JSONEncoder 在 Swift 中还是非常常用的，最近项目中有需要将APP数据转换为JSON格式之后，再发送给服务器的需求，测试过程中，然后报了如下错误：

invalidValue(Optional(1), 
Swift.EncodingError.Context(codingPath: [], debugDescription: "Top-level Optional<Int> encoded as number JSON fragment.", underlyingError: nil))

移除业务逻辑的话，代码大概长这样：

class ViewController: UIViewController {

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

    override func viewDidLoad() {
        super.viewDidLoad()
        self.view.backgroundColor = .red
        // Do any additional setup after loading the view.
        sendData(1, res: nil)
        sendData(User(name: "韦弦zhy", age: 18), res: nil)
    }

    func sendData<T: Codable>(_ model: T?, res: ((_ error: Error?) -> Void)?) {
        guard let json = model.json() else {
            print("json error ")
            return
        }
        
        print("encoded json: \(json)")
        // begin send data
        // res?(error)
    }
}

extension Encodable {
    /// 将model转换为json
    /// - Returns: json?
    func json() -> String? {
        var modelJson: String? = nil
        do {
            let data = try JSONEncoder().encode(self)
            modelJson = String(data: data, encoding: .utf8)
        } catch {
            print(error)
        }
        return modelJson
    }
}

最初开发的时候一切都很正常，不管我传递的 model 是 1 还是 "1" 或者是 一个 User， 代码跑起来打印如下：

encoded json: 1
encoded json: {"name":"韦弦zhy","age":18}

问题开始

当开始兼容性测试时，iOS 13 系统以下，业务突然完全无法实现，查看 log:

invalidValue(Optional(1), Swift.EncodingError.Context(codingPath: [], debugDescription: "Top-level Optional<Int> encoded as number JSON fragment.", underlyingError: nil))

json error 

encoded json: {"name":"韦弦zhy","age":18}

后续测试发现：只有类似 User 这样的结构体或类才能正常编码，而基础类型 Int , Double, String 等，均无法编码成功，可是查看encode 接口并没有相关描述：

open class JSONEncoder {
    ...

    /// Encodes the given top-level value and returns its JSON representation.
    ///
    /// - parameter value: The value to encode.
    /// - returns: A new `Data` value containing the encoded JSON data.
    /// - throws: `EncodingError.invalidValue` if a non-conforming floating-point value is encountered during encoding, and the encoding strategy is `.throw`.
    /// - throws: An error if any value throws an error during encoding.
    open func encode<T>(_ value: T) throws -> Data where T : Encodable
}

在 Swift JSONEncoder 的源码中也翻了翻，也是没找到关于iOS 版本相关描述，方法实现如下:

open class JSONEncoder {
    ...

    /// Encodes the given top-level value and returns its JSON representation.
    ///
    /// - parameter value: The value to encode.
    /// - returns: A new `Data` value containing the encoded JSON data.
    /// - throws: `EncodingError.invalidValue` if a non-conforming floating-point value is encountered during encoding, and the encoding strategy is `.throw`.
    /// - throws: An error if any value throws an error during encoding.
    open func encode<T : Encodable>(_ value: T) throws -> Data {
        let encoder = _JSONEncoder(options: self.options)

        guard let topLevel = try encoder.box_(value) else {
            throw EncodingError.invalidValue(value, EncodingError.Context(codingPath: [], debugDescription: "Top-level \(T.self) did not encode any values."))
        }

        if topLevel is NSNull {
            throw EncodingError.invalidValue(value, EncodingError.Context(codingPath: [], debugDescription: "Top-level \(T.self) encoded as null JSON fragment."))
        } else if topLevel is NSNumber {
            throw EncodingError.invalidValue(value, EncodingError.Context(codingPath: [], debugDescription: "Top-level \(T.self) encoded as number JSON fragment."))
        } else if topLevel is NSString {
            throw EncodingError.invalidValue(value, EncodingError.Context(codingPath: [], debugDescription: "Top-level \(T.self) encoded as string JSON fragment."))
        }

        let writingOptions = JSONSerialization.WritingOptions(rawValue: self.outputFormatting.rawValue)
        do {
           return try JSONSerialization.data(withJSONObject: topLevel, options: writingOptions)
        } catch {
            throw EncodingError.invalidValue(value, EncodingError.Context(codingPath: [], debugDescription: "Unable to encode the given top-level value to JSON.", underlyingError: error))
        }
    }
}

内部实现会先调用 box_方法封装，得到topLevel， 实际上 box_ 内部主要又是调用 box 方法将基础类型转换为NSString 和 NSNumber(这里只关注基础类型，其他的可以自行查看源码) 所以才有了encode 中的判断 NSNumber 和 NSString 然后抛出异常。。。

问题来了。。。iOS 13 之后怎么就可以了，没找到代码。。。有人找到望同步一下

最终，为了代码能够正常运行，改了一下扩展方法， 经过测试，已经可以表现正常，因为不知道具体生效的版本（万一是12.x呢），所以判断写在了抛出异常的地方，否则可以写在encode之前：

extension Encodable {
    /// 将model转换为json
    /// - Returns: json?
    func json() -> String? {
        var modelJson: String? = nil
        do {
            let data = try JSONEncoder().encode(self)
            modelJson = String(data: data, encoding: .utf8)
        } catch {
            /// see: https://github.com/apple/swift/blob/56a1663c9859f1283904cb0be4774a4e79d60a22/stdlib/public/SDK/Foundation/JSONEncoder.swift
            /// 从源码也找不到具体是从哪个版本才支持对 Int Double String 等基本类型的的支持
            if (self is NSNumber) || (self is NSString) {
                return "\(self)"
            }
            
            print(error)
        }
        return modelJson
    }
}

问题是解决了，可是，原因还是没找到。。。。