A protocol that allows you to load a view from a xib file into another xib or storyboard. https://stackoverflow.com/a/47295926

NibLoadable.swift

public protocol NibLoadable {
    static var nibName: String { get }
}

public extension NibLoadable where Self: UIView {
    
    public static var nibName: String {
        return String(describing: Self.self)
    }
    
    public static var nib: UINib {
        let bundle = Bundle(for: Self.self)
        return UINib(nibName: Self.nibName, bundle: bundle)
    }
    
    func setupFromNib() {
        guard let view = Self.nib.instantiate(withOwner: self, options: nil).first as? UIView else { fatalError("Error loading \(self) from nib") }
        addSubview(view)
        view.translatesAutoresizingMaskIntoConstraints = false
        view.leadingAnchor.constraint(equalTo: self.safeAreaLayoutGuide.leadingAnchor, constant: 0).isActive = true
        view.topAnchor.constraint(equalTo: self.safeAreaLayoutGuide.topAnchor, constant: 0).isActive = true
        view.trailingAnchor.constraint(equalTo: self.safeAreaLayoutGuide.trailingAnchor, constant: 0).isActive = true
        view.bottomAnchor.constraint(equalTo: self.safeAreaLayoutGuide.bottomAnchor, constant: 0).isActive = true
    }
}

Usage:

1. Create a .swift file for your custom view and a .xib file with the same name.
2. Set the File's owner of the .xib file to the .swift file.
3. Do what ever you want with the .xib file.
4. Implement the NibLoadable for your custom view as below:

@IBDesignable
class CustomView: UIView, NibLoadable {
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        setupFromNib()
    }

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

Now you can load the CustomView in your xib/storyboard.