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

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

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

- (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;
}

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;
}

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