Swift如何依次加载视图控制器？无按钮自动跳转实现

我希望以编程方式先加载第一个视图控制器SLLoadingViewController，再自动从它加载嵌套在导航控制器中的StoreViewController。

现有失效代码 #

SceneDelegate 代码 #

func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        
        if let windowScene = (scene as? UIWindowScene) {
            let window = UIWindow(windowScene: windowScene)
            window.rootViewController = SLLoadingViewController()
            self.window = window
            window.makeKeyAndVisible()
        }
    }

SLLoadingViewController 自动跳转代码（失效） #

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

func load() {
    
    let nav = SLNavigationViewController()
    let mainView = StoreViewController()
    nav.navigationBar.frame.size = nav.navigationBar.sizeThatFits(CGSize(width: nav.navigationBar.frame.size.width, height: 2))
    nav.viewControllers = [mainView]
    
    present(nav, animated: false, completion: nil)
}

这段代码无法生效，最终的StoreViewController无法显示在屏幕上，但通过按钮触发的代码可以正常运行：

按钮触发的有效代码 #

override func viewDidLoad() {
    super.viewDidLoad()
    button.frame = CGRect(x: 100, y: 200, width: 30, height: 20)
    button.backgroundColor = .green
    view.addSubview(button)
    button.addTarget(self, action: #selector(self.load), for: .touchUpInside) 
}

@objc func load() {
    
    let nav = SLNavigationViewController()
    let mainView = StoreViewController()
    nav.navigationBar.frame.size = nav.navigationBar.sizeThatFits(CGSize(width: nav.navigationBar.frame.size.width, height: 2))
    nav.viewControllers = [mainView]
    
    present(nav, animated: false, completion: nil)
}

解决方案 #

问题出在viewDidLoad阶段，此时视图控制器的视图还未完成布局和显示，调用present无法正确触发跳转。可以通过以下几种方式解决：

方法1：移至viewDidAppear执行 #

将跳转逻辑放到viewDidAppear方法中，确保视图已经完全显示后再执行跳转：

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

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
    load()
}

func load() {
    let nav = SLNavigationViewController()
    let mainView = StoreViewController()
    nav.navigationBar.frame.size = nav.navigationBar.sizeThatFits(CGSize(width: nav.navigationBar.frame.size.width, height: 2))
    nav.viewControllers = [mainView]
    
    present(nav, animated: false, completion: nil)
}

方法2：延迟执行跳转逻辑 #

如果不想等待viewDidAppear，可以在viewDidLoad中延迟调用跳转方法，给视图足够的初始化时间：

override func viewDidLoad() {
    super.viewDidLoad()
    DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) {
        self.load()
    }
}

func load() {
    let nav = SLNavigationViewController()
    let mainView = StoreViewController()
    nav.navigationBar.frame.size = nav.navigationBar.sizeThatFits(CGSize(width: nav.navigationBar.frame.size.width, height: 2))
    nav.viewControllers = [mainView]
    
    present(nav, animated: false, completion: nil)
}

方法3：直接替换根控制器（更高效） #

如果不需要保留SLLoadingViewController，可以直接替换窗口的根控制器，无需使用present：

func load() {
    let nav = SLNavigationViewController()
    let mainView = StoreViewController()
    nav.navigationBar.frame.size = nav.navigationBar.sizeThatFits(CGSize(width: nav.navigationBar.frame.size.width, height: 2))
    nav.viewControllers = [mainView]
    
    if let window = UIApplication.shared.windows.first {
        window.rootViewController = nav
    }
}

内容的提问来源于stack exchange，提问作者user25462420