Swift 4 中默认情况下的可编码枚举

Question

我定义了一个enum，如下：

enum Type: String, Codable {
    case text = "text"
    case image = "image"
    case document = "document"
    case profile = "profile"
    case sign = "sign"
    case inputDate = "input_date"
    case inputText = "input_text"
    case inputNumber = "input_number"
    case inputOption = "input_option"

    case unknown
}

映射一个 JSON 字符串属性。 自动序列化和反序列化工作正常，但我发现如果遇到不同的字符串，反序列化失败。

是否可以定义一个映射任何其他可用案例的unknown 案例？

这可能非常有用，因为这些数据来自 RESTFul API，未来可能会发生变化。

Answer 1

您必须实现 init(from decoder: Decoder) throws 初始化程序并检查有效值：

struct SomeStruct: Codable {

    enum SomeType: String, Codable {
        case text
        case image
        case document
        case profile
        case sign
        case inputDate = "input_date"
        case inputText = "input_text"
        case inputNumber = "input_number"
        case inputOption = "input_option"

        case unknown
    }

    var someType: SomeType

    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        someType = (try? values.decode(SomeType.self, forKey: .someType)) ?? .unknown
    }

}

Answer 2

您可以扩展您的 Codable 类型并在失败的情况下分配默认值：

enum Type: String {
    case text,
         image,
         document,
         profile,
         sign,
         inputDate = "input_date",
         inputText = "input_text" ,
         inputNumber = "input_number",
         inputOption = "input_option",
         unknown
}
extension Type: Codable {
    public init(from decoder: Decoder) throws {
        self = try Type(rawValue: decoder.singleValueContainer().decode(RawValue.self)) ?? .unknown
    }
}

---

编辑/更新：

Xcode 11.2 • Swift 5.1 或更高版本

创建一个默认为 CaseIterable & Decodable 枚举的最后一种情况的协议：

protocol CaseIterableDefaultsLast: Decodable & CaseIterable & RawRepresentable
where RawValue: Decodable, AllCases: BidirectionalCollection { }

extension CaseIterableDefaultsLast {
    init(from decoder: Decoder) throws {
        self = try Self(rawValue: decoder.singleValueContainer().decode(RawValue.self)) ?? Self.allCases.last!
    }
}

---

游乐场测试：

enum Type: String, CaseIterableDefaultsLast {
    case text, image, document, profile, sign, inputDate = "input_date", inputText = "input_text" , inputNumber = "input_number", inputOption = "input_option", unknown
}

---

let types = try! JSONDecoder().decode([Type].self , from: Data(#"["text","image","sound"]"#.utf8))  // [text, image, unknown]

Answer 3

您可以删除 Type 的原始类型，并设置处理关联值的 unknown 大小写。但这是有代价的。您以某种方式需要您的案例的原始值。受this 和this SO 答案的启发，我想出了这个优雅的解决方案来解决您的问题。

为了能够存储原始值，我们将维护另一个枚举，但是是私有的：

enum Type {
    case text
    case image
    case document
    case profile
    case sign
    case inputDate
    case inputText
    case inputNumber
    case inputOption
    case unknown(String)

    // Make this private
    private enum RawValues: String, Codable {
        case text = "text"
        case image = "image"
        case document = "document"
        case profile = "profile"
        case sign = "sign"
        case inputDate = "input_date"
        case inputText = "input_text"
        case inputNumber = "input_number"
        case inputOption = "input_option"
        // No such case here for the unknowns
    }
}

将encoding & decoding 部分移至扩展：

可解码部分：

extension Type: Decodable {
    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        // As you already know your RawValues is String actually, you decode String here
        let stringForRawValues = try container.decode(String.self) 
        // This is the trick here...
        switch stringForRawValues { 
        // Now You can switch over this String with cases from RawValues since it is String
        case RawValues.text.rawValue:
            self = .text
        case RawValues.image.rawValue:
            self = .image
        case RawValues.document.rawValue:
            self = .document
        case RawValues.profile.rawValue:
            self = .profile
        case RawValues.sign.rawValue:
            self = .sign
        case RawValues.inputDate.rawValue:
            self = .inputDate
        case RawValues.inputText.rawValue:
            self = .inputText
        case RawValues.inputNumber.rawValue:
            self = .inputNumber
        case RawValues.inputOption.rawValue:
            self = .inputOption

        // Now handle all unknown types. You just pass the String to Type's unknown case. 
        // And this is true for every other unknowns that aren't defined in your RawValues
        default: 
            self = .unknown(stringForRawValues)
        }
    }
}

可编码部分：

extension Type: Encodable {
    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        switch self {
        case .text:
            try container.encode(RawValues.text)
        case .image:
            try container.encode(RawValues.image)
        case .document:
            try container.encode(RawValues.document)
        case .profile:
            try container.encode(RawValues.profile)
        case .sign:
            try container.encode(RawValues.sign)
        case .inputDate:
            try container.encode(RawValues.inputDate)
        case .inputText:
            try container.encode(RawValues.inputText)
        case .inputNumber:
            try container.encode(RawValues.inputNumber)
        case .inputOption:
            try container.encode(RawValues.inputOption)

        case .unknown(let string): 
            // You get the actual String here from the associated value and just encode it
            try container.encode(string)
        }
    }
}

示例：

我只是将它包装在一个容器结构中（因为我们将使用 JSONEncoder/JSONDecoder）作为：

struct Root: Codable {
    let type: Type
}

对于未知大小写以外的值：

let rootObject = Root(type: Type.document)
do {
    let encodedRoot = try JSONEncoder().encode(rootObject)
    do {
        let decodedRoot = try JSONDecoder().decode(Root.self, from: encodedRoot)
        print(decodedRoot.type) // document
    } catch {
        print(error)
    }
} catch {
    print(error)
}

对于大小写未知的值：

let rootObject = Root(type: Type.unknown("new type"))
do {
    let encodedRoot = try JSONEncoder().encode(rootObject)
    do {
        let decodedRoot = try JSONDecoder().decode(Root.self, from: encodedRoot)
        print(decodedRoot.type) // unknown("new type")
    } catch {
        print(error)
    }
} catch {
    print(error)
}

我将示例与本地对象放在一起。您可以尝试使用 REST API 响应。

Answer 4

这是基于nayem 的答案的替代方案，它通过使用内部RawValues 初始化的可选绑定提供了稍微更精简的语法：

enum MyEnum: Codable {

    case a, b, c
    case other(name: String)

    private enum RawValue: String, Codable {

        case a = "a"
        case b = "b"
        case c = "c"
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        let decodedString = try container.decode(String.self)

        if let value = RawValue(rawValue: decodedString) {
            switch value {
            case .a:
                self = .a
            case .b:
                self = .b
            case .c:
                self = .c
            }
        } else {
            self = .other(name: decodedString)
        }
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()

        switch self {
        case .a:
            try container.encode(RawValue.a)
        case .b:
            try container.encode(RawValue.b)
        case .c:
            try container.encode(RawValue.c)
        case .other(let name):
            try container.encode(name)
        }
    }
}

如果您确定所有现有枚举案例名称都与它们所代表的基础字符串值匹配，您可以将RawValue 简化为：

private enum RawValue: String, Codable {

    case a, b, c
}

...和encode(to:) 到：

func encode(to encoder: Encoder) throws {
    var container = encoder.singleValueContainer()

    if let rawValue = RawValue(rawValue: String(describing: self)) {
        try container.encode(rawValue)
    } else if case .other(let name) = self {
        try container.encode(name)
    }
}

这是一个使用此方法的实际示例，例如，您想对具有要建模为枚举的属性的 SomeValue 建模：

struct SomeValue: Codable {

    enum MyEnum: Codable {

        case a, b, c
        case other(name: String)

        private enum RawValue: String, Codable {

            case a = "a"
            case b = "b"
            case c = "letter_c"
        }

        init(from decoder: Decoder) throws {
            let container = try decoder.singleValueContainer()
            let decodedString = try container.decode(String.self)

            if let value = RawValue(rawValue: decodedString) {
                switch value {
                case .a:
                    self = .a
                case .b:
                    self = .b
                case .c:
                    self = .c
                }
            } else {
                self = .other(name: decodedString)
            }
        }

        func encode(to encoder: Encoder) throws {
            var container = encoder.singleValueContainer()

            switch self {
            case .a:
                try container.encode(RawValue.a)
            case .b:
                try container.encode(RawValue.b)
            case .c:
                try container.encode(RawValue.c)
            case .other(let name):
                try container.encode(name)
            }
        }
    }

}

let jsonData = """
[
    { "value": "a" },
    { "value": "letter_c" },
    { "value": "c" },
    { "value": "Other value" }
]
""".data(using: .utf8)!

let decoder = JSONDecoder()

if let values = try? decoder.decode([SomeValue].self, from: jsonData) {
    values.forEach { print($0.value) }

    let encoder = JSONEncoder()

    if let encodedJson = try? encoder.encode(values) {
        print(String(data: encodedJson, encoding: .utf8)!)
    }
}


/* Prints:
 a
 c
 other(name: "c")
 other(name: "Other value")
 [{"value":"a"},{"value":"letter_c"},{"value":"c"},{"value":"Other value"}]
 */

Answer 5

添加此扩展并设置 YourEnumName 。

extension <#YourEnumName#>: Codable {
    public init(from decoder: Decoder) throws {
        self = try <#YourEnumName#>(rawValue: decoder.singleValueContainer().decode(RawValue.self)) ?? .unknown
    }
}

Answer 6

@LeoDabus 感谢您的回答。我对它们进行了一些修改，以制作一个似乎对我有用的字符串枚举协议：

protocol CodableWithUnknown: Codable {}
extension CodableWithUnknown where Self: RawRepresentable, Self.RawValue == String {
    init(from decoder: Decoder) throws {
        do {
            try self = Self(rawValue: decoder.singleValueContainer().decode(RawValue.self))!
        } catch {
            if let unknown = Self(rawValue: "unknown") {
                self = unknown
            } else {
                throw error
            }
        }
    }
}

Answer 7

enum Type: String, Codable, Equatable {
    case image
    case document
    case unknown

    public init(from decoder: Decoder) throws {
        guard let rawValue = try? decoder.singleValueContainer().decode(String.self) else {
            self = .unknown
            return
        }
        self = Type(rawValue: rawValue) ?? .unknown
    }
}

Answer 8

让我们从一个测试用例开始。我们希望这会通过：

    func testCodableEnumWithUnknown() throws {
        enum Fruit: String, Decodable, CodableEnumWithUnknown {
            case banana
            case apple

            case unknown
        }
        struct Container: Decodable {
            let fruit: Fruit
        }
        let data = #"{"fruit": "orange"}"#.data(using: .utf8)!
        let val = try JSONDecoder().decode(Container.self, from: data)
        XCTAssert(val.fruit == .unknown)
    }

我们的协议CodableEnumWithUnknown 表示支持unknown 案例，如果数据中出现未知值，解码器应使用该案例。

然后解决办法：

public protocol CodableEnumWithUnknown: Codable, RawRepresentable {
    static var unknown: Self { get }
}

public extension CodableEnumWithUnknown where Self: RawRepresentable, Self.RawValue == String {

    init(from decoder: Decoder) throws {
        self = (try? Self(rawValue: decoder.singleValueContainer().decode(RawValue.self))) ?? Self.unknown
    }
}

诀窍是让您的枚举使用CodableEnumWithUnknown 协议实现并添加unknown 案例。

我赞成使用其他帖子中提到的.allCases.last! 实现的上述解决方案，因为我发现它们有点脆弱，因为编译器没有对它们进行类型检查。

Answer 9

您可以使用此扩展程序进行编码/解码 （此 sn-p 支持 Int 和 String RawValue 类型的枚举，但可以轻松扩展以适应其他类型）

extension NSCoder {
    
    func encodeEnum<T: RawRepresentable>(_ value: T?, forKey key: String) {
        guard let rawValue = value?.rawValue else {
            return
        }
        if let s = rawValue as? String {
            encode(s, forKey: key)
        } else if let i = rawValue as? Int {
            encode(i, forKey: key)
        } else {
            assert(false, "Unsupported type")
        }
    }
    
    func decodeEnum<T: RawRepresentable>(forKey key: String, defaultValue: T) -> T {
        if let s = decodeObject(forKey: key) as? String, s is T.RawValue {
            return T(rawValue: s as! T.RawValue) ?? defaultValue
        } else {
            let i = decodeInteger(forKey: key)
            if i is T.RawValue {
                return T(rawValue: i as! T.RawValue) ?? defaultValue
            }
        }
        return defaultValue
    }
    
}

比使用它

// encode
coder.encodeEnum(source, forKey: "source")
// decode
source = coder.decodeEnum(forKey: "source", defaultValue: Source.home)