Swift iOS : 解决了一个委托不工作的情况

在解决一个遗产代码的过程中，我希望对UIAlertView稍作封装，以便从巨大的ViewController内分离出特定的AlertView的代码，我这样做的：

import UIKit
@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    var window : UIWindow?
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        window = UIWindow()
        window!.rootViewController = UIViewController()
        window!.rootViewController!.view.backgroundColor = .blue
        window!.makeKeyAndVisible()
        let a = AlertView()
        a.show()
        return true
    }
}
class AlertView : UIViewController,UIAlertViewDelegate{
    var done : ((_ buttonIndex: Int)->Void)?
    func show(){
        var createAccountErrorAlert: UIAlertView = UIAlertView()
        createAccountErrorAlert.delegate = self
        createAccountErrorAlert.title = "Oops"
        createAccountErrorAlert.message = "Could not create account!"
        createAccountErrorAlert.addButton(withTitle: "Dismiss")
        createAccountErrorAlert.addButton(withTitle: "Retry")
        createAccountErrorAlert.show()
    }
    func alertView(_ alertView: UIAlertView, clickedButtonAt buttonIndex: Int){
            print("Why delegate of alert view does not work?")
    }
}

出乎意料的是，点击按钮的时候，alertView:clickedButtonAt:就是不执行。然后很快问题解决，原因在于，AlertView实例是局部的，代码执行后实例已经被释放，委托也就跟着没了。

解决方法很简单，把：

let a = AlertView()

改成

    var a : AlertView
    ...
    a = AlertView()

即可。