tap tab to change element view controller

TabViewController.swift

import UIKit

class ButtonViewController: UIViewController {
    var tapAction: (() -> Void)?
    private lazy var button: UIButton = {
        let v = UIButton.init(frame: .init(x: 0, y: 0, width: 60, height: 60))
        v.backgroundColor = .orange
        v.addTarget(self, action: #selector(didTapAction), for: .touchUpInside)
        return v
    }()
    @objc
    func didTapAction() {
        tapAction?()
    }
    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(button)
    }
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        button.frame = .init(x: (view.bounds.width - button.frame.width) * 0.5,
                             y: (view.bounds.height - button.frame.height) * 0.5,
                             width: button.frame.width,
                             height: button.frame.height)
    }
}

class AViewController: ButtonViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .red
    }
}

class B1ViewController: ButtonViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .blue
    }
}

class B2ViewController: UIViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .green
    }
}

class TabController: UITabBarController {

    let tabA = (item: UITabBarItem(title: "A", image: nil, tag: 0), index: 0)
    let tabB = (item: UITabBarItem(title: "B", image: nil, tag: 1), index: 1)
    
    let aVC = AViewController()
    let b1VC = B1ViewController()
    let b2VC = B2ViewController()

    override func viewDidLoad() {
        super.viewDidLoad()
        aVC.tabBarItem = tabA.item
        b1VC.tabBarItem = tabB.item
        b2VC.tabBarItem = tabB.item
        setViewControllers([aVC, b1VC], animated: false)
        aVC.tapAction = { [weak self] in
            if let idx = self?.tabB.index {
                self?.selectedIndex = idx
            }
        }
        b1VC.tapAction = { [weak self] in
            guard let strongSelf = self else {
                return
            }
            strongSelf.setViewControllers([strongSelf.aVC, strongSelf.b2VC], animated: false)
        }
    }
    
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
    }
}