iOS：Facebook Access令牌为零

Question

为什么即使在测试用户登录后，viewdidload中的Facebook访问令牌也为零？

override func viewDidLoad() {
    super.viewDidLoad()
let loginButton = FBSDKLoginButton()
    loginButton.center = self.view.center
    self.view.addSubview(loginButton)
    loginButton.readPermissions = ["public_profile", "email", "user_friends"]

    if (FBSDKAccessToken.current() != nil) {
        // User is logged in, do work such as go to next view controller.
        let vc = MainViewController()
        self.present(vc, animated: true, completion: nil)
    }
}