ios - 为什么我们不能在以编程方式设置初始 viewController 时以编程方式在 viewController 之间跳转？

Question

我只使用了两个 viewController，比如 viewControllerA 和 viewControllerB。通过在AppDelegate.swift中使用如下代码，当App第一次启动时，会将viewControllerA设置为初始viewController，从第二次启动开始， viewControllerB 将被设置为初始 viewController。

class AppDelegate: UIResponder, UIApplicationDelegate {
var window: UIWindow?
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool
{
    let defaultss = UserDefaults.standard
    if (!defaultss.bool(forKey: "firstTimes")) {  
        self.window = UIWindow(frame: UIScreen.main.bounds)

        let storyboard = UIStoryboard(name: "Main", bundle: nil)

        let initialViewController = storyboard.instantiateViewController(withIdentifier: "ViewControllerA")

        self.window?.rootViewController = initialViewController
        self.window?.makeKeyAndVisible()

    defaultss.set(true, forKey: "firstTimes") //set the key so it never executes again
    } else
    {

        self.window = UIWindow(frame: UIScreen.main.bounds)

        let storyboard = UIStoryboard(name: "Main", bundle: nil)

        let initialViewController = storyboard.instantiateViewController(withIdentifier: "ViewControllerB")

        self.window?.rootViewController = initialViewController
        self.window?.makeKeyAndVisible()
    }
     return true

}

它工作正常。但是如果我使用这段代码，那么我无法以编程方式在 viewController 之间跳转，无论是从 A 到 B 还是从 B 到 A，使用以下代码。

import UIKit

class ViewControllerA: UIViewController {


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

    @IBAction func buttonA(_ sender: Any)
    {
            let ViewControllerB = storyboard?.instantiateViewController(withIdentifier: "ViewControllerB") as! ViewControllerB
            navigationController?.pushViewController(ViewControllerB, animated: true)
    }


}

import UIKit

class ViewControllerB: UIViewController {

    override func viewDidLoad()
    {
        super.viewDidLoad()

    }


    @IBAction func buttonB(_ sender: Any)
    {
         let ViewControllerA = storyboard?.instantiateViewController(withIdentifier: "ViewControllerA") as! ViewControllerA
            navigationController?.pushViewController(ViewControllerA, animated: true)
    }

}

我怎样才能同时实现这两者，我应该能够检测到第一次启动并设置初始 viewController，而且我还应该能够以编程方式在 viewController 之间跳转。

我正在使用 Xcode 8.2、Swift 3.0。

提前致谢。

Answer 1

在 App delegate 中执行此操作

let navC = UINavigationController(rootViewController: initialViewController)
self.window?.rootViewController = navC

代替

self.window?.rootViewController = initialViewController