In the xib IB, make the used custom view class (subclass of NibView) the File’s Owner and connect the view outlet to the xib view.

NibView.swift

import UIKit

/**
 In the xib IB, make the used custom view class (subclass of NibView)
 the File’s Owner and connect the view outlet to the xib view.
 */
class NibView: UIView {

    @IBOutlet weak private var view: UIView!
    var nibName: String {
        guard let identifier = NSStringFromClass(self.classForCoder).components(separatedBy: ".").last else {
            assertionFailure("View could not be loaded from a nib. Make sure that the nib file shares the same name as the view class that is the file's owner and that the content view is mapped to owner's view IBOutlet.")
            return ""
        }
        return identifier
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)

        initializeSubviews()
    }

    override init(frame: CGRect) {
        super.init(frame: frame)

        view = initializeSubviews()
    }

    override func prepareForInterfaceBuilder() {
        super.prepareForInterfaceBuilder()

        initializeSubviews()
    }

    @discardableResult private func initializeSubviews() -> UIView {
        let view: UIView = Bundle.main.loadNibNamed(nibName, owner: self, options: nil)?.first as? UIView ?? UIView()
        view.frame = self.bounds
        addSubview(view)

        return view
    }
}