对Swift最大的吐槽就是不允许多重继承，通过extension扩展某一个类带来的问题是全局有效的，想要控制它的可见范围，就要针对不同的场景隔离出不同的framework，如此在无形中提高了复杂度；所以最近野蛮地在团队里规定，如果是针对某一个类进行的功能扩展，优先使用：protocol + extension的方式，下面以在UINavigationController添加返回按钮为例进行说明：

import UIKit
import SnapKit

protocol EnableNavigationGoBack where Self:UIViewController {
    func enableNavigationGoBack()
    func onNavigationGoBackViewClicked()
}

extension EnableNavigationGoBack where Self:UIViewController {

    func enableNavigationGoBack() {
        let goBackView = UIButton(type: UIButtonType.custom)
        if #available(iOS 11, *) {
            goBackView.snp.makeConstraints { (make) in
                make.width.equalTo(13)
                make.height.equalTo(21)
            }
        } else {
            goBackView.frame = CGRect(x: 0, y: 0, width: 13, height: 21)
        }
        goBackView.setImage(UIImage.init(named: "go_back"), for: UIControlState.normal)
        goBackView.setOnClickedListener { [unowned self] (_) in
            self.onNavigationGoBackViewClicked()
        } // 参见：https://www.jianshu.com/p/0b2d5a03cd33
        self.navigationItem.leftBarButtonItem = UIBarButtonItem.init(customView: goBackView)
    }

    func onNavigationGoBackViewClicked() {
        if let rootViewController = self.navigationController?.viewControllers.first {
            if rootViewController.isEqual(self) {
                self.navigationController?.dismiss(animated: true, completion: nil)
                return
            }
        }
        self.navigationController?.popViewController(animated: true)
    }
}

class UserViewController: UIViewController, EnableNavigationGoBack {

    override func viewDidLoad() {
        super.viewDidLoad()
        enableNavigationGoBack()
    }
}