var b = UIBarButtonItem(title: "Cancel", style: .Plain, target: self, action: "Cancel clicked")

如果希望该方法将sender作为参数，则应在结尾处放置冒号：

var b = UIBarButtonItem(title: "Cancel", style: .Plain, target: self, action: "CancelClicked:")

func CancelClicked(sender: UIBarButtonItem) {
}

使用IB将NavigationBar（和NavigationItem）添加到情节提要后，对navigationItem var的引用将自动Map到ViewController文件。添加下面的代码以将取消按钮附加到navigationItem，从而消除模态视图控制器：

@IBOutlet weak var navigationBar: UINavigationBar!

override func viewDidLoad() {
    super.viewDidLoad()
    navigationItem.leftBarButtonItem = UIBarButtonItem(title: "Cancel", style: .plain, target: self, action: #selector(cancelClicked(sender:)))
}

func cancelClicked(sender: UIBarButtonItem) {
    print("Cancel clicked!")
    self.dismiss(animated: true, completion: nil)
}

试试看：

@IBAction func close(sender: AnyObject) {
    self.dismissViewControllerAnimated(true, completion: nil)
}

您的代码在一个小的修复后工作正常。Dismiss是在呈现控制器上调用的。self.dismissViewController将解除当前vc呈现的子视图控制器。但如果您想解除现有vc，则应在父视图控制器上调用dismiss

@IBAction func cancel(sender: UIBarButtonItem) {
      presentingViewController.dismissViewControllerAnimated(true, completion: nil)
}