为什么我的模板不适用于 JSONDecodable

Question

我正在使用新的 Swift 4 Codable 接口从 Web 服务中简单地获取 JSON 数据。我试图实现一个通用类型方法来处理解码（因此我不需要自定义方法）但我一直收到错误。这是代码

extension StarWarsAPI {
public func decodeJson<T: Codable>(fetchUrl: URL, modelType: T, completion: @escaping (_ modelObject: Codable?, _ error:StarWarsErrorType?) -> Void){

    //guard modelType is Codable else {return completion(nil,nil)}

    var fetchRequest = URLRequest(url: fetchUrl, cachePolicy: .useProtocolCachePolicy, timeoutInterval: 10.0)

    fetchRequest.httpMethod = "GET"
    fetchRequest.allHTTPHeaderFields = [
        "content-type": "application/json",
        "cache-control": "no-cache",
    ]

    let session = URLSession.shared
    let fetchDataTask = session.dataTask(with: fetchRequest) { (data, response, error) in

        guard error == nil else {
            return  completion(nil, StarWarsErrorType.urlResponseError(error: error))
        }
        guard let httpResponse = response as? HTTPURLResponse else {
            return  completion(nil, StarWarsErrorType.NilUrlResponseError())
        }
        guard let data = data else {
            return  completion(nil, StarWarsErrorType.noDataFound)
        }
        guard httpResponse.statusCode > 199 && httpResponse.statusCode < 300 else {
            return  completion(nil, StarWarsErrorType.httpErrorCode(code: httpResponse.statusCode))
        }
        var modelObject:Codable?
        do {

            let jsonDecoder = JSONDecoder()
            modelObject = try jsonDecoder.decode(modelType.self, from: data)
            return completion(modelObject, nil)
        }catch{ // do nothing }

            DispatchQueue.main.async {
                completion(nil, nil)
            }
        }
    }

    fetchDataTask.resume()
}

错误显示为 "Cannot invoke 'decode' with an argument list of type '(T, from: Data)'"。这是错误的屏幕截图。

我错过了什么？谢谢！

Answer 1

传递给decode(_:from:)的类型需要静态知道。 modelType.self 是 modelType 的动态实例，但它的类型不受静态约束；您需要使用 T.self，这是静态已知类型。

为什么我的模板不适用于 JSONDecodable

Why doesn't my template work with JSONDecodable

swift

templates

swift4

codable