swift - 通用 FetchRequest

Question

我有一个扩展 NSManagedObject 的静态函数得到这样的对象......

NSManagedObject.get(type: MYUser.self, with: ("id", "SomeUserId"), in: context)

extension NSManagedObject {

    static func get<M: NSManagedObject>(type: M.Type, with kvp: (String, CVarArg), in context: NSManagedObjectContext) -> M? {

     guard let name = entity().name else { return nil }
     guard M.entity().propertiesByName[kvp.0] != nil else { Assert("\(name) does not have \(kvp.0)"); return nil }

     let fetchRequest = NSFetchRequest<M>(entityName: name)
     fetchRequest.predicate = NSPredicate(format: "\(kvp.0) == %@", kvp.1)

     do {
            let object = try context.fetch(fetchRequest)
            if let foundObject = object.first { return foundObject }
            return nil
        } catch {
            return nil
        }
    }
}    

我想要的语法是

MYUser.get(with: ("id", "SomeUserId"), in: context)

并从发出调用的类中推断出类型...但我不确定用什么来代替这里的泛型

NSFetchRequest<M>(entityName: name)

NSFetchRequest<???>(entityName: name)

提前致谢

Answer 1

如果你不介意写两次MYUser，你可以去掉type参数并指定类型，这样Swift就可以推断出M :

extension NSManagedObject {

    static func get<M: NSManagedObject>(with kvp: (String, CVarArg), in context: NSManagedObjectContext) -> M? {

        guard let name = entity().name else { return nil }
        guard M.entity().propertiesByName[kvp.0] != nil else {
            print("\(name) does not have \(kvp.0)")
            return nil
        }

        let fetchRequest = NSFetchRequest<M>(entityName: name)
        fetchRequest.predicate = NSPredicate(format: "\(kvp.0) == %@", kvp.1)

        do {
            let object = try context.fetch(fetchRequest)
            if let foundObject = object.first { return foundObject }
            return nil
        } catch {
            return nil
        }
    }
}

// usage:
let user: MYUser? = MYUser.get(with: ("id", "SomeUserId"), in: context)

如果你不想写两次MYUser，那我想不出任何解决办法。如果 NSManagedObject 是一个协议(protocol)，您可以在其中使用 Self。