Question

我设法让JSON和plist编码和解码都能正常工作，但只能直接调用特定对象上的编码/解码函数。

例如：

struct Test: Codable {
    var someString: String?
}

let testItem = Test()
testItem.someString = "abc"

let result = try JSONEncoder().encode(testItem)

这很有效且没有问题。

但是，我试图获得一个只接受Codable协议一致性的函数并保存该对象。

func saveObject(_ object: Encodable, at location: String) {
    // Some code

    let data = try JSONEncoder().encode(object)

    // Some more code
}

这会导致以下错误：

无法调用＆＃39;编码＆＃39;使用类型＆＃39;（可编码）＆＃39;
的参数列表

看一下编码函数的定义，似乎它应该能够接受Encodable，除非Value是一些我不知道的奇怪类型。

open func encode<Value>(_ value: Value) throws -> Data where Value : Encodable

Answer 1

使用约束为Encodable

的通用类型

func saveObject<T : Encodable>(_ object: T, at location: String) {
    //Some code

    let data = try JSONEncoder().encode(object)

    //Some more code
}

Answer 2

我将使用一种与您可能需要的所有实例方法扩展Encodable协议的方法不同。进一步扩展您可以在方法中添加参数以传递自定义编码器，并为所有方法提供默认编码器：

extension Encodable {
    func data(using encoder: JSONEncoder = JSONEncoder()) throws -> Data {
        return try encoder.encode(self)
    }
    func string(using encoder: JSONEncoder = JSONEncoder()) throws -> String {
        return try String(data: encoder.encode(self), encoding: .utf8)!
    }
}

用法

let message = ["key":["a","b","c"]]

let jsonData = try! message.data() // 21 bytes [123, 34, 107, 101, 121, 34, 58, 91, 34, 97, 34, 44, 34, 98, 34, 44, 34, 99, 34, 93, 125]
let jsonString = try! message.string()  // "{"key":["a","b","c"]}"

使用默认编码器传递日期时的示例。请注意，使用的dateEncodingStrategy是默认值（代表timeIntervalSinceReferenceDate的Double）：

let message = ["createdAt": Date()]

let jsonData = try! message.data() // 33 bytes -> [123, 34, 99, 114, 101, 97, 116, 101, 97, 100, 65, 116, 34, 58, 53, 55, 49, 54, 49, 55, 56, 52, 49, 46, 52, 53, 48, 55, 52, 52, 48, 51, 125]
let jsonString = try! message.string()  // {"createdAt":571617841.45074403}"

现在，您可以将自定义编码器传递给您的方法，以将日期格式设置为易于理解的格式：

let message = ["createdAt": Date()]
let encoder = JSONEncoder()
encoder.dateEncodingStrategy = .iso8601
let jsonString = try! message.string(using: encoder)  // "{"createdAt":"2019-02-11T22:48:19Z"}"

现在使用自定义消息结构

struct Message: Codable {
    let id: Int
    let createdAt: Date
    let sender, title, body: String
}

extension Encodable {
    func sendDataToServer(using encoder: JSONEncoder = JSONEncoder()) throws {
        print(self, terminator: "\n\n")
        // Don't handle the error here. Propagate the error.
        let data = try self.data(using: encoder)
        print(String(data: data, encoding: .utf8)!)
        // following the code to upload the data to the server
        print("Message was successfully sent")
    }
}

let message = Message(id: 1, createdAt: Date(), sender: "user@company.com", title: "Lorem Ipsum", body: """
Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum.
""")

let iso8601 = JSONEncoder()
iso8601.dateEncodingStrategy = .iso8601
iso8601.outputFormatting = .prettyPrinted
do {
    try message.sendDataToServer(using: iso8601)
} catch {
    // handle all errors
    print(error)
}

这将打印

Message(id: 1, createdAt: 2019-02-11 23:57:31 +0000, sender: "user@company.com", title: "Lorem Ipsum", body: "Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry\'s standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum.")

{
  "body" : "Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum.",
  "id" : 1,
  "sender" : "user@company.com",
  "title" : "Lorem Ipsum",
  "createdAt" : "2019-02-11T23:57:31Z"
}
now just add the code to send the json data to the server

Answer 3

您需要使用泛型类型Encodable

的泛型函数

你不能

func toData(object: Encodable) throws -> Data {
  let encoder = JSONEncoder()
  return try encoder.encode(object) // Cannot invoke 'encode' with an argument list of type '(Encodable)'
}

你可以

func toData<T: Encodable>(object: T) throws -> Data {
  let encoder = JSONEncoder()
  return try encoder.encode(object)
}

使用JSONEncoder将Codable变量编码为

3 个答案: