JSONEncoder 在 Swift 中還是非常常用的，最近項(xiàng)目中有需要將APP數(shù)據(jù)轉(zhuǎn)換為JSON格式之后，再發(fā)送給服務(wù)器的需求，測(cè)試過程中，然后報(bào)了如下錯(cuò)誤：

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

移除業(yè)務(wù)邏輯的話，代碼大概長(zhǎng)這樣：

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: "韋弦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 {
    /// 將model轉(zhuǎn)換為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
    }
}

最初開發(fā)的時(shí)候一切都很正常，不管我傳遞的 model 是 1 還是 "1" 或者是 一個(gè) User， 代碼跑起來打印如下：

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

問題開始

當(dāng)開始兼容性測(cè)試時(shí)，iOS 13 系統(tǒng)以下，業(yè)務(wù)突然完全無法實(shí)現(xiàn)，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":"韋弦zhy","age":18}

后續(xù)測(cè)試發(fā)現(xiàn)：只有類似 User 這樣的結(jié)構(gòu)體或類才能正常編碼，而基礎(chǔ)類型 Int , Double, String 等，均無法編碼成功，可是查看encode 接口并沒有相關(guān)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 的源碼中也翻了翻，也是沒找到關(guān)于iOS 版本相關(guān)描述，方法實(shí)現(xiàn)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))
        }
    }
}

內(nèi)部實(shí)現(xiàn)會(huì)先調(diào)用 box_方法封裝，得到topLevel， 實(shí)際上 box_ 內(nèi)部主要又是調(diào)用 box 方法將基礎(chǔ)類型轉(zhuǎn)換為NSString 和 NSNumber(這里只關(guān)注基礎(chǔ)類型，其他的可以自行查看源碼) 所以才有了encode 中的判斷 NSNumber 和 NSString 然后拋出異常。。。

問題來了。。。iOS 13 之后怎么就可以了，沒找到代碼。。。有人找到望同步一下

最終，為了代碼能夠正常運(yùn)行，改了一下擴(kuò)展方法， 經(jīng)過測(cè)試，已經(jīng)可以表現(xiàn)正常，因?yàn)椴恢谰唧w生效的版本（萬一是12.x呢），所以判斷寫在了拋出異常的地方，否則可以寫在encode之前：

extension Encodable {
    /// 將model轉(zhuǎn)換為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
            /// 從源碼也找不到具體是從哪個(gè)版本才支持對(duì) Int Double String 等基本類型的的支持
            if (self is NSNumber) || (self is NSString) {
                return "\(self)"
            }
            
            print(error)
        }
        return modelJson
    }
}

問題是解決了，可是，原因還是沒找到。。。。