/*
    Custom View
*/

class CustomView: UIView() {

    @IBOutlet weak var actionButton: UIButton!

    public var buttonAction:  (() -> Void)?

    // init coder, frame

    private func initialize(){
        //...
        //=== actionButton
        self.actionButton.addTarget(self, action: #selector(self.actionButtonTouch(_:)), for: .touchUpInside)
    }

    @objc func actionButtonTouch(_ sender: UIButton) {
        buttonAction?()
    }
}

/*
    ViewController
*/

    func initContentView() {
        let customView = ...

        customView.buttonAction = { [weak self] in
            self?.x()
            let mainViewController = MainViewController()
            self?.navigationController?.pushViewController(mainViewController, animated: true)
        }
    }