如何以自定义方式编码已经符合编码(例如TimeInterval)的基础类型

Question

以自定义方式编码自定义类型的JSON在Swift 4中很容易，并且有很好的文档可用。但是我在以自定义方式编码Foundation类型时遇到了麻烦。我的自定义类型具有TimeInterval类型的属性，需要用JSON格式为Codable，显示出这样的分钟和几秒钟：

// my type:
struct MyType: Codable {
    let time: TimeInterval = 65.12
}
// required JSON representation:
"myType": {
    "time": 
    {
        "minutes": 1,
        "seconds": 5.12
    }
}

这可以正常工作，如果我为我的每种类型提供了encode(to:)和init(decoder:)的自定义实现。但是，我想不重复自己，而只是为TimeInterval提供这些方法的自定义版本。所以我尝试了这样的扩展：

extension TimeInterval {
    enum CodingKeys: String, CodingKey {
        case minutes
        case seconds
    }
    func encode(to encoder: Encoder) throws {
        let minutes = (NSInteger(self) / 60) % 60
        let seconds = self.truncatingRemainder(dividingBy: 60)
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(minutes, forKey: .minutes)
        try container.encode(seconds, forKey: .seconds)
    }
    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        guard let minutes = try? values.decode(Int.self, forKey: .minutes), let seconds = try? values.decode(Double.self, forKey: .seconds)  else {
            throw NSError()
        }
        self = TimeInterval(Double(minutes) * 60.0 + seconds)
    }
}

不幸的是，此代码从未被调用：编码TimeInterval值总是使用编码Double的默认实现，该实现符合Codable。

let myType = MyType(time: 65.12) 
let encoded = try! JSONEncoder().encode(myType)
let json = String(data: encoded, encoding: .utf8)!
print(json)
// prints:
// "myType":
// {
//    "time": 65.12
// }

我该如何解决？

编辑：就像我在第一句话中写的那样，我不是在寻找解决方案，如何编码自己的自定义类型。这已经在我的代码中正常工作。我正在尝试更改基础类型TimeInterval的编码行为。

Answer 1

您可以使时间间隔仅读取计算的属性，并添加一个定制初始化器，该属性需要timeInterval并初始化时间子结构属性：

struct Root: Codable  {
    let myType: MyType
}
struct MyType: Codable {
    let time: Time
    struct Time: Codable {
        let minutes: Int
        let seconds: Double
    }
    init(time: TimeInterval) {
        self.time = Time(minutes: (Int(time) / 60) % 60, seconds: time.truncatingRemainder(dividingBy: 60))
    }
    var timeInterval: TimeInterval {
        return TimeInterval(time.minutes * 60) + time.seconds
    }
}

---

let root = Root(myType: MyType(time: 65.12))
do {
    let encoded = try JSONEncoder().encode(root)
    print(String(data: encoded, encoding: .utf8)!) // "{"myType":{"time":{"minutes":1,"seconds":5.1200000000000045}}}n
} catch {
    print(error)
}

---

编辑/更新：

如果您需要将时间属性添加到更多的结构中，则可以将结构的时间移出自定义类型并创建定时协议：

struct Time: Codable {
    let minutes: Int
    let seconds: Double
}
protocol Timed {
    var time: Time { get }
}

扩展了定时协议，添加了TimeInterval读取的唯一计算属性

extension Timed {
    var timeInterval: TimeInterval {
        return TimeInterval(time.minutes * 60) + time.seconds
    }
}

添加自定义初始器延长时间：

extension Time {
    init(time: TimeInterval) {
        self.minutes = (Int(time) / 60) % 60
        self.seconds = time.truncatingRemainder(dividingBy: 60)
    }
}

然后，您只需要将定时协议添加到具有时间属性的结构：

struct Root: Codable {
    let myType: MyType
}
struct MyType: Codable, Timed  {
    let time: Time
}

---

测试：

let root = Root(myType: MyType(time: Time(time: 65.12)))
root.myType.timeInterval
do {
    let encoded = try JSONEncoder().encode(root)
    print(String(data: encoded, encoding: .utf8)!) // "{"myType":{"time":{"minutes":1,"seconds":5.1200000000000045}}}n
} catch {
    print(error)
}

Answer 2

我修改了您的代码（例如），这有效。

struct MyType {
    let time: TimeInterval = 65.12
    enum CodingKeys: String, CodingKey {
        case minutes
        case seconds
    }
}
extension MyType: Encodable {
    func encode(to encoder: Encoder) throws {
        let minutes = 20
        let seconds = 30
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(minutes, forKey: .minutes)
        try container.encode(seconds, forKey: .seconds)
    }
}
let myType = MyType()
let encoded = try! JSONEncoder().encode(myType)
let json = String(data: encoded, encoding: .utf8)!
print(json)

---

enum MyCodingKeys: String, CodingKey {
    case minutes
    case seconds
}
protocol TimeIntervalEncodable: Encodable {
    var time: TimeInterval {get set}
}
extension TimeIntervalEncodable {
    func encode(to encoder: Encoder) throws {
        let minutes = (Int(time) / 60) % 60
        let seconds = time.truncatingRemainder(dividingBy: 60)
        var container = encoder.container(keyedBy: MyCodingKeys.self)
        try container.encode(minutes, forKey: .minutes)
        try container.encode(seconds, forKey: .seconds)
    }
}
struct MyType: TimeIntervalEncodable {
    var time: TimeInterval = 65.12
}
let myType = MyType()
let encoded = try! JSONEncoder().encode(myType)
let json = String(data: encoded, encoding: .utf8)!
print(json)