- (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;
}

(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)
        }
    }
    
}

理想情况下，由于它已经被弃用，我建议你将窗口存储在SceneDelegate中。但是，如果您确实想要一个临时的解决方案，您可以创建一个过滤器并像这样检索keyWindow。

let window = UIApplication.shared.windows.filter {$0.isKeyWindow}.first

如果你的应用还没有更新到采用基于场景的应用生命周期，另一种获得活动窗口对象的简单方法是通过UIApplicationDelegate:

let window = UIApplication.shared.delegate?.window
let rootViewController = window??.rootViewController

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;
            }
        }
    }
}

通常使用

斯威夫特5

UIApplication.shared.windows.filter {$0.isKeyWindow}.first

另外，在UIViewController中:

self.view.window

视图。窗口是当前场景的窗口

WWDC 2019:

关键的窗口 手动跟踪窗口

在iPad上介绍多个Windows - WWDC 2019 -视频-苹果开发人员 支持iPad上的多个Windows |苹果开发者文档