Swift 4 ,must be used from main thread only warning

You’re making this call on a background queue. To fix, try something like…

public var context: NSManagedObjectContext

DispatchQueue.main.async {

    var appDelegate = UIApplication.shared.delegate as! AppDelegate
    context = appDelegate.persistentContainer.viewContext        
}

Although this is a pretty bad way to do this… you’re using your App Delegate as a global variable (which we all know is bad!)

You should look at passing the managed object context from view controller to view controller…

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: 

    (window?.rootViewController as? MyViewController)?.moc = persistentContainer.viewContext
}

and so on