前言：
多個 UIButton 合併在一起的用法

正文：
先建立幾個 UIButton

全部選取，然後點擊 Stack View

將 １２３４ 這4個 UIButton 連接到 Outlet Collection

利用Control鍵 + 滑鼠左鍵 將 firstButton 拖曳到 Safe Area 點選 Equal Hieghts

最後加上約束，隱藏１２３４ 這幾個 UIButton

以下為程式碼：
ViewController

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var firstButton: UIButton!
    //將１２３４ UIButton 連結到options
    @IBOutlet var options: [UIButton]!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
    }
    //將 firstButton UIButton 連結到 optionpressed
    @IBAction func selectOptions(_ sender: UIButton) {
        for option in options{
            UIView.animate(withDuration: 0.3, animations: {
                option.isHidden = !option.isHidden
            })
        }
    }
    //將１２３４ UIButton 連結到 optionpressed
    @IBAction func optionpressed(_ sender: UIButton) {
        let number = sender.currentTitle ?? ""
        firstButton.setTitle("\(number)", for: .normal)
        for option in options{
            UIView.animate(withDuration: 0.3, animations: {
                option.isHidden = !option.isHidden
            })
        }
    }

}

成果圖：