为什么我们不能在以编程方式设置初始视图控制器时以编程方式在视图控制器之间跳转？

Question

我只使用了两个视图控制器，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，将viewControllerA设置为初始viewController，从第二次开始，viewControllerB将设置为初始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，如果我使用此代码，则无法使用以下代码以编程方式在视图控制器之间从 A 跳转到 B 或 B 到 A。

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，我应该能够检测到第一次启动并设置初始 viewController，我也应该能够以编程方式在 viewController 之间跳转。

我使用的是 Xcode 8.2、Swift 3.0。

提前致谢。

Answer 1

在应用委托中执行此操作

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

而不是

self.window?.rootViewController = initialViewController