[英]Storing data from an asynchronous closure during URLSession.shared.dataTask
[英]URLSession.shared.dataTask correct way to receive data
美好的一天!
我在尝试寻找正确的顺序来检查从dataTask收到的(数据,响应,错误)并进行一些特殊的错误处理时有些困惑。
通常我们的URLSession看起来像这样:
class HTTPRequest {
static func request(urlStr: String, parameters: [String: String], completion: @escaping (_ data: Data?,_ response: URLResponse?, _ error: Error?) -> ()) {
var url = OpenExchange.base_URL + urlStr
url += getParameters(param: parameters)
let request = URLRequest(url: URL(string: url)!)
let task = URLSession.shared.dataTask(with: request) { (data, response, error) in
if error != nil {
print("URLSession Error: \(String(describing: error?.localizedDescription))")
completion(nil,nil,error)
} else {
completion(data,response,nil)
}
}
task.resume()
}
static func getParameters(param: [String: String]) -> String {
var data = [String]()
for (key,value) in param {
data.append(key + "=\(value)")
}
return data.map { String($0) }.joined(separator: "&")
}
}
我还有另一个内部带有HTTPRequest的函数,用于将所有内容包装到我正在使用的对象类型中:
static func networkOperation(urlStr: String, parameters: [String: String], completion: @escaping (ReturnedData) -> () ) {
var recieved = ReturnedData()
HTTPRequest.request(urlStr: urlStr, parameters: parameters) { (data, resp, err) in
if let data = data, let response = resp {
// TODO: try JSONDecoder() if data is API Error Struct; Moderate this section depending on results of decoding;
recieved.data = data
recieved.response = response
recieved.result = .Success
completion(recieved)
return
} else if err == nil {
recieved.result = .ErrorUnknown
completion(recieved)
return
}
recieved.error = err as NSError?
completion(recieved)
}
}
public struct ReturnedData {
public var data: Data?
public var response: URLResponse?
public var error: Error?
public var result: RequestResult = .ErrorHTTP
}
public enum RequestResult: String {
case Success
case ErrorAPI
case ErrorHTTP
case ErrorUnknown
}
使用上面的代码,我可以轻松地创建不同的networkOperation调用,以执行不同的API方法并处理返回的不同数据模型。 我正在尝试实现的是API错误检查。 由于我的API有一些错误描述,例如当您输入错误的APP_ID或当前的APP_ID无权获取信息等时。因此,如果发生任何此类情况,数据将如下所示:
{
"error": true,
"status": 401,
"message": "invalid_app_id",
"description": "Invalid App ID provided - please sign up at https://openexchangerates.org/signup, or contact support@openexchangerates.org."
}
我认为尝试用networkOperations“ // TODO”标记中的Error结构解码每个接收到的数据是不正确的,也许有一些好的方法可以实现这一点?
您应该让API错误返回错误对象。
例如,您可以这样做:
enum NetworkRequestError: Error {
case api(_ status: Int, _ code: ApiResultCode, _ description: String)
}
在哪里将响应编码到称为ApiResultCode
的enum
ApiResultCode
所示:
enum ApiResultCode {
case invalidAppId
case recordNotFound // just an example
...
case unknown(String)
}
extension ApiResultCode {
static func code(for string: String) -> ApiResultCode {
switch string {
case "invalid_app_id": return .invalidAppId
case "record_not_found": return .recordNotFound
...
default: return .unknown(string)
}
}
}
该枚举使您可以检查message
代码,而不会在字符串文字中乱扔代码。
而且,如果您解析API错误,则可以返回该错误。 例如
if responseObject.error {
let error = NetworkRequestError.api(responseObject.status, ApiResultCode.code(for: responseObject.message), responseObject.description)
... now pass this `error`, just like any other `Error` object
}
如果您愿意进行更广泛的重新设计,我个人建议
RequestResult
以提取那些单独的错误类型(调用方只想知道它是成功还是失败...如果失败,则应查看Error
对象以确定失败的原因); Result
枚举包含关联的值,即成功Data
和失败Error
; 和 ReturnedData
。 因此,首先,让我们扩展RequestResult
使其包含失败时的错误和成功时的有效负载:
public enum Result {
case success(Data)
case failure(Error)
}
实际上,现代惯例是使此通用,上面的代码使用以下内容变为Result<Data, Error>
:
public enum Result<T, U> {
case success(T)
case failure(U)
}
(Swift 5实际上包含此泛型。)
然后,我将展开ResultError
来处理API错误以及所有未知错误:
enum NetworkRequestError: Error {
case api(_ status: Int, _ code: ApiResultCode, _ description: String)
case unknown(Data?, URLResponse?)
}
因此,完成此操作后,您可以更改request
以传回Result<Data, Error>
:
static func request(urlString: String, parameters: [String: String], completion: @escaping (Result<Data, Error>) -> ()) {
let request = URLRequest(url: URL(string: urlString)!)
let task = URLSession.shared.dataTask(with: request) { data, response, error in
guard let responseData = data, error == nil else {
completion(.failure(error ?? NetworkRequestError.unknown(data, response)))
return
}
completion(.success(responseData))
}
task.resume()
}
然后,呼叫者将执行以下操作:
request(...) { result in
switch result {
case .failure(let error):
// do something with `error`
case .success(let data):
// do something with `data`
}
}
此Result
泛型的优点在于,它成为您可以在整个代码中使用的一致模式。 例如,假设您有某种方法Foo
request
返回的Data
中解析出Foo
对象:
func retrieveFoo(completion: @escaping (Result<Foo, Error>) -> Void) {
request(...) { result in
switch result {
case .failure(let error):
completion(.failure(error))
case .success(let data):
do {
let responseObject = try JSONDecoder().decode(ResponseObject.self, from: data)
if responseObject.error {
completion(.failure(NetworkRequestError.api(responseObject.status, ApiResultCode.code(for: responseObject.message), responseObject.description)))
return
}
let foo = responseObject.foo
completion(.success(foo))
} catch {
completion(.failure(error))
}
}
}
}
或者,如果您想测试特定的API错误,例如.recordNotFound
:
retrieveFoo { result in
switch result {
case .failure(NetworkRequestError.api(_, .recordNotFound, _)):
// handle specific “record not found” error here
case .failure(let error):
// handle all other errors here
case .success(let foo):
// do something with `foo`
}
}
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