在 Swift 4 中使用 Decodable 和继承

问题描述

如果使用类继承会破坏类的可解码性.比如下面的代码

类服务器:可编码{变量 id:整数?}类开发:服务器{变量名称:字符串?var userId : 整数?}var json = "{"id" : 1,"name" : "大型建筑开发"}"让 jsonDecoder = JSONDecoder()let item = try jsonDecoder.decode(Development.self, from:json.data(using: .utf8)!) 作为开发print(item.id ?? "id is nil")print(item.name ?? "name is nil") 这里

输出是:

<代码>1名字为零

现在如果我反转这个，name 会解码，但 id 不会.

类服务器{变量 id:整数?}类开发:服务器，可编码{变量名称:字符串?var userId : 整数?}var json = "{"id" : 1,"name" : "大型建筑开发"}"让 jsonDecoder = JSONDecoder()let item = try jsonDecoder.decode(Development.self, from:json.data(using: .utf8)!) 作为开发print(item.id ?? "id is nil")print(item.name ?? "name is nil")

输出是:

id 为 nil大型建筑开发

而且你不能在这两个类中表达 Codable.

相信在继承的情况下你必须自己实现Coding.也就是说，您必须在超类和子类中指定 CodingKeys 并实现 init(from:) 和 encode(to:).根据

必需的初始化(来自解码器:解码器)抛出 {//为这个子类的编码键获取我们的容器let container = try decoder.container(keyedBy: CodingKeys.self)myVar = 尝试 container.decode(MyType.self, forKey: .myVar)//其他变量 = ...//获取超类的 superDecoder 并用它调用 super.init(from:)让 superDecoder = 尝试 container.superDecoder()尝试 super.init(来自:superDecoder)}

视频似乎没有显示编码方面(但它是 encode(to:) 方面的 container.superEncoder() )但它在很多方面都有效encode(to:) 实现中的方式相同.我可以确认这在这个简单的情况下有效(参见下面的操场代码).

我自己仍然在为一些奇怪的行为而苦苦挣扎，我自己是从 NSCoding 转换而来的更复杂的模型，它有很多新嵌套的类型(包括 struct 和 enum) 表现出这种意外的 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.

嵌套类型似乎在我的测试操场上运行良好；我现在怀疑自引用类(想想树节点的子节点)有问题，它本身的集合还包含该类的各种子类的实例.一个简单的自引用类的测试可以很好地解码(即没有子类)，所以我现在将精力集中在子类案例失败的原因上.

2017 年 6 月 25 日更新:我最终向 Apple 提交了一个关于此问题的错误.rdar://32911973 - 不幸的是，包含 Subclass: Superclass 元素的 Superclass 数组的编码/解码循环将导致数组中的所有元素都被解码为 超类(子类的init(from:)永远不会被调用，导致数据丢失或更糟).

//:完全实现的继承类FullSuper:可编码{变量 ID:UUID?在里面() {}私有枚举 CodingKeys: String, CodingKey { case id }所需的初始化(来自解码器:解码器)抛出 {let container = try decoder.container(keyedBy: CodingKeys.self)id = 尝试 container.decode(UUID.self, forKey: .id)}func 编码(到编码器:编码器)抛出 {var container = encoder.container(keyedBy: CodingKeys.self)尝试 container.encode(id, forKey: .id)}}类FullSub:FullSuper {变量字符串:字符串?私有枚举 CodingKeys: String, CodingKey { case string }覆盖 init() { super.init() }所需的初始化(来自解码器:解码器)抛出 {let container = try decoder.container(keyedBy: CodingKeys.self)让 superdecoder = 尝试 container.superDecoder()尝试 super.init(来自:superdecoder)字符串 = 尝试 container.decode(String.self, forKey: .string)}覆盖 func 编码(到编码器:编码器)抛出 {var container = encoder.container(keyedBy: CodingKeys.self)尝试 container.encode(string, forKey: .string)让 superencoder = container.superEncoder()尝试 super.encode(to: superencoder)}}让 fullSub = FullSub()fullSub.id = UUID()fullSub.string = "FullSub"让 fullEncoder = PropertyListEncoder()让 fullData = 尝试 fullEncoder.encode(fullSub)让 fullDecoder = PropertyListDecoder()let fullSubDecoded: FullSub = try fullDecoder.decode(FullSub.self, from: fullData)

在fullSubDecoded中恢复超类和子类属性.

Should the use of class inheritance break the Decodability of class. For example, the following code

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

output is:

1
name is nil

Now if I reverse this, name decodes but id does not.

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")

output is:

id is nil
Large Building Development

And you can't express Codable in both classes.

I believe in the case of inheritance you must implement Coding yourself. That is, you must specify CodingKeys and implement init(from:) and encode(to:) in both superclass and subclass. Per the WWDC video (around 49:28, pictured below), you must call super with the super encoder/decoder.

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)

}

The video seems to stop short of showing the encoding side (but it's container.superEncoder() for the encode(to:) side) but it works in much the same way in your encode(to:) implementation. I can confirm this works in this simple case (see playground code below).

I'm still struggling with some odd behavior myself with a much more complex model I'm converting from NSCoding, which has lots of newly-nested types (including struct and enum) that's exhibiting this unexpected nil behavior and "shouldn't be". Just be aware there may be edge cases that involve nested types.

Edit: Nested types seem to work fine in my test playground; I now suspect something wrong with self-referencing classes (think children of tree nodes) with a collection of itself that also contains instances of that class' various subclasses. A test of a simple self-referencing class decodes fine (that is, no subclasses) so I'm now focusing my efforts on why the subclasses case fails.

Update June 25 '17: I ended up filing a bug with Apple about this. rdar://32911973 - Unfortunately an encode/decode cycle of an array of Superclass that contains Subclass: Superclass elements will result in all elements in the array being decoded as Superclass (the subclass' init(from:) is never called, resulting in data loss or worse).

//: 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)

Both the super- and subclass properties are restored in fullSubDecoded.

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