Objective C解决方案:

UIWindow *foundWindow = nil;
NSSet *scenes=[[UIApplication sharedApplication] connectedScenes];
NSArray *windows;
for(id aScene in scenes){  // it's an NSSet so you can't use the first object
    windows=[aScene windows];
    if([aScene activationState]==UISceneActivationStateForegroundActive)
         break;
}
for (UIWindow  *window in windows) {
    if (window.isKeyWindow) {
        foundWindow = window;
        break;
    }
}
 // and to find the parent viewController:
UIViewController* parentController = foundWindow.rootViewController;
while( parentController.presentedViewController &&
      parentController != parentController.presentedViewController ){
    parentController = parentController.presentedViewController;
}

我为一个视图分配了一个新窗口，并设置它[newWindow makeKeyAndVisible]; 当完成使用时，设置它[newWindow resignKeyWindow]; 然后尝试通过[UIApplication sharedApplication]. keywindow直接显示原始的键-窗口。

在iOS 12上一切正常，但在iOS 13上原始的键窗口不能正常显示。它显示整个白色屏幕。

我通过以下方法解决了这个问题:

UIWindow *mainWindow = nil;
if ( @available(iOS 13.0, *) ) {
   mainWindow = [UIApplication sharedApplication].windows.firstObject;
   [mainWindow makeKeyWindow];
} else {
    mainWindow = [UIApplication sharedApplication].keyWindow;
}

一个UIApplication扩展:

extension UIApplication {

    /// The app's key window taking into consideration apps that support multiple scenes.
    var keyWindowInConnectedScenes: UIWindow? {
        return windows.first(where: { $0.isKeyWindow })
    }

}

用法:

let myKeyWindow: UIWindow? = UIApplication.shared.keyWindowInConnectedScenes

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 })

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;
}

支持iOS 13及以上版本。

为了继续使用与旧iOS版本相似的语法，UIApplication.shared.keyWindow创建以下扩展:

extension UIApplication {
    var mainKeyWindow: UIWindow? {
        get {
            if #available(iOS 13, *) {
                return connectedScenes
                    .flatMap { ($0 as? UIWindowScene)?.windows ?? [] }
                    .first { $0.isKeyWindow }
            } else {
                return keyWindow
            }
        }
    }
}

使用

if let keyWindow = UIApplication.shared.mainKeyWindow {
    // Do Stuff
}