UIApplication 的 keyWindow

@property(nullable, nonatomic,readonly) UIWindow *keyWindow 
API_DEPRECATED("Should not be used for applications that support multiple scenes as it returns a key window across all connected scenes", ios(2.0, 13.0));

AppDelegate.window
== [[UIApplication sharedApplication] keyWindow]
== [[UIApplication sharedApplication] windows][0]

需要在window上添加视图时，可以使用

UIWindow *window = [[UIApplication sharedApplication] windows][0];