Is there a way to assign delegate to more than one object of the same class just one time in Swift?

There is no explicit way. You can store 20 buttons into an array for productively use next time.

buttons = [btn0, ......., btn19]

for btn in buttons {
    btn.delegate = self
}

Another way, you can make static delegate. When the button event will fire, check your static delegate type, and control everything.

import UIKit

class ViewController1: UIViewController {
    var buttons: [CustomButton] = [CustomButton]()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        loadUI()
    }
    
    func loadUI()  {
        buttons = (0..<5).map({ (index) -> CustomButton in
            let button = CustomButton()
            button.tag = index
            button.setTitle("\(index)", for: .normal)
            button.setTitleColor(.black, for: .normal)
            button.backgroundColor = UIColor.green
            return button
        })
        
        for (index, button) in buttons.enumerated()  {
            view.addSubview(button)
            button.translatesAutoresizingMaskIntoConstraints = false
            button.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
            if index == 0 {
                button.leftAnchor.constraint(equalTo: view.leftAnchor).isActive = true
            }else {
                button.leftAnchor.constraint(equalTo: (buttons[index-1]).rightAnchor).isActive = true
            }
            if index == buttons.count - 1 {
                button.rightAnchor.constraint(equalTo: view.rightAnchor).isActive = true
            }
            button.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
            button.widthAnchor.constraint(equalTo: (buttons[0]).widthAnchor).isActive = true
            button.addTarget(self, action: #selector(self.buttonEvent(_:)), for: .touchUpInside)
        }
        
        CustomButton.delegate = self
    }
    
    @objc func buttonEvent(_ button: UIButton) {
        if let viewController = CustomButton.delegate as? ViewController1 {
            print("tag: \(button.tag) viewController: \(viewController)")
        }
    }
}

extension ViewController1: CustomButtonProtocol {
}

protocol CustomButtonProtocol {
}

class CustomButton: UIButton {
    public static var delegate: CustomButtonProtocol?
}

There are a couple of approaches:

1. Personally, I'd make the delegate property an @IBOutlet and then I could specify the delegate right in IB’s “Connection Inspector”, not having to do anything in code at all, e.g.

   @objc protocol CustomButtonDelegate: class { ... }
   
   class CustomButton: UIButton {
   
       @IBOutlet weak var delegate: CustomButtonDelegate?
   
       ...
   }
   

   

2. Alternatively, if you really want to do this programmatically, when you create outlets for the buttons in IB, you can also create a “outlet collection”:

   @IBOutlet var buttons: [CustomButton]!
   

   Then you can control-drag from the control in IB to this outlet collection in the “Assistant” view.

   

   Once you do that, you can iterate through the outlet collection:

   for button in buttons {
       button.delegate = self
   }
   

   Clearly, you can manually populate the array of buttons yourself, but again, I would hook up the buttons to this “outlet collection” array right in IB.

You can use IBOutletCollection with different tags for your buttons.