facebook登录currentAccessToken返回null

Question

使用Facebook SDK登录时出现问题。 代码在那里。在AppDeledate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    BOOL flag = [[FBSDKApplicationDelegate sharedInstance] application:application
                                         didFinishLaunchingWithOptions:launchOptions];
    …………
   return flag
}
- (BOOL)application:(UIApplication *)application
            openURL:(NSURL *)url
  sourceApplication:(NSString *)sourceApplication
         annotation:(id)annotation{
    BOOL handled = [[FBSDKApplicationDelegate sharedInstance] application:application
                                                                  openURL:url
                                                        sourceApplication:sourceApplication
                                                               annotation:annotation
                    ];
    return handled;
}

并且在客户VC中

- (void)viewDidLoad {
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor whiteColor];
    // Do any additional setup after loading the view.
    FBSDKLoginButton *loginButton = [[FBSDKLoginButton alloc] init];
    loginButton.delegate = self;
    loginButton.readPermissions = @[@"public_profile", @"email"];
    loginButton.center = self.view.center;
    [self.view addSubview:loginButton];
}

当我在loginbutton点击并登录成功后调用[FBSDKAccessToken currentAccessToken]时（在我的facebook设置应用程序中我发现应用程序存在），令牌始终为null