Referencing AppDelegate instance variables

YourAppDelegate *appDelegate = (YourAppDelegate *)[[UIApplication sharedApplication] delegate];
//and then access the variable by appDelegate.variable

UIApplication* myApplication = [UIApplication sharedApplication];
UIWindow* mainWindow = [myApplication keyWindow];
UIViewController* rootViewController = [mainWindow rootViewController];

UIApplication* myApplication = [UIApplication sharedApplication];
UIWindow* mainWindow = [myApplication keyWindow];
UIViewController* rootViewController = [mainWindow rootViewController];