NSManagedObject和Codable用于存储在服务器和本地存储中的类 [英] NSManagedObject and Codable for class stored in server and local storage

问题描述

我正在创建一个类，该类代表本地存储中或来自在线的数据。在运行时，我将使用get请求从服务器获取这些对象的数组，并将它们与CoreData中的现有对象进行比较，以确定将用于应用程序并存储的一组新对象。

I am creating a class that represents data that is either in local storage or comes in from online. On runtime I will use a get request to get an array of these objects from a server and compare them to the existing ones in CoreData in order to determine a new set of objects to be used for the app and to be stored.

这样，我创建了一个CoreData托管类，该类代表我的对象，因此它继承了 NSManagedObject 。这将使我能够处理本地存储，从而将阵列存储到存储中，并在应用程序完成后放回新存储。

As such I have created a CoreData managed class that represents my object as such it inherits NSManagedObject. This will allow me to deal with the local storage getting the array in storage and putting back a new one after the app has finished.

然后，我想做这个类可以这样编码，我可以实现所需的公共init（来自解码器：解码器）方法，该方法允许我在get请求中获取这样的对象数组。 JSONDecoder（）。decode（[DataClass] .self，from：data！）。

I would then like to make this class Codable that way I can implement the required public init(from decoder: Decoder) method that allows me to get my array of objects like this in the get request. JSONDecoder().decode([DataClass].self, from: data!).

有两种方法我正在尝试执行此操作，但它们都不起作用。

There are two ways I am trying to do this and neither of them are working.

问题：

我在类范围内有一个私有枚举CodingKeys：String，CodingKey {...} 并使用它进行编码非常有用，但是由于某种原因会触发错误提示该类没有实现我无意实现的编码。我不确定为什么这么重要。

I have a private enum CodingKeys: String, CodingKey {...} in class scope and using that to encode works great but for whatever reason that triggers an error that says that the class doesn't implement encode which I have no intention of implementing. I am not sure why that matters.

我可以将枚举值放在初始化程序的范围内，但如果这样做，则会弹出一个新问题。我必须调用 NSManagedObject 的指定初始化程序。这是一个问题，因为该调用需要一个我没有的 NSManagedObjectContext 。

I can put the enum inside the scope of the initializer but if I do that a new problem pops up. I must call a designated initializer of NSManagedObject. This is a problem because that call requires a NSManagedObjectContext which I dont have.

作为替代：

我已经创建了一种方法 func getDataClass（来自解码器：Decoder）-> DataClass？，但这也带来了两个问题。我仍然必须以某种方式初始化 DataClass 对象，而且我不知道如何将其带入 JSONDecoder（）。decode（[DataClass] .self，来自：数据！）行。

I have create a method func getDataClass(from decoder: Decoder) -> DataClass? however this also brings with it two problems. I still must initialize a DataClass object somehow and I have no clue how to bring that into the JSONDecoder().decode([DataClass].self, from: data!) line.

推荐答案

您必须创建 DataClass 符合 Codable 。

首先创建两个扩展名，以便能够通过 JSONDecoder 的 userInfo 词典中的 NSManagedObjectContext 。

First of all create two extensions to be able to pass the NSManagedObjectContext in the userInfo dictionary of the JSONDecoder.

extension CodingUserInfoKey {
    static let context = CodingUserInfoKey(rawValue: "context")!
}

extension JSONDecoder {
    convenience init(context: NSManagedObjectContext) {
        self.init()
        self.userInfo[.context] = context
    }
}

---

下面的示例仅在 DataClass 中使用一个属性 name 。

Codable 方法必须位于 CoreDataProperties 扩展名之外的类中。

---

The following example uses only one attribute name in DataClass.
The Codable methods must be in the class not in the CoreDataProperties extension.

class DataClass: NSManagedObject, Codable {

    private enum CodingKeys: String, CodingKey { case name }

    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(name, forKey: .name)
    }

    required convenience init(from decoder: Decoder) throws {
        guard let context = decoder.userInfo[.context] as? NSManagedObjectContext else { fatalError("Error: NSManagedObject not specified!") }
        let entity = NSEntityDescription.entity(forEntityName: "DataClass", in: context)!
        self.init(entity: entity, insertInto: context)
        let values = try decoder.container(keyedBy: CodingKeys.self)
        name = try values.decode(String.self, forKey: .name)
    }
}

要使用解码器，您必须使用初始化程序

To use the decoder you have to use the initializer

let context = // get the managed object context
let decoder = JSONDecoder(context: context)

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