enum PostType: Decodable {
init(from decoder: Decoder) throws {
// What do i put here?
}
case Image
enum CodingKeys: String, CodingKey {
case image
}
}
我要怎么做才能完成这个?
此外，让我们说我把情况改为这样:
case image(value: Int)
我如何使这符合解码?
这是我的完整代码(不工作)
let jsonData = """
{
"count": 4
}
""".data(using: .utf8)!
do {
let decoder = JSONDecoder()
let response = try decoder.decode(PostType.self, from: jsonData)
print(response)
} catch {
print(error)
}
}
}
enum PostType: Int, Codable {
case count = 4
}
此外，它将如何处理这样的枚举?
enum PostType: Decodable {
case count(number: Int)
}
为了扩展@Toka的回答，你也可以在enum中添加一个原始的可表示值，并使用默认的可选构造函数来构建没有开关的enum:
enum MediaType: String, Decodable {
case audio = "AUDIO"
case multipleChoice = "MULTIPLE_CHOICES"
case other
init(from decoder: Decoder) throws {
let label = try decoder.singleValueContainer().decode(String.self)
self = MediaType(rawValue: label) ?? .other
}
}
它可以使用一个允许重构构造函数的自定义协议进行扩展:
protocol EnumDecodable: RawRepresentable, Decodable {
static var defaultDecoderValue: Self { get }
}
extension EnumDecodable where RawValue: Decodable {
init(from decoder: Decoder) throws {
let value = try decoder.singleValueContainer().decode(RawValue.self)
self = Self(rawValue: value) ?? Self.defaultDecoderValue
}
}
enum MediaType: String, EnumDecodable {
static let defaultDecoderValue: MediaType = .other
case audio = "AUDIO"
case multipleChoices = "MULTIPLE_CHOICES"
case other
}
如果指定了无效的enum值，也可以轻松地扩展它以抛出错误，而不是默认值。这一变化的要点可以在这里找到:https://gist.github.com/stephanecopin/4283175fabf6f0cdaf87fef2a00c8128。
使用Swift 4.1/Xcode 9.3编译和测试代码。
@proxpero响应的一个更简洁的变体是将解码器表述为:
public init(from decoder: Decoder) throws {
let values = try decoder.container(keyedBy: CodingKeys.self)
guard let key = values.allKeys.first else { throw err("No valid keys in: \(values)") }
func dec<T: Decodable>() throws -> T { return try values.decode(T.self, forKey: key) }
switch key {
case .count: self = try .count(dec())
case .title: self = try .title(dec())
}
}
func encode(to encoder: Encoder) throws {
var container = encoder.container(keyedBy: CodingKeys.self)
switch self {
case .count(let x): try container.encode(x, forKey: .count)
case .title(let x): try container.encode(x, forKey: .title)
}
}
这允许编译器详尽地验证这些情况，并且在编码值与键的期望值不匹配的情况下也不会抑制错误消息。
如果遇到未知的enum值，Swift会抛出一个. datacorurt错误。如果你的数据来自服务器，它可以在任何时候给你一个未知的枚举值(错误服务器端，在API版本中添加的新类型，你希望你的应用程序的前版本能优雅地处理这种情况，等等)，你最好做好准备，并编码“防御性风格”来安全地解码你的枚举。
这里有一个关于如何使用或不使用相关值的示例
enum MediaType: Decodable {
case audio
case multipleChoice
case other
// case other(String) -> we could also parametrise the enum like that
init(from decoder: Decoder) throws {
let label = try decoder.singleValueContainer().decode(String.self)
switch label {
case "AUDIO": self = .audio
case "MULTIPLE_CHOICES": self = .multipleChoice
default: self = .other
// default: self = .other(label)
}
}
}
以及如何在封闭结构中使用它:
struct Question {
[...]
let type: MediaType
enum CodingKeys: String, CodingKey {
[...]
case type = "type"
}
extension Question: Decodable {
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
[...]
type = try container.decode(MediaType.self, forKey: .type)
}
}
为了扩展@Toka的回答，你也可以在enum中添加一个原始的可表示值，并使用默认的可选构造函数来构建没有开关的enum:
enum MediaType: String, Decodable {
case audio = "AUDIO"
case multipleChoice = "MULTIPLE_CHOICES"
case other
init(from decoder: Decoder) throws {
let label = try decoder.singleValueContainer().decode(String.self)
self = MediaType(rawValue: label) ?? .other
}
}
它可以使用一个允许重构构造函数的自定义协议进行扩展:
protocol EnumDecodable: RawRepresentable, Decodable {
static var defaultDecoderValue: Self { get }
}
extension EnumDecodable where RawValue: Decodable {
init(from decoder: Decoder) throws {
let value = try decoder.singleValueContainer().decode(RawValue.self)
self = Self(rawValue: value) ?? Self.defaultDecoderValue
}
}
enum MediaType: String, EnumDecodable {
static let defaultDecoderValue: MediaType = .other
case audio = "AUDIO"
case multipleChoices = "MULTIPLE_CHOICES"
case other
}
如果指定了无效的enum值，也可以轻松地扩展它以抛出错误，而不是默认值。这一变化的要点可以在这里找到:https://gist.github.com/stephanecopin/4283175fabf6f0cdaf87fef2a00c8128。
使用Swift 4.1/Xcode 9.3编译和测试代码。
实际上，上面的答案真的很好，但它们遗漏了许多人在持续开发客户端/服务器项目中需要的一些细节。我们开发一个应用程序，而我们的后端随着时间不断发展，这意味着一些枚举情况将改变这种发展。因此，我们需要一个枚举解码策略，能够解码包含未知情况的枚举数组。否则，解码包含数组的对象就会失败。
我所做的很简单:
enum Direction: String, Decodable {
case north, south, east, west
}
struct DirectionList {
let directions: [Direction]
}
extension DirectionList: Decodable {
public init(from decoder: Decoder) throws {
var container = try decoder.unkeyedContainer()
var directions: [Direction] = []
while !container.isAtEnd {
// Here we just decode the string from the JSON which always works as long as the array element is a string
let rawValue = try container.decode(String.self)
guard let direction = Direction(rawValue: rawValue) else {
// Unknown enum value found - ignore, print error to console or log error to analytics service so you'll always know that there are apps out which cannot decode enum cases!
continue
}
// Add all known enum cases to the list of directions
directions.append(direction)
}
self.directions = directions
}
}
奖励:隐藏实现>使其成为一个集合
隐藏实现细节总是一个好主意。为此，您只需要再编写一点代码。诀窍是使DirectionsList符合Collection，并使你的内部列表数组私有:
struct DirectionList {
typealias ArrayType = [Direction]
private let directions: ArrayType
}
extension DirectionList: Collection {
typealias Index = ArrayType.Index
typealias Element = ArrayType.Element
// The upper and lower bounds of the collection, used in iterations
var startIndex: Index { return directions.startIndex }
var endIndex: Index { return directions.endIndex }
// Required subscript, based on a dictionary index
subscript(index: Index) -> Element {
get { return directions[index] }
}
// Method that returns the next index when iterating
func index(after i: Index) -> Index {
return directions.index(after: i)
}
}
您可以在John Sundell的这篇博客文章中阅读更多关于遵循自定义集合的内容:https://medium.com/@johnsundell/ creating-customization - colls-in swift-a344e25d0bb0
如何使枚举与相关类型符合可编码
这个答案类似于@Howard Lovatt的答案，但避免了创建PostTypeCodableForm结构体，而是使用苹果提供的KeyedEncodingContainer类型作为编码器和解码器的属性，这减少了样板文件。
enum PostType: Codable {
case count(number: Int)
case title(String)
}
extension PostType {
private enum CodingKeys: String, CodingKey {
case count
case title
}
enum PostTypeCodingError: Error {
case decoding(String)
}
init(from decoder: Decoder) throws {
let values = try decoder.container(keyedBy: CodingKeys.self)
if let value = try? values.decode(Int.self, forKey: .count) {
self = .count(number: value)
return
}
if let value = try? values.decode(String.self, forKey: .title) {
self = .title(value)
return
}
throw PostTypeCodingError.decoding("Whoops! \(dump(values))")
}
func encode(to encoder: Encoder) throws {
var container = encoder.container(keyedBy: CodingKeys.self)
switch self {
case .count(let number):
try container.encode(number, forKey: .count)
case .title(let value):
try container.encode(value, forKey: .title)
}
}
}
这段代码适用于我的Xcode 9b3。
import Foundation // Needed for JSONEncoder/JSONDecoder
let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted
let decoder = JSONDecoder()
let count = PostType.count(number: 42)
let countData = try encoder.encode(count)
let countJSON = String.init(data: countData, encoding: .utf8)!
print(countJSON)
// {
// "count" : 42
// }
let decodedCount = try decoder.decode(PostType.self, from: countData)
let title = PostType.title("Hello, World!")
let titleData = try encoder.encode(title)
let titleJSON = String.init(data: titleData, encoding: .utf8)!
print(titleJSON)
// {
// "title": "Hello, World!"
// }
let decodedTitle = try decoder.decode(PostType.self, from: titleData)
