CoreData DBManager用于检索和输入记录代码的错误

Question

我是IOS的新手，试图实现一个数据库管理器类来检索行和保存。

获取错误＆＃34;对NSEntityDescription，NSManagedObject，NSFetchRequest使用未解析的标识符。

我该如何解决这个问题？

class DatabaseManager: NSObject {

    var pizzas: [NSManagedOject] = []
    let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
    func addRow( name:String, price:Int16) {
        // set the core data to access the PizzaTable Entity
        guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else {
            return }
        let managedContext = appDelegate.persistentContainer.viewContext
        let entity = NSEntityDescription.entity(forEntityName: "PizzaTable", in: managedContext)
        let pizzas = NSManagedObject(entity: entity!, insertInto: managedContext)
        pizzas.setValue(name,  forKey: "name")
        pizzas.setValue(price, forKey: "price")
            do {
            try managedContext.save()
            pizzas.append(pizzas)
            //     showMessage("Information is added")
            }
        catch let error as NSError {
            //   showMessage("Error While Adding to Core Data")
        }

    }

    func retrieveRows() -> [String] {  // return array of Strings
        guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else {
            return [""]}
        let managedContext = appDelegate.persistentContainer.viewContext
        let fetchRequest = NSFetchRequest<NSManagedObject>(entityName: "PizzaTable")
        do {
            pizzas  = try managedContext.fetch(fetchRequest)
        }
        catch let error as NSError{
            // show something
        }
        var msg : [String] = []
        var count = 0
        for pizzas in pizzas {
            msg.append((pizzas.value(forKeyPath: "name") as? String)!)
            count += 1
        }
        //  msg = String(count)
        return msg
    }



}