JSONEncoder 基礎類型編碼失敗的解決方法

JSONEncoder 在 Swift 中還是非常常用的，最近項目中有需要將APP數據轉換爲JSON格式之後，再發送給服務器的需求，測試過程中，然後報瞭如下錯誤：

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

移除業務邏輯的話，代碼大概長這樣：

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轉換爲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
    }
}

最初開發的時候一切都很正常，不管我傳遞的 model 是 1 還是 "1" 或者是 一個 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

當開始兼容性測試時，iOS 13 系統以下，業務突然完全無法實現，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}

後續測試發現：只有類似 User 這樣的結構體或類才能正常編碼，而基礎類型 Int , Double, String 等，均無法編碼成功，可是查看encode 接口並沒有相關描述：

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，也是沒找到關於iOS 版本相關描述，方法實現如下:

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))
        }
    }
}

內部實現會先調用 box_方法封裝，得到topLevel， 實際上 box_ 內部主要又是調用 box 方法將基礎類型轉換爲NSString 和 NSNumber(這裏只關注基礎類型，其他的可以自行查看源碼) 所以纔有了encode 中的判斷 NSNumber 和 NSString 然後拋出異常。。。

問題來了。。。iOS 13 之後怎麼就可以了，沒找到代碼。。。有人找到望同步一下

最終，爲了代碼能夠正常運行，改了一下擴展方法， 經過測試，已經可以表現正常，因爲不知道具體生效的版本（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轉換爲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版本才支持對 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。。。。