试试这个:

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

NSSet *connectedScenes = [UIApplication sharedApplication].connectedScenes;
for (UIScene *scene in connectedScenes) {
    if (scene.activationState == UISceneActivationStateForegroundActive && [scene isKindOfClass:[UIWindowScene class]]) {
        UIWindowScene *windowScene = (UIWindowScene *)scene;
        for (UIWindow *window in windowScene.windows) {
            UIViewController *viewController = window.rootViewController;
            // Get the instance of your view controller
            if ([viewController isKindOfClass:[YOUR_VIEW_CONTROLLER class]]) {
                // Your code here...
                break;
            }
        }
    }
}

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 })

如果你使用SwiftLint的'first_where'规则，并想静音交战:

UIApplication.shared.windows.first(where: { $0.isKeyWindow })

当.foregroundActive场景为空时，我遇到了这个问题

这是我的变通办法

public extension UIWindow {
    @objc
    static var main: UIWindow {
        // Here we sort all the scenes in order to work around the case
        // when no .foregroundActive scenes available and we need to look through
        // all connectedScenes in order to find the most suitable one
        let connectedScenes = UIApplication.shared.connectedScenes
            .sorted { lhs, rhs in
                let lhs = lhs.activationState
                let rhs = rhs.activationState
                switch lhs {
                case .foregroundActive:
                    return true
                case .foregroundInactive:
                    return rhs == .background || rhs == .unattached
                case .background:
                    return rhs == .unattached
                case .unattached:
                    return false
                @unknown default:
                    return false
                }
            }
            .compactMap { $0 as? UIWindowScene }

        guard connectedScenes.isEmpty == false else {
            fatalError("Connected scenes is empty")
        }
        let mainWindow = connectedScenes
            .flatMap { $0.windows }
            .first(where: \.isKeyWindow)

        guard let window = mainWindow else {
            fatalError("Couldn't get main window")
        }
        return window
    }
}

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;
}