Swift4之Codable协议

最近公司新项目采用了Swift开发,在处理JSON数据时,一开始是准备采用阿里开源的HandyJSON,后来了解了下,随着Swift4的到来,苹果为我们带来了Codable协议,用于数据的编解码,虽然现在的Codable并不完善,使用上也并没有那么的友好,但是毕竟是官方出品,后期肯定会越来越完善的,所以决定采用Codable而弃用HandyJSON。

基础

首先,Codable是Decodable协议和Encodable协议的组合类型,它们分别定义了init(from decoder: Decoder) throwsfunc encode(to encoder: Encoder) throws方法。如果我们只需单向转换,选择其一即可。

/// A type that can encode itself to an external representation.
public protocol Encodable {

    /// Encodes this value into the given encoder.
    ///
    /// If the value fails to encode anything, `encoder` will encode an empty
    /// keyed container in its place.
    ///
    /// This function throws an error if any values are invalid for the given
    /// encoder's format.
    ///
    /// - Parameter encoder: The encoder to write data to.
    public func encode(to encoder: Encoder) throws
}

/// A type that can decode itself from an external representation.
public protocol Decodable {

    /// Creates a new instance by decoding from the given decoder.
    ///
    /// This initializer throws an error if reading from the decoder fails, or
    /// if the data read is corrupted or otherwise invalid.
    ///
    /// - Parameter decoder: The decoder to read data from.
    public init(from decoder: Decoder) throws
}

/// A type that can convert itself into and out of an external representation.
public typealias Codable = Decodable & Encodable

源码

可以在Swift源码目录/stdlib/public/SDK/Foundation/JSONEncoder.swift看到苹果这该功能的实现。

JSON转Model

最理想的情况下,当然是服务器返回的JSON数据中key和我们Model中定义的key保持一致,那么我们只需要将Model声明为遵守Codable协议即可。

struct Person: Codable {
    var name: String
    var age: Int
}

接下来就可以直接对JSON数据进行解码操作了:

let data = "{\"name\":\"xiaopin\", \"age\": 18}".data(using: .utf8)!
let model = try? JSONDecoder().decode(Person.self, from: data)

看,是不是so easy!代码简直是6到没朋友啊。

自定义键值

  • CodingKey协议

当然了,大多数情况下我们都会遇到服务器返回的Key与Model属性名称不一致的情况,此时我们就需要自己定义映射关系了。Codable也为我们提供了解决方案,我们只需要定义一个名称为CodingKeys的嵌套枚举,关联值类型为String,并遵守CodingKey协议即可。

假设我们需要给Person增加一个学校信息,服务器返回的Key为school_name,而我们Model中则为schoolName,我们只需简单修改Person的定义即可:

struct Person: Codable {
    enum CodingKeys: String, CodingKey {
        case name, age
        case schoolName = "school_name"
    }

    var name: String
    var age: Int
    var schoolName: String
}

通过CodingKeys这个枚举,我们便可轻易完成JSON Key和Model之间的映射关系。

默认情况下,编译器会为我们自动生成CodingKeys,并提供init(from decoder: Decoder) throwsfunc encode(to encoder: Encoder) throws的默认实现。

  • keyDecodingStrategy属性

随着Swift4.1的更新,Apple给JSONDecoder/JSONEncoder扩展了一个keyDecodingStrategy属性,为我们带来更加方便的自定义键值映射功能。

keyDecodingStrategy是一个枚举值,有个case是convertFromSnakeCase,可以在对象/结构体的Camel Case格式的属性名和JSON的Snake Case格式的key之间转换(即在Decoder时会自动将JSON中的school_name映射成Person中的schoolName),这个是核心功能内置的,就不需要我们额外写代码处理了。上面加上的枚举CodingKeys也可以去掉了,只需要在JSONDecoder这个实例设置这个属性就行。

let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase

枚举

Codable提供对枚举的支持,只需定义好枚举的关联值类型,并遵守Codable协议即可。

现在我们给Person增加一个性别属性:

struct Person: Codable {
    enum CodingKeys: String, CodingKey {
        case name, age, gender
        case schoolName = "school_name"
    }

    enum Gender: String, Codable {
        case male, female
    }

    var name: String
    var age: Int
    var gender: Gender
    var schoolName: String
}

只需简单修改,Codable就会自动为我们将JSON中的gender字符串转换为对应的Gender枚举值。

日期处理

安利一个网站www.nsdateformatter.com,你可以查看各种日期格式的字符串表示。

JSON 没有数据类型表示日期格式,因此需要客户端和服务端对序列化进行约定。

JSONDecoder提供了一个枚举类型来处理日期格式,其定义如下:

/// The strategy to use for decoding `Date` values.
public enum DateDecodingStrategy {

    /// Defer to `Date` for decoding. This is the default strategy.
    case deferredToDate

    /// Decode the `Date` as a UNIX timestamp from a JSON number.
    case secondsSince1970

    /// Decode the `Date` as UNIX millisecond timestamp from a JSON number.
    case millisecondsSince1970

    /// Decode the `Date` as an ISO-8601-formatted string (in RFC 3339 format).
    case iso8601

    /// Decode the `Date` as a string parsed by the given formatter.
    case formatted(DateFormatter)

    /// Decode the `Date` as a custom value decoded by the given closure.
    case custom((Decoder) throws -> Date)
}

可以看到JSONDecoder内置了几种日期处理方式,前四种没什么可说的,直接赋值拿来用就行了,着重说一下.formatted(DateFormatter).custom((Decoder) throws -> Date)

  • .formatted(DateFormatter)

当服务器返回的是一个非标准格式的日期字符串时,我们可以提供一个DateFormatter来指定日期格式

let decoder = JSONDecoder()
let formatter = DateFormatter()
formatter.dateFormat = "yyyy-MM-dd HH:mm:ss"
formatter.timeZone = TimeZone(abbreviation: "UTC")
decoder.dateDecodingStrategy = .formatted(formatter)
  • .custom((Decoder) throws -> Date)

当DateFormatter也不能满足我们的需求时,我们可以自己指定如何解析。

let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .custom({ (decoder) -> Date in
    let container = try decoder.singleValueContainer()
    if let str = try? container.decode(String.self) {
        // 如有必要,这里还可以判断字符串是否为时间戳,最终转换成Date
        let formatter = DateFormatter()
        formatter.dateFormat = "yyyy-MM-dd HH:mm:ss"
        formatter.timeZone = TimeZone(abbreviation: "UTC")
        if let date = formatter.date(from: str) {
            return date
        }
    }
    if let double = try? container.decode(Double.self) {
        // 可根据服务器返回的时间戳是相对于1970.1.1 00:00:00还是2001.1.1 00:00:00进行相应的转换
        return Date(timeIntervalSinceReferenceDate: double)
    }
    throw DecodingError.dataCorruptedError(in: container, debugDescription: "Cannot decode date.")
})
...

我们可以进一步优化代码,通过给JSONDecoder.DateDecodingStrategy扩展一个customDateConvert()方法,之后在多个地方进行日期的转换也将变得很简单。

decoder.dateDecodingStrategy = .customDateConvert()
extension JSONDecoder.DateDecodingStrategy {

    /// 自定义解析日期
    /// 使用方式: JSONDecoder().dateDecodingStrategy = .customDateConvert()
    ///
    /// - Returns: .custom((Decoder) -> Date)
    static func customDateConvert() -> JSONDecoder.DateDecodingStrategy {
        return .custom({ (decoder) -> Date in
            let container = try decoder.singleValueContainer()
            if let str = try? container.decode(String.self) {
                let formatter = DateFormatter()
                formatter.timeZone = TimeZone(abbreviation: "UTC")
                for dateFormat in ["yyyy-MM-dd HH:mm:ss", "yyyy-MM-dd"] {
                    formatter.dateFormat = dateFormat
                    guard let date = formatter.date(from: str) else { continue }
                    return date
                }
            }
            if let double = try? container.decode(Double.self) {
                // 根据服务器返回的时间戳是相对于1970.1.1 00:00:00还是2001.1.1 00:00:00进行相应的转换
                return Date(timeIntervalSince1970: double)
            }
            throw DecodingError.dataCorruptedError(in: container, debugDescription: "Cannot decode date.")
        })
    }

}

自定义解析

理想情况下我们当然不需要自己实现init(from decoder: Decoder) throws方法,但是总有一些特殊情况,或者说是给代码增加容错机制,我们不得不实现该方法,自己实现解码操作。

编译器的默认实现中,都是使用public func decode<T>(_ type: T.Type, forKey key: KeyedDecodingContainer.Key) throws -> T where T : Decodable方法进行解码的。

默认实现有一定的局限性

  • 我们在上面中定义的gender属性,如果JSON数据中并不存在gender这个Key时则会抛出下面的异常信息:

    keyNotFound(ETNavBarTransparentDemo.Person.(CodingKeys in _34A23078E52B5E180CB78F393D171183).gender, Swift.DecodingError.Context(codingPath: [], debugDescription: "No value associated with key gender (\"gender\").", underlyingError: nil))
  • 如果Model中属性的类型是Int型,但是JSON中却是String类型的数字,那也会抛出异常:

    typeMismatch(Swift.Int, Swift.DecodingError.Context(codingPath: [ETNavBarTransparentDemo.Person.(CodingKeys in _34A23078E52B5E180CB78F393D171183).age], debugDescription: "Expected to decode Int but found a string/data instead.", underlyingError: nil))
  • Bool类型,只能处理true/false,不能处理字符串的”true”/“false”以及数字0/1

  • Int和Double的区别,当JSON中的value为88.0时,Int和Double均可以解码,但是如果value为88.01时,Int解码失败,抛出typeMismatch异常信息

对于以上情况,如果你们App觉得无所谓,那就没什么了;但是如果你想自己处理这些情况,保证能够正常的解码JSON数据,那你就只能自行实现init(from decoder: Decoder) throws了。

自定义解析:

struct DataModel: Codable {
    enum CodingKeys: String, CodingKey {
        case id, flag
    }

    var id: Int
    var flag: Bool

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        id = (try container.decodeIntIfPresent(.id)) ?? 0
        flag = (try container.decodeBoolIfPresent(.flag)) ?? true
    }
}

针对上面的情况,我给KeyedDecodingContainer扩展了几个方法,已可满足目前项目的需求,后期可视情况再新增:

// MARK: - 强制解码,如果key不存在则抛出异常`DecodingError.keyNotFound`
extension KeyedDecodingContainer {

    /// 解码CGFloat
    func decodeCGFloat(_ key: K) throws -> CGFloat {
        do {
            let d = try decodeDouble(key)
            return CGFloat(d)
        } catch {
            throw error
        }
    }

    /// 解码Double
    func decodeDouble(_ key: K) throws -> Double {
        do {
            return try decode(Double.self, forKey: key)
        } catch {
            if let i = try? decode(Int.self, forKey: key) {
                return Double(i)
            }
            if let s = try? decode(String.self, forKey: key), let d = Double(s) {
                return d
            }
            if let b = try? decode(Bool.self, forKey: key) {
                return b ? 1.0 : 0.0
            }
            throw error
        }
    }

    /// 解码Int
    func decodeInt(_ key: K) throws -> Int {
        do {
            return try decode(Int.self, forKey: key)
        } catch {
            if let d = try? decode(Double.self, forKey: key) {
                return Int(d)
            }
            if let s = try? decode(String.self, forKey: key), let i = Int(s) {
                return i
            }
            if let b = try? decode(Bool.self, forKey: key) {
                return b ? 1 : 0
            }
            throw error
        }
    }

    /// 解码String
    func decodeString(_ key: K) throws -> String {
        do {
            return try decode(String.self, forKey: key)
        } catch {
            if let i = try? decode(Int.self, forKey: key) {
                return String(i)
            }
            if let d = try? decode(Double.self, forKey: key) {
                return String(d)
            }
            if let b = try? decode(Bool.self, forKey: key) {
                return b ? "true" : "false"
            }
            throw error
        }
    }

    /// 解码Bool
    func decodeBool(_ key: K) throws -> Bool {
        do {
            return try decode(Bool.self, forKey: key)
        } catch {
            if let s = try? decode(String.self, forKey: key) {
                if s.isEmpty || s == "0" || s.lowercased() == "false" {
                    return false
                }
                return true
            }
            if let i = try? decode(Int.self, forKey: key) {
                return (i == 0) ? false : true
            }
            if let d = try? decode(Double.self, forKey: key) {
                return (d == 0.0) ? false : true
            }
            throw error
        }
    }

    /// 解码Date
    func decodeDate(_ key: K) throws -> Date {
        guard let date = try decodeDateIfPresent(key) else {
            let context = DecodingError.Context(codingPath: [key], debugDescription: "No value associated with key `\(key)`")
            throw DecodingError.keyNotFound(key, context)
        }
        return date
    }

}

// MARK: - 忽略`DecodingError.keyNotFound`异常信息,当key不存在时返回`nil`
extension KeyedDecodingContainer {

    /// 解码CGFloat数据
    func decodeCGFloatIfPresent(_ key: K) throws -> CGFloat? {
        if let d = try decodeDoubleIfPresent(key) {
            return CGFloat(d)
        }
        return nil
    }

    /// 解码Double数据
    func decodeDoubleIfPresent(_ key: K) throws -> Double? {
        do {
            return try decodeIfPresent(Double.self, forKey: key)
        } catch {
            // Int -> Double
            do {
                if let i = try decodeIfPresent(Int.self, forKey: key) {
                    return Double(i)
                }
            } catch {}
            // String -> Double
            do {
                if let s = try decodeIfPresent(String.self, forKey: key) {
                    return Double(s)
                }
            } catch {}
            // Bool -> Double
            do {
                if let b = try decodeIfPresent(Bool.self, forKey: key) {
                    return b ? 1.0 : 0.0
                }
            } catch {}

            // failure
            throw error
        }
    }

    /// 解码Int数据
    func decodeIntIfPresent(_ key: K) throws -> Int? {
        do {
            return try decodeIfPresent(Int.self, forKey: key)
        } catch {
            // Double -> Int
            do {
                if let d = try decodeIfPresent(Double.self, forKey: key) {
                    return Int(d)
                }
            } catch {}
            // String -> Int
            do {
                if let s = try decodeIfPresent(String.self, forKey: key) {
                    return Int(s)
                }
            } catch {}
            // Bool -> Int
            do {
                if let b = try decodeIfPresent(Bool.self, forKey: key) {
                    return b ? 1 : 0
                }
            } catch {}
            // decode failure.
            throw error
        }
    }

    /// 解码String数据
    func decodeStringIfPresent(_ key: K) throws -> String? {
        do {
            return try decodeIfPresent(String.self, forKey: key)
        } catch {
            // Int -> String
            do {
                if let i = try decodeIfPresent(Int.self, forKey: key) {
                    return String(i)
                }
            } catch {}
            // Double -> String
            do {
                if let d = try decodeIfPresent(Double.self, forKey: key) {
                    return String(d)
                }
            } catch {}
            // Bool -> String
            do {
                if let b = try decodeIfPresent(Bool.self, forKey: key) {
                    return b ? "true" : "false"
                }
            } catch {}
            // decode failure.
            throw error
        }
    }

    /// 解码Bool数据
    func decodeBoolIfPresent(_ key: K) throws -> Bool? {
        do {
            return try decodeIfPresent(Bool.self, forKey: key)
        } catch {
            // String -> Bool
            do {
                if let s = try decodeIfPresent(String.self, forKey: key) {
                    if s.isEmpty || s == "0" || s.lowercased() == "false" {
                        return false
                    }
                    return true
                }
            } catch {}
            // Int -> Bool
            do {
                if let i = try decodeIfPresent(Int.self, forKey: key) {
                    return (i == 0) ? false : true
                }
            } catch {}
            // Double -> Bool
            do {
                if let d = try decodeIfPresent(Double.self, forKey: key) {
                    return (d == 0.0) ? false : true
                }
            } catch {}
            // decode failure.
            throw error
        }
    }

    /// 解码CGSize数据
    func decodeCGSizeIfPresent(_ key: K) throws -> CGSize? {
        do {
            return try decodeIfPresent(CGSize.self, forKey: key)
        } catch {
            do {
                if let s = try decodeIfPresent(String.self, forKey: key) {
                    // 例: 宽x高 宽,高 宽X高
                    for seprator in ["x", ",", "X"] {
                        let array = s.components(separatedBy: seprator)
                        if array.count == 2 {
                            if let w = Double(array.first!), let h = Double(array.last!) {
                                return CGSize(width: w, height: h)
                            }
                        }
                    }
                }
            } catch {}
            throw error
        }
    }

    /// 解码Date
    func decodeDateIfPresent(_ key: K) throws -> Date? {
        do {
            return try decodeIfPresent(Date.self, forKey: key)
        } catch {
            if let s = try? decode(String.self, forKey: key) {
                let formatter = DateFormatter()
                formatter.timeZone = TimeZone(abbreviation: "UTC")
                for dateFormat in ["yyyy-MM-dd HH:mm:ss", "yyyy-MM-dd"] {
                    formatter.dateFormat = dateFormat
                    guard let date = formatter.date(from: s) else { continue }
                    return date
                }
            }
            throw error
        }
    }

}

有了这些扩展方法,我们就事半功倍了。

有趣的发现

前提条件:

  • 定义了JSONDecoder.DateDecodingStrategycustomDateConvert()扩展方法
  • 定义了KeyedDecodingContainer的扩展方法decodeDate(_:)decodeDateIfPresent(_:)

伪代码(详细代码请看上面):

extension JSONDecoder.DateDecodingStrategy {
    static func customDateConvert() -> JSONDecoder.DateDecodingStrategy {
        ...
    }
}

extension KeyedDecodingContainer {
    func decodeDate(_ key: K) throws -> Date {...}
    func decodeDateIfPresent(_ key: K) throws -> Date? {
        do {
            return try decodeIfPresent(Date.self, forKey: key)
        } catch {...}
    }
}

下面是测试代码:

struct DataModel: Codable {
    var time: Date

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        time = try container.decodeDate(.time)
    }
}

do {
    let data = "{\"time\":\"2018-04-11 17:34:23\"}".data(using: .utf8)!
    let decoder = JSONDecoder()
    decoder.dateDecodingStrategy = .customDateConvert()
    let model = try decoder.decode(DataModel.self, from: data)
} catch {
    print(error)
}

从测试代码中我们可以看到,不仅仅实现了init(from:)方法,并且还指定了dateDecodingStrategy属性。

在初始化方法中我们调用了上面的扩展方法decodeDate(_:)来获取日期数据,而其内部是通过调用decodeDateIfPresent(_:)来获取日期,这看似正常,并没什么问题。关键在于decodeDateIfPresent(_:)中首先尝试通过系统方法去获取Date,即try decodeIfPresent(Date.self, forKey: key),该行代码会导致JSONDecoder使用我们的customDateConvert()来解码日期。

也就是形成了这么一条调用链: decodeDate(_:) -> decodeDateIfPresent(_:) -> customDateConvert()

更多信息请参考: