facebook_sync_authenticator.rb

  def authenticate &block
    account_store = ACAccountStore.alloc.init
    fb_account_type = account_store.accountTypeWithAccountTypeIdentifier( ACAccountTypeIdentifierFacebook)
    facebook_options = {}
    facebook_options[ACFacebookAppIdKey] = @application_id
    facebook_options[ACFacebookPermissionsKey] = ["email"]

    account_store.requestAccessToAccountsWithType(fb_account_type, options:facebook_options, completion: lambda do |granted, error|
      if granted
        accounts = account_store.accountsWithAccountType(fb_account_type)
        fb_account = accounts.last
        @access_token = fb_account.credential.oauthToken
        NSUserDefaults.standardUserDefaults.setObject @access_token, forKey:access_token_key
        block.call fb_account
      else
        Dispatch::Queue.main.async do
          alert = UIAlertView.alloc.initWithTitle("Account Error",
                                          message:"There is no Facebook Accounts configured. You can configure a Facebook acount in Settings.",
                                          delegate:nil,
                                          cancelButtonTitle:"Ok",
                                          otherButtonTitles: nil)
          alert.show
        end
      end
    end)
  end