如何在swift中实现popViewControllerAnimated

Question

我正在尝试回到我的应用程序中的上一个视图，并且我正在使用popViewControllerAnimated方法，但是当按下后退按钮时，这不会执行任何操作。我在这里错过了什么吗？

import UIKit

class SubjectsViewController: UIViewController {

    @IBOutlet var myTableView: UITableView!

    @IBOutlet var noBooksLabel: UILabel!

    @IBOutlet var navigationBarTitle: UINavigationBar!

    func backAction() -> Void {

        if let navController = self.navigationController {
            navController.popViewControllerAnimated(true)
        }

        //self.navigationController?.popViewControllerAnimated(true)

    }

    override func viewWillAppear(animated: Bool) {

        self.noBooksLabel.hidden = true

    }

    override func viewDidLoad() {
        super.viewDidLoad()

        descriptions.removeAll(keepCapacity: true)
        usernames.removeAll(keepCapacity: true)
        imageFiles.removeAll(keepCapacity: true)

        println("Subject view controller")

        self.navigationBarTitle.topItem?.title = "\(selectedSubject)"

        let backButton = UIBarButtonItem(title: "Back", style: UIBarButtonItemStyle.Plain, target: self, action: "backAction")

        self.navigationBarTitle.topItem?.leftBarButtonItem = backButton