我已经解决了:

let scenes = UIApplication.shared.connectedScenes
let windowScene = scenes.first as? UIWindowScene
let window = windowScene?.windows.first

灵感来自berni的回答

let keyWindow = Array(UIApplication.shared.connectedScenes)
        .compactMap { $0 as? UIWindowScene }
        .flatMap { $0.windows }
        .first(where: { $0.isKeyWindow })

- (UIWindow *)mainWindow {
    NSEnumerator *frontToBackWindows = [UIApplication.sharedApplication.windows reverseObjectEnumerator];
    for (UIWindow *window in frontToBackWindows) {
        BOOL windowOnMainScreen = window.screen == UIScreen.mainScreen;
        BOOL windowIsVisible = !window.hidden && window.alpha > 0;
        BOOL windowLevelSupported = (window.windowLevel >= UIWindowLevelNormal);
        BOOL windowKeyWindow = window.isKeyWindow;
        if(windowOnMainScreen && windowIsVisible && windowLevelSupported && windowKeyWindow) {
            return window;
        }
    }
    return nil;
}

对于Objective-C解决方案

+ (UIWindow *)keyWindow
{
    NSArray<UIWindow *> *windows = [[UIApplication sharedApplication] windows];
    for (UIWindow *window in windows) {
        if (window.isKeyWindow) {
            return window;
        }
    }
    return nil;
}

试试这个:

UIApplication.shared.windows.filter { $0.isKeyWindow }.first?.rootViewController!.present(alert, animated: true, completion: nil)

(在运行于Xcode 13.2.1的iOS 15.2上测试)

extension UIApplication {
    
    var keyWindow: UIWindow? {
        // Get connected scenes
        return UIApplication.shared.connectedScenes
            // Keep only active scenes, onscreen and visible to the user
            .filter { $0.activationState == .foregroundActive }
            // Keep only the first `UIWindowScene`
            .first(where: { $0 is UIWindowScene })
            // Get its associated windows
            .flatMap({ $0 as? UIWindowScene })?.windows
            // Finally, keep only the key window
            .first(where: \.isKeyWindow)
    }
    
}

如果你想在关键的UIWindow中找到呈现的UIViewController，这是另一个你可以发现有用的扩展:

extension UIApplication {
    
    var keyWindowPresentedController: UIViewController? {
        var viewController = self.keyWindow?.rootViewController
        
        // If root `UIViewController` is a `UITabBarController`
        if let presentedController = viewController as? UITabBarController {
            // Move to selected `UIViewController`
            viewController = presentedController.selectedViewController
        }
        
        // Go deeper to find the last presented `UIViewController`
        while let presentedController = viewController?.presentedViewController {
            // If root `UIViewController` is a `UITabBarController`
            if let presentedController = presentedController as? UITabBarController {
                // Move to selected `UIViewController`
                viewController = presentedController.selectedViewController
            } else {
                // Otherwise, go deeper
                viewController = presentedController
            }
        }
        
        return viewController
    }
    
}

你可以把它放在任何你想要的地方，但我个人把它作为UIViewController的扩展。

这让我可以添加更多有用的扩展，比如更容易地呈现UIViewControllers:

extension UIViewController {
    
    func presentInKeyWindow(animated: Bool = true, completion: (() -> Void)? = nil) {
        DispatchQueue.main.async {
            UIApplication.shared.keyWindow?.rootViewController?
                .present(self, animated: animated, completion: completion)
        }
    }
    
    func presentInKeyWindowPresentedController(animated: Bool = true, completion: (() -> Void)? = nil) {
        DispatchQueue.main.async {
            UIApplication.shared.keyWindowPresentedController?
                .present(self, animated: animated, completion: completion)
        }
    }
    
}