未调用PFLogInViewControllerDelegate函数

Question

当使用PFLogInViewController（或它的子类）时，看起来PFLogInViewControllerDelegate中的所有函数都没有被调用。虽然主ViewController声明符合PFLogInViewControllerDelegate协议，但PFLogInViewController对象的委托已设置。

还有别的事吗？

以下是相关代码，但我认为它不会带来比上面所写的更多：

class ViewController: UIViewController, PFLogInViewControllerDelegate {
    var loginVC:PFLogInViewController!
    ....
    func someFunc() {
        if (loginVC == nil) {
            loginVC = PFLogInViewController()
            loginVC.delegate = self
        }

        self.navigationController?.pushViewController(loginVC, animated: true)
    }
    ....
    // MARK: - PFLogInViewControllerDelegate

    // Sent to the delegate to determine whether the log in request should be submitted to the server.
    func logInViewController(logInController: PFLogInViewController,
                             shouldBeginLogInWithUsername username:String,
                             password: String) -> Bool {
        print(#function) // THIS IS NEVER CALLED!!!

        return true
    }


    // Sent to the delegate when a PFUser is logged in.
    func logInViewController(logInController: PFLogInViewController, didLogInUser user: PFUser) {
        print(#function) // THIS IS NEVER CALLED!!!
    }
}