iphone - AFHttpClient による認証の問題

Question

Instagram 用の AFHTTPClient クラスを作成しました。アクセス トークンを iOS キーチェーンに保存して、アプリを起動するたびに認証する必要がないようにしたいと考えています。ただし、アクセストークンを認証して保存できるのは、一度ログインした後だけです。コードは次のとおりです。

#define kAccessTokenInstagram    @"Token"
#define INSTAGRAM_AUTH_URL_FORMAT @"https://instagram.com/oauth/authorize/?client_id=%@&redirect_uri=%@&response_type=token"

#import "InstagramClient.h"
#import "Lockbox.h"

@interface InstagramClient ()
@property (nonatomic, copy) NSString *accessToken;
@end

@implementation InstagramClient

+ (instancetype)sharedClient {
    static InstagramClient *_sharedClient = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSURL *baseURL = [NSURL URLWithString:@"https://api.instagram.com/v1/"];
        _sharedClient = [[InstagramClient alloc] initWithBaseURL:baseURL];
    });

    return _sharedClient;
}

- (id)initWithBaseURL:(NSURL *)url {
    self = [super initWithBaseURL:url];
    if (self) {
        [self registerHTTPOperationClass:[AFJSONRequestOperation class]];
        [self setDefaultHeader:@"Accept" value:@"application/json"];
    }
    return self;
}

- (void)authenticateWithClientID:(NSString *)clientId callbackURL:(NSString *)callbackUrl {
    NSString *urlString = [NSString stringWithFormat:INSTAGRAM_AUTH_URL_FORMAT, clientId, callbackUrl];
    NSURL *url = [NSURL URLWithString:urlString];
    [[UIApplication sharedApplication] openURL:url];
}

- (void)handleOAuthCallbackWithURL:(NSURL *)url {
    NSError *regexError = nil;
    NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:@"^[^#]*#access_token=(.*)$"
                                                                           options:0
                                                                             error:&regexError];
    NSString *input = [url description];
    [regex enumerateMatchesInString:input
                            options:0
                              range:NSMakeRange(0, [input length])
                         usingBlock:^(NSTextCheckingResult *result, NSMatchingFlags flags, BOOL *stop) {
                             if ([result numberOfRanges] > 1) {
                                 NSRange accessTokenRange = [result rangeAtIndex:1];
                                 self.accessToken = [input substringWithRange:accessTokenRange];
                                 NSLog(@"Access Token: %@", self.accessToken);
                                 NSString *token = self.accessToken;
                                [Lockbox setString:token forKey:kAccessTokenInstagram];
                             }
                         }];

}

- (NSString *)accessToken {
    return [Lockbox stringForKey:kAccessTokenInstagram];
}

-(AFHTTPRequestOperation *)HTTPRequestOperationWithRequest:(NSURLRequest *)urlRequest success:(void (^)(AFHTTPRequestOperation *, id))success failure:(void (^)(AFHTTPRequestOperation *, NSError *))failure {
    NSMutableURLRequest *request = [urlRequest mutableCopy];
    NSString *separator = [request.URL query] ? @"&" : @"?";
    NSString *newURLString = [NSString stringWithFormat:@"%@%@access_token=%@", [request.URL absoluteString], separator, self.accessToken];
    NSURL *newURL = [[NSURL alloc] initWithString:newURLString];
    [request setURL:newURL];
    return [super HTTPRequestOperationWithRequest:request success:success failure:failure];

}

@end

問題を引き起こしているコードは次のとおりです...：

- (NSString *)accessToken {
    return [Lockbox stringForKey:kAccessTokenInstagram];
}