在Swift中序列化和解码JSON

Question

收到错误时，我正在从数据转换JSON：

  

“ NSInvalidArgumentException”，原因：“ *** + [NSJSONSerialization dataWithJSONObject：options：error：]：JSON写入中的无效顶级类型”

数据按预期方式打印，但JSON没有打印。我认为代码在do语句后的第一行中失败。

do {
    var str: String = "http://localhost:5000/requirements/\(Array.joined(separator: "_"))"
    let url = URL(string: str)

    URLSession.shared.dataTask(with: url!) { (data, response, error) in
        guard let data = data else { return }

        print(data)
        do {
            let json = try? JSONSerialization.data(withJSONObject: data)
            print(json)
            if let json = json {
                print(json)
                let decodedList = try? JSONDecoder().decode(List.self, from: json)
                print(decodedList)
                if let items = decodedList {
                    listToDisplay = items.itemsAsString.components(separatedBy: ",")
                    print(listToDisplay)
                }
            }
        } catch let jsonErr {
            print("Error serializing json", jsonErr)
        }
    }.resume()

    return listToDisplay
}