An @IBDesignable iOS class that can be used in Interface Builder as a custom segue to dismiss a modally presented view controller.

DismissSegue.swift

@IBDesignable class DismissSegue: UIStoryboardSegue {

    var completion: (() -> Void)?

    override func perform() {
        self.source.presentingViewController?.dismiss(animated: UIView.areAnimationsEnabled, completion: self.completion)
    }

}