Simple custom view delegate example

CustomView.swift

import UIKit

protocol CustomViewDelegate {
    func customView(_ customView: CustomView, tappedButton button: UIButton)
}

class CustomView: UIView {
    
    weak var delegate: CustomViewDelegate?
    
    // <control-drag> from .XIB's button to here
    @IBAction func buttonTapped(_ sender: Any) {
        guard let button = sender as? UIButton else { return }
        delegate?.customView(self, tappedButton: button)
    }
    
    // ...
}

ViewController.swift

import UIKit

class ViewController: UIViewController {
    
    // <control-drag> from storyboard to here
    @IBOutlet weak var customView: CustomView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        customView.delegate = self
    }
    
    // ...
}

extension ViewController: CustomViewDelegate {
    
    func customView(_ customView: CustomView, tappedButton button: UIButton) {
        print("Button in custom view was tapped")
    }
}