上下文闭包类型“(Data?, URLResponse?, Error?) -> Void”需要 3 个参数，但在闭包主体中使用了 1 个

Question

我正在学习本教程，但出现此错误：

Contextual closure type '(Data?, URLResponse?, Error?) -> Void' expects 3 arguments, but 1 was used in closure body

行：urlSession.dataTask(with: url) { (result) in

private func fetchResources<T: Decodable>(url: URL, completion: @escaping (Result<T, APIServiceError>) -> Void) {
    guard var urlComponents = URLComponents(url: url, resolvingAgainstBaseURL: true) else {
        completion(.failure(.invalidEndpoint))
        return
    }
    let queryItems = [URLQueryItem(name: "api_key", value: apiKey)]
    urlComponents.queryItems = queryItems
    guard let url = urlComponents.url else {
        completion(.failure(.invalidEndpoint))
        return
    }

    urlSession.dataTask(with: url) { (result)  in
        switch result {
            case .success(let (response, data)):
                guard let statusCode = (response as? HTTPURLResponse)?.statusCode, 200..<299 ~= statusCode else {
                    completion(.failure(.invalidResponse))
                    return
                }
                do {
                    let values = try self.jsonDecoder.decode(T.self, from: data)
                    completion(.success(values))
                } catch {
                    completion(.failure(.decodeError))
                }
            case .failure(let error):
                completion(.failure(.apiError))
            }
     }.resume()
}

Answer 1

这个 dataTask 的闭包需要三个参数，一个 Data?、一个 URLResponse? 和一个 Error?。你的代码写得好像这个闭包只有一个参数，Result。看起来你认为它是 Result<(Data, URLResponse), Error>，但它不是。这个 Result 枚举是我们在自己的代码中使用的枚举，但未被 URLSession 使用。

因此，更改 dataTask 以使用具有三个参数的闭包：

private func fetchResources<T: Decodable>(url: URL, completion: @escaping (Result<T, APIServiceError>) -> Void) {
    guard var urlComponents = URLComponents(url: url, resolvingAgainstBaseURL: true) else {
        completion(.failure(.invalidEndpoint))
        return
    }
    urlComponents.queryItems = [URLQueryItem(name: "api_key", value: apiKey)]
    guard let url = urlComponents.url else {
        completion(.failure(.invalidEndpoint))
        return
    }

    urlSession.dataTask(with: url) { data, response, error in
        guard error == nil else {
            completion(.failure(.apiError))
            return
        }

        guard 
            let data = data,
            let response = response as? HTTPURLResponse,
            200...299 ~= response.statusCode 
        else {
            completion(.failure(.invalidResponse))
            return
        }

        do {
            let values = try self.jsonDecoder.decode(T.self, from: data)
            completion(.success(values))
        } catch {
            completion(.failure(.decodeError))
        }
    }.resume()
}

上下文闭包类型“(Data?, URLResponse?, Error?) -> Void”需要 3 个参数，但在闭包主体中使用了 1 个

Contextual closure type '(Data?, URLResponse?, Error?) -> Void' expects 3 arguments, but 1 was used in closure body

closures

ios

nsurlsession

swift

urlsession