogeId completion:(void (^)(NSDictionary *results, NSError *error))block;
@end

Hoge.m

#import "Hoge.h"

static NSString * const APIBaseURLString = @"http://example.com/";

@implementation Hoge

+ (Hoge*)sharedClient
{
    static Hoge* _sharedClient = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _sharedClient = [[Hoge alloc] init];
    });
    return _sharedClient;
}

- (NSURL *)url
{
    return [[NSURL URLWithString:APIBaseURLString] URLByAppendingPathComponent:@"api/hoges"];
}

- (void)hogeWithId:(NSString *)hogeId completion:(void (^)(NSDictionary *results, NSError *error))block
{
    AFHTTPSessionManager *manager = [AFHTTPSessionManager manager];
    [manager GET:[[self url] absoluteString]
       parameters:nil
          success:^(NSURLSessionDataTask *task, id responseObject) {
              if (block) block(responseObject, nil);
          }
          failure:^(NSURLSessionDataTask *task, NSError *error) {
              if (block) block(nil, error);
          }];
}
@end

MainViewController.m

#import "MainViewController.h"
#import "Hoge.h"

<略>

- (IBAction)getHoge:(id)sender {
    NSString* id = _hogeId.text;

    [[Authentication sharedClient]
     hogeWithId:id
     completion:^(NSDictionary *results, NSError *error) { //通信完了時のViewなどの処理はblockにして渡す
         if (error) { NSLog(@"Error %@", error); return; }
         if ([results[@"success"] isEqual: @(YES)]) {
             // 成功時の処理
         } else {
             // 失敗時の処理
         }
     }];
}

<略>