如何制作JSONDecoder实例的副本？

Question

我希望从默认实例复制不同的JSONDecoder实例。JSONDecoder似乎是一个class，但是它没有.copy() API。

extension JSONDecoder {
    static let `default` = JSONDecoder()
}

let decoder2: JSONDecoder = .default
decoder2.keyDecodingStrategy = .convertFromSnakeCase

print(JSONDecoder.default.keyDecodingStrategy) // convertFromSnakeCase

在本例中，decoder2突变适用于JSONDecoder.default。在变异之前，我如何复制到decoder2呢？

Answer 1

没有副本的原因是JSONDecoder不符合NSCopying。如果您关注线程安全，并且能够在没有引用的情况下传递对象，则需要包装器结构而不是类，并且不公开声明为私有的解码器。这样，您就可以初始化自定义对象，但无法更改解码器属性：

struct JSONDecoderWrapper {
    private let decoder = JSONDecoder()
}

extension JSONDecoderWrapper {
    init(dataDecodingStrategy: JSONDecoder.DataDecodingStrategy = .base64,
         dateDecodingStrategy: JSONDecoder.DateDecodingStrategy = .deferredToDate,
         keyDecodingStrategy: JSONDecoder.KeyDecodingStrategy = .useDefaultKeys,
         nonConformingFloatDecodingStrategy: JSONDecoder.NonConformingFloatDecodingStrategy = .throw,
         userInfo: [CodingUserInfoKey : Any] = [:]) {
        decoder.dataDecodingStrategy = dataDecodingStrategy
        decoder.dateDecodingStrategy = dateDecodingStrategy
        decoder.keyDecodingStrategy = keyDecodingStrategy
        decoder.nonConformingFloatDecodingStrategy = nonConformingFloatDecodingStrategy
        decoder.userInfo = userInfo
    }
}

extension JSONDecoderWrapper {
    /// Summary:
    /// Decodes a top-level value of the given type from the given JSON representation.
    func decode<T>(_ type: T.Type, from data: Data) throws -> T where T : Decodable {
        try decoder.decode(type, from: data)
    }
}

extension JSONDecoderWrapper {
    var dataDecodingStrategy: JSONDecoder.DataDecodingStrategy { decoder.dataDecodingStrategy }
    var dateDecodingStrategy: JSONDecoder.DateDecodingStrategy { decoder.dateDecodingStrategy }
    var keyDecodingStrategy: JSONDecoder.KeyDecodingStrategy { decoder.keyDecodingStrategy }
    var nonConformingFloatDecodingStrategy: JSONDecoder.NonConformingFloatDecodingStrategy { decoder.nonConformingFloatDecodingStrategy }
    var userInfo: [CodingUserInfoKey : Any] { decoder.userInfo }
}

extension JSONDecoderWrapper {
    func copy(dataDecodingStrategy: JSONDecoder.DataDecodingStrategy? = nil,
              dateDecodingStrategy: JSONDecoder.DateDecodingStrategy? = nil,
              keyDecodingStrategy: JSONDecoder.KeyDecodingStrategy? = nil,
              nonConformingFloatDecodingStrategy: JSONDecoder.NonConformingFloatDecodingStrategy? = nil,
              userInfo: [CodingUserInfoKey : Any]? = nil) -> Self {
       .init(dataDecodingStrategy: dataDecodingStrategy ?? decoder.dataDecodingStrategy, dateDecodingStrategy: dateDecodingStrategy ?? decoder.dateDecodingStrategy, keyDecodingStrategy: keyDecodingStrategy ?? decoder.keyDecodingStrategy, nonConformingFloatDecodingStrategy:  nonConformingFloatDecodingStrategy ?? decoder.nonConformingFloatDecodingStrategy, userInfo: userInfo ?? decoder.userInfo)
    }
}

操场测试

extension JSONDecoderWrapper {
    static let `default` = JSONDecoderWrapper()
}

JSONDecoderWrapper.default

// let decoder2 = JSONDecoderWrapper.default
// decoder2.keyDecodingStrategy = .convertFromSnakeCase // error Cannot assign to property: 'keyDecodingStrategy' is a get-only property
// decoder2.decoder.keyDecodingStrategy = .convertFromSnakeCase // error 'decoder' is inaccessible due to 'private' protection level

// you can make a copy changing one or mpore specific properties
let decoder2 = JSONDecoderWrapper.default.copy(keyDecodingStrategy: .convertFromSnakeCase)

print(decoder2.dataDecodingStrategy) // "base64\n"
print(decoder2.dateDecodingStrategy) // deferredToDate\n"
print(decoder2.keyDecodingStrategy) // "convertFromSnakeCase\n"
print(decoder2.nonConformingFloatDecodingStrategy) // "throw\n"
print(decoder2.userInfo) // "[:]\n"

Answer 2

您可以随意编写自己的复制方法，没有那么多属性可复制。或者，如果您希望从同一模板开始，则可以使用工厂方法来创建具有默认配置的解码器。

extension JSONDecoder {
    //copy
    func copy() -> JSONDecoder {
        let decoder = JSONDecoder()
        decoder.dataDecodingStrategy = self.dataDecodingStrategy
        decoder.dateDecodingStrategy = self.dateDecodingStrategy
        //...
        return decoder
    }

    //factory
    static func createDefault() -> JSONDecoder {
        let decoder = JSONDecoder()
        decoder.dataDecodingStrategy = .base64
        decoder.dateDecodingStrategy = .iso8601
        //...

        return decoder
    }
}