使协议可编码并将其存储在数组中

Question

我有一个带有 2 个符合它的结构的 Animal 协议和一个存储动物列表的 Farm 结构。然后，我让它们都符合 Codable 将其存储在一个文件中，但它抛出了错误cannot automatically synthesize 'Encodable' because '[Animal]' does not conform to 'Encodable'

我明白为什么会发生这种情况，但我找不到好的解决方案。我怎样才能让数组只接受 Codable 和 Animal，而不将 Animal 标记为 Codable 所以这个问题不会发生，比如var animals = [Codable & Animal]？（或任何其他变通方法）。谢谢

protocol Animal: Codable {
    var name: String { get set }
    var sound: String { get set }
}

struct Cow: Animal {
    var name = "Cow"
    var sound = "Moo!"
}

struct Duck: Animal {
    var name = "Duck"
    var sound = "Quack!"
}

struct Farm: Codable {

    var name = "Manor Farm"
    // this is where the error is shown
    var animals = [Animal]()

}

--edit-- 当我将它们更改为一个类时，它看起来像这样：

class Animal: Codable {
    var name = ""
    var sound = ""
}

class Duck: Animal {
    var beakLength: Int

    init(beakLength: Int) {
        self.beakLength = beakLength
        super.init()

        name = "Duck"
        sound = "Quack!"
    }

    required init(from decoder: Decoder) throws {
        // works, but now I am required to manually do this?
        fatalError("init(from:) has not been implemented")
    }
}

如果我没有其他属性，它会起作用，但是一旦我添加了一个属性，我就需要引入一个初始化程序，然后这需要我包含来自解码器初始化程序的 init，它删除了 Codable 提供的自动转换。因此，要么我为我扩展的每个类手动执行它，要么我可以强制转换变量（如var beakLength: Int!）以删除对初始化程序的要求。但是还有其他方法吗？这似乎是一个简单的问题，但解决它的方法使它非常混乱，我不喜欢。另外，当我使用这种方法从文件中保存/加载时，似乎没有保存数据

Answer 1

您可以通过两种方式执行此操作：

1 种解决方案 - 使用包装器：

protocol Animal {}

struct Cow: Animal, Codable {
}

struct Duck: Animal, Codable {
}

struct Farm: Codable {
    let animals: [Animal]

    private enum CodingKeys: String, CodingKey {
        case animals
    }

    func encode(to encoder: Encoder) throws {
        let wrappers = animals.map { AnimalWrapper($0) }
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(wrappers, forKey: .animals)
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let wrappers = try container.decode([AnimalWrapper].self, forKey: .animals)
        self.animals = wrappers.map { $0.animal }
    }
}

fileprivate struct AnimalWrapper: Codable {
    let animal: Animal

    private enum CodingKeys: String, CodingKey {
        case base, payload
    }

    private enum Base: Int, Codable {
        case cow
        case duck
    }

    init(_ animal: Animal) {
        self.animal = animal
    }

    public init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let base = try container.decode(Base.self, forKey: .base)

        switch base {
            case .cow:
                self.animal = try container.decode(Cow.self, forKey: .payload)
            case .duck:
                self.animal = try container.decode(Duck.self, forKey: .payload)
        }
    }

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

        switch animal {
            case let payload as Cow:
                try container.encode(Base.cow, forKey: .base)
                try container.encode(payload, forKey: .payload)
            case let payload as Duck:
                try container.encode(Base.duck, forKey: .base)
                try container.encode(payload, forKey: .payload)
            default:
                break
        }
    }
}

2 解决方案 - 使用枚举

struct Cow: Codable {
}

struct Duck: Codable {
}

enum Animal {
    case cow(Cow)
    case duck(Duck)
}

extension Animal: Codable {
    private enum CodingKeys: String, CodingKey {
        case base, payload
    }

    private enum Base: Int, Codable {
        case cow
        case duck
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let base = try container.decode(Base.self, forKey: .base)
        switch base {
            case .cow:
                self = .cow(try container.decode(Cow.self, forKey: .payload))
            case .duck:
                self = .duck(try container.decode(Duck.self, forKey: .payload))
        }
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        switch self {
            case .cow(let payload):
                try container.encode(Base.cow, forKey: .base)
                try container.encode(payload, forKey: .payload)
            case .duck(let payload):
                try container.encode(Base.duck, forKey: .base)
                try container.encode(payload, forKey: .payload)
        }
    }
}