Facebook API Windows Universal 获取 WebAuthentication 结果

我想得到登录对话框的响应，如何得到它？我的目标是确保用户成功授予权限，并且我通过访问令牌获得与之交换的代码：

我的 GET 请求网址：

public async void getAccessByLogin()
{
    string request_string = string.Format("https://www.facebook.com/dialog/oauth?client_id={0}&display={1}&response_type={2}&redirect_uri={3}&scope={4}",
    Uri.EscapeDataString(RequestConstants.app_id.ToString()),
    Uri.EscapeDataString("popup"),
    Uri.EscapeDataString("token"),
    Uri.EscapeDataString(WebAuthenticationBroker.GetCurrentApplicationCallbackUri().ToString()),
    Uri.EscapeDataString("email"));
    Uri request_uri = new Uri(request_string);
    await WebAuthenticationBroker.AuthenticateAsync(WebAuthenticationOptions.None, request_uri);
}

要登录，我在getAccessByLogin方法中调用此方法：

await WebAuthenticationBroker.AuthenticateAsync(WebAuthenticationOptions.None, request_uri);

WebAuthenticationResult result = await WebAuthenticationBroker.AuthenticateAsync(WebAuthenticationOptions.None, request_uri);
if (result.ResponseStatus != WebAuthenticationStatus.Success)
    await new MessageDialog("Problem!").ShowAsync();
else // do something