在Swift 4中使用Decodable继承

Question

是否应该使用类继承来破坏类的可解码性.例如,以下代码

class Server : Codable {
    var id : Int?
}

class Development : Server {
    var name : String?
    var userId : Int?
}

var json = "{\"id\" : 1,\"name\" : \"Large Building Development\"}"
let jsonDecoder = JSONDecoder()
let item = try jsonDecoder.decode(Development.self, from:json.data(using: .utf8)!) as Development

print(item.id ?? "id is nil")
print(item.name ?? "name is nil") here

输出是:

1
name is nil

现在,如果我反转这个,名称解码但id没有.

class Server {
    var id : Int?
}

class Development : Server, Codable {
    var name : String?
    var userId : Int?
}

var json = "{\"id\" : 1,\"name\" : \"Large Building Development\"}"
let jsonDecoder = JSONDecoder()
let item = try jsonDecoder.decode(Development.self, from:json.data(using: .utf8)!) as Development

print(item.id ?? "id is nil")
print(item.name ?? "name is nil")

输出是:

id is nil
Large Building Development

并且你不能在两个班级中表达Codable.

Answer 1

我相信继承的情况你必须Coding自己实现.也就是说,你必须指定CodingKeys和执行init(from:),并encode(to:)在这两个超和子类.根据WWDC视频(大约49:28,如下图所示),您必须使用超级编码器/解码器调用super.

required init(from decoder: Decoder) throws {

  // Get our container for this subclass' coding keys
  let container = try decoder.container(keyedBy: CodingKeys.self)
  myVar = try container.decode(MyType.self, forKey: .myVar)
  // otherVar = ...

  // Get superDecoder for superclass and call super.init(from:) with it
  let superDecoder = try container.superDecoder()
  try super.init(from: superDecoder)

}

视频似乎停止短显示编码端的(但它container.superEncoder()的encode(to:)一侧),但它在几乎相同的方式在encode(to:)执行.我可以在这个简单的例子中确认这是有效的(参见下面的游乐场代码).

我仍在努力处理一些奇怪的行为,我正在使用一个更加复杂的模型来转换NSCoding,它有许多新嵌套的类型(包括struct和enum),它们表现出这种意想不到的nil行为并且"不应该".请注意,可能存在涉及嵌套类型的边缘情况.

编辑:嵌套类型似乎在我的测试操场中正常工作; 我现在怀疑自引用类(想想树节点的子节点)有一个错误,它有一个自身的集合,它也包含该类的各个子类的实例.对一个简单的自引用类的测试解码很好(也就是说,没有子类)所以我现在正在集中精力研究子类案例失败的原因.

更新于2017年6月25日:我最终向Apple提交了一个关于此问题的错误.rdar:// 32911973 - 不幸的是Superclass,包含Subclass: Superclass元素的数组的编码/解码周期将导致数组中的所有元素被解码为Superclass(子类' init(from:)永远不被调用,导致数据丢失或更糟).

//: Fully-Implemented Inheritance

class FullSuper: Codable {

    var id: UUID?

    init() {}

    private enum CodingKeys: String, CodingKey { case id }

    required init(from decoder: Decoder) throws {

        let container = try decoder.container(keyedBy: CodingKeys.self)
        id = try container.decode(UUID.self, forKey: .id)

    }

    func encode(to encoder: Encoder) throws {

        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(id, forKey: .id)

    }

}

class FullSub: FullSuper {

    var string: String?
    private enum CodingKeys: String, CodingKey { case string }

    override init() { super.init() }

    required init(from decoder: Decoder) throws {

        let container = try decoder.container(keyedBy: CodingKeys.self)
        let superdecoder = try container.superDecoder()
        try super.init(from: superdecoder)

        string = try container.decode(String.self, forKey: .string)

    }

    override func encode(to encoder: Encoder) throws {

        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(string, forKey: .string)

        let superencoder = container.superEncoder()
        try super.encode(to: superencoder)

    }
}

let fullSub = FullSub()
fullSub.id = UUID()
fullSub.string = "FullSub"

let fullEncoder = PropertyListEncoder()
let fullData = try fullEncoder.encode(fullSub)

let fullDecoder = PropertyListDecoder()
let fullSubDecoded: FullSub = try fullDecoder.decode(FullSub.self, from: fullData)

超级和子类属性都将恢复fullSubDecoded.

Answer 2

找到此链接 - 转到继承部分

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

对于解码,我这样做了:

 required init(from decoder: Decoder) throws {

    try super.init(from: decoder)

    let values = try decoder.container(keyedBy: CodingKeys.self)
    total = try values.decode(Int.self, forKey: .total)
  }

private enum CodingKeys: String, CodingKey
{
    case total

}

Answer 3

Swift 在 5.1 中引入了 Property Wrappers 我实现了一个名为SerializedSwift的库，它使用属性包装器的强大功能将 JSON 数据解码和编码为对象。

我的主要目标之一是，使继承的对象开箱即用地解码，而无需额外的init(from decoder: Decoder)覆盖。

import SerializedSwift

class User: Serializable {

    @Serialized
    var name: String
    
    @Serialized("globalId")
    var id: String?
    
    @Serialized(alternateKey: "mobileNumber")
    var phoneNumber: String?
    
    @Serialized(default: 0)
    var score: Int
    
    required init() {}
}

// Inherited object
class PowerUser: User {
    @Serialized
    var powerName: String?

    @Serialized(default: 0)
    var credit: Int
}

它还支持自定义编码键、备用键、默认值、自定义转换类以及未来将包含的更多功能。

可在GitHub (SerializedSwift) 上获得。

Answer 4

我能够通过使我的基类和子类符合Decodable而不是Codable. 如果我使用Codable它，它会以奇怪的方式崩溃，例如EXC_BAD_ACCESS在访问子类的字段时获取 a ，但调试器可以毫无问题地显示所有子类值。

此外，将 superDecoder 传递给基类super.init()不起作用。我刚刚将解码器从子类传递到基类。

Answer 5

使用下面的方法怎么样？

protocol Parent: Codable {
    var inheritedProp: Int? {get set}
}

struct Child: Parent {
    var inheritedProp: Int?
    var title: String?

    enum CodingKeys: String, CodingKey {
        case inheritedProp = "inherited_prop"
        case title = "short_title"
    }
}

有关组合的其他信息：http://mikebuss.com/2016/01/10/interfaces-vs-inheritance/

Answer 6

这是一个库TypePreservingCodingAdapter来做到这一点（可以与 Cocoapods 或 SwiftPackageManager 一起安装）。

下面的代码编译并与 Swift 一起工作得很好4.2。不幸的是，对于每个子类，您都需要自己实现属性的编码和解码。

import TypePreservingCodingAdapter
import Foundation

// redeclared your types with initializers
class Server: Codable {
    var id: Int?

    init(id: Int?) {
        self.id = id
    }
}

class Development: Server {
    var name: String?
    var userId: Int?

    private enum CodingKeys: String, CodingKey {
        case name
        case userId
    }

    init(id: Int?, name: String?, userId: Int?) {
        self.name = name
        self.userId = userId
        super.init(id: id)
    }

    required init(from decoder: Decoder) throws {
        try super.init(from: decoder)
        let container = try decoder.container(keyedBy: CodingKeys.self)

        name = try container.decodeIfPresent(String.self, forKey: .name)
        userId = try container.decodeIfPresent(Int.self, forKey: .userId)
    }

    override func encode(to encoder: Encoder) throws {
        try super.encode(to: encoder)
        var container = encoder.container(keyedBy: CodingKeys.self)

        try container.encode(name, forKey: .name)
        try container.encode(userId, forKey: .userId)
    }

}

// create and adapter
let adapter = TypePreservingCodingAdapter()
let encoder = JSONEncoder()
let decoder = JSONDecoder()

// inject it into encoder and decoder
encoder.userInfo[.typePreservingAdapter] = adapter
decoder.userInfo[.typePreservingAdapter] = adapter

// register your types with adapter
adapter.register(type: Server.self).register(type: Development.self)


let server = Server(id: 1)
let development = Development(id: 2, name: "dev", userId: 42)

let servers: [Server] = [server, development]

// wrap specific object with Wrap helper object
let data = try! encoder.encode(servers.map { Wrap(wrapped: $0) })

// decode object back and unwrap them force casting to a common ancestor type
let decodedServers = try! decoder.decode([Wrap].self, from: data).map { $0.wrapped as! Server }

// check that decoded object are of correct types
print(decodedServers.first is Server)     // prints true
print(decodedServers.last is Development) // prints true