I'm facing compile error.
I trying to decode API response into custom class(struct actually).
Our API returns several type of error (sometime it contain string but sometime it returns code (tuple [String: Int]))
So I would like to decode into another class if first decoding failed.
If I try decoding into just one class, it is okay but if I try to decode into another class in catch scope, it produces compile error
Invalid conversion from throwing function of type '(Data?, URLResponse?, Error?) throws -> Void' to non-throwing function type '(Data?, URLResponse?, Error?) -> Void'
Do you know how to solve this?
I'm useing Swift5, Xcode 12.4
struct ErrorInfo: Codable {
public let message: String?
public let details: String?
public let debugInformation: String?
}
struct ErrorInfoWithErrorCode: Codable {
public let message: [String: Int] // this is the difference.
public let details: String?
public let debugInformation: String?
}
let request = URLRequest(url: URL(string: "https://www.google.com/")!)
let task = URLSession.shared.dataTask(with: request) { data, response, error in
if let httpResponse = response as? HTTPURLResponse, !(200..<300).contains(httpResponse.statusCode) {
do {
let errorMessage = try decoder.decode(ErrorInfo.self, from: data)
print(errorMessage)
} catch {
let errorMessage = try decoder.decode(ErrorInfoWithErrorCode.self, from: data)
print(errorMessage)
} catch {
fatalError()
}
}
}
task.resume()
Error message in playground.
error: MyPlayground.playground:28:54: error: invalid conversion from throwing function of type '(Data?, URLResponse?, Error?) throws -> Void' to non-throwing function type '(Data?, URLResponse?, Error?) -> Void'
let task = URLSession.shared.dataTask(with: request) { data, response, error in
The other nested try needs a do also , You can try
do {
let errorMessage = try decoder.decode(ErrorInfo.self, from: data)
print(errorMessage)
} catch {
do {
let errorMessage = try decoder.decode(ErrorInfoWithErrorCode.self, from: data)
print(errorMessage)
} catch {
fatalError()
}
}
Related
I'm writing a transform function that would take network request results and try to parse them automatically using a dict to Model transformer(not Decodable due to several backend reasons).
So the chain should look like this:
func getModel -> Single<Model> {
return networkRequest(requestParameters).parse(modelTranslator)
}
The translator is a generic protocol:
public protocol Translator {
associatedtype Model
func translateFrom(dictionary json: [String: Any]) throws -> Model
}
Single is a wrapper around Deferred and Future:
public typealias Single<T> = Deferred<Future<T, Error>>
The problematic parse extension method here is:
public extension Publisher {
func parse<T: Translator, M>(translator: T) -> Single<M> where T.Model == M {
return self.flatMap { (data: Data) -> Single<M> in
return Deferred {
return Future<M, any Error> { promise in
guard
let json = try? JSONSerialization.jsonObject(with: data, options: []),
let dict = json as? [String : Any]
else {
let error: any Error = TranslatorError.invalidJSONObject
return promise(Result.failure(error))
}
do {
let translatedModel: M = translator.translateFrom(dictionary: dict)
return promise(Result.success(translatedModel))
} catch let error {
return promise(Result.failure(error))
}
}
}
}
}
}
It won't compile. It shows 2 errors on the .flatmap row:
No 'flatMap' candidates produce the expected contextual result type 'Single' (aka 'Deferred<Future<M, any Error>>')
No exact matches in call to instance method 'flatMap'
I believe that it has something to do with a type mismatch?
Could you please help me see the problem?
Thank you in advance!
You are trying too hard. A simple tryMap is all you need to parse your [String: Any] into the appropriate model type. Here is a complete example:
func getFoo(_ requestParameters: RequestParameters) -> AnyPublisher<Foo, Error> {
getModel(requestParameters, modelTranslator: FooTranslator())
}
func getModel<T>(_ requestParameters: RequestParameters, modelTranslator: T) -> AnyPublisher<T.Model, Error> where T: Translator {
networkRequest(requestParameters)
.tryMap { try modelTranslator.translateFrom(dictionary: $0) }
.eraseToAnyPublisher()
}
The above assumes the following declarations:
func networkRequest(_ params: RequestParameters) -> Single<[String: Any]> ...
struct FooTranslator: Translator {
func translateFrom(dictionary json: [String : Any]) throws -> Foo ...
}
I'm trying to migrate my project from Alamofire 4.9 to 5.3 and I'm having a hard time with error handling. I would like to use Decodable as much as possible, but my API endpoints return one JSON structure when everything goes well, and a different JSON structure when there is an error, the same for all errors across all endpoints. The corresponding Codable in my code is ApiError.
I would like to create a custom response serializer that can give me a Result<T, ApiError> instead of the default Result<T, AFError>. I found this article that seems to explain the general process but the code in there does not compile.
How can I create such a custom ResponseSerializer?
I ended up making it work with the following ResponseSerializer:
struct APIError: Error, Decodable {
let message: String
let code: String
let args: [String]
}
final class TwoDecodableResponseSerializer<T: Decodable>: ResponseSerializer {
lazy var decoder: JSONDecoder = {
let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .iso8601
return decoder
}()
private lazy var successSerializer = DecodableResponseSerializer<T>(decoder: decoder)
private lazy var errorSerializer = DecodableResponseSerializer<APIError>(decoder: decoder)
public func serialize(request: URLRequest?, response: HTTPURLResponse?, data: Data?, error: Error?) throws -> Result<T, APIError> {
guard error == nil else { return .failure(APIError(message: "Unknown error", code: "unknown", args: [])) }
guard let response = response else { return .failure(APIError(message: "Empty response", code: "empty_response", args: [])) }
do {
if response.statusCode < 200 || response.statusCode >= 300 {
let result = try errorSerializer.serialize(request: request, response: response, data: data, error: nil)
return .failure(result)
} else {
let result = try successSerializer.serialize(request: request, response: response, data: data, error: nil)
return .success(result)
}
} catch(let err) {
return .failure(APIError(message: "Could not serialize body", code: "unserializable_body", args: [String(data: data!, encoding: .utf8)!, err.localizedDescription]))
}
}
}
extension DataRequest {
#discardableResult func responseTwoDecodable<T: Decodable>(queue: DispatchQueue = DispatchQueue.global(qos: .userInitiated), of t: T.Type, completionHandler: #escaping (Result<T, APIError>) -> Void) -> Self {
return response(queue: .main, responseSerializer: TwoDecodableResponseSerializer<T>()) { response in
switch response.result {
case .success(let result):
completionHandler(result)
case .failure(let error):
completionHandler(.failure(APIError(message: "Other error", code: "other", args: [error.localizedDescription])))
}
}
}
}
And with that, I can call my API like so:
AF.request(request).validate().responseTwoDecodable(of: [Item].self) { response in
switch response {
case .success(let items):
completion(.success(items))
case .failure(let error): //error is an APIError
log.error("Error while loading items: \(String(describing: error))")
completion(.failure(.couldNotLoad(underlyingError: error)))
}
}
I simply consider that any status code outside of the 200-299 range corresponds to an error.
ResponseSerializers have a single requirement. Largely you can just copy the existing serializers. For example, if you wanted to parse a CSV (with no response checking):
struct CommaDelimitedSerializer: ResponseSerializer {
func serialize(request: URLRequest?, response: HTTPURLResponse?, data: Data?, error: Error?) throws -> [String] {
// Call the existing StringResponseSerializer to get many behaviors automatically.
let string = try StringResponseSerializer().serialize(request: request,
response: response,
data: data,
error: error)
return Array(string.split(separator: ","))
}
}
You can read more in Alamofire's documentation.
In swift how do I throw an error within a completion handler like this:
let task = URLSession.shared.dataTask(with: request as URLRequest, completionHandler: {
(data, response, error) in
do {
//something
completion(result)
} catch let jsonError {
throw CustomError.myerror //THIS DOESN'T WORK
}
})
task.resume()
as the error is
Invalid conversion from throwing function of type '(_, _, _) throws ->
()' to non-throwing function type '(Data?, URLResponse?, Error?) ->
Void'
Short story: You can't throw in a dataTask completion closure
You could return two values in the completion handler
...completion: #escaping (ResultType?, Error?)->Void
and return
completion(result, nil)
completion(nil, CustomError.myerror)
or more convenient use an enum with associated type
enum Result {
case success(ResultType), failure(Error)
}
...completion: #escaping (Result)->Void
and return
completion(.success(result))
completion(.failure(CustomError.myerror))
You can process the result
foo() { result in
switch result {
case .success(let resultType): // do something with the result
case .failure(let error): // Handle the error
}
}
Update:
In Swift 5 using the new built-in Result type it's even more comfortable because Result can capture the result of the throwing expression
...completion: #escaping (Result<MyType,Error>)->Void
let task = URLSession.shared.dataTask(with: request as URLRequest, completionHandler: {
(data, response, error) in
completion(Result { try something()})
})
task.resume()
Update 2:
With async/await completion handlers are gone
do {
let (data, response) = try await URLSession.shared.data(for: request)
} catch {
throw CustomError.myerror
}
I'm attempting to define a custom model serialization for Alamofire 4.0. So far I'm following the model presented used by responseJson and friends. Specifically, what I have so far is:
extension Alamofire.Request {
public static func serializeResponseModel<T:ModelObject>(response:HTTPURLResponse?, data:Data?, error:Error?) -> Alamofire.Result<T> {
switch serializeResponseJSON(options: [], response: response, data: data, error: error) {
case .success(let jsonObject):
do {
return .success(try T(json:jsonObject as! JSONObject))
}
catch {
return .failure(error)
}
case .failure(let error):
return .failure(error)
}
}
}
extension Alamofire.DataRequest {
public static func serializeResponseModel<T:ModelObject>() -> DataResponseSerializer<T> {
return DataResponseSerializer { _, response, data, error in
return Request.serializeResponseConcierge(response: response, data: data, error: error)
}
}
#discardableResult
public func responseModel<T:ModelObject>(queue: DispatchQueue? = nil, completionHandler: #escaping (DataResponse<T>) -> Void) -> Self
{
return response(
queue: queue,
responseSerializer: DataRequest.serializeResponseModel(),
completionHandler: completionHandler
)
}
}
Unfortunately, the framework is somewhat poorly implemented and the line return response( is finding the response property (defined in Request) and not the appropriate response method (defined in DataRequest), which leads to the compile error:
Cannot call value of non-function type 'HTTPURLResponse?'
What am I missing here that allows this to work in the responseJson case, but not in my case?
Apparently the problem arose from over-generalization, and the compiler not being able to generate an appropriate type for DataRequest.serializeResponseModel() When I changed responseModel to the following and specified the appropriate type, things work as expected:
#discardableResult
public func responseModel<T:ModelObject>(queue: DispatchQueue? = nil, completionHandler: #escaping (DataResponse<T>) -> Void) -> Self
{
return response(
queue: queue,
responseSerializer: DataRequest.modelResponseSerializer() as DataResponseSerializer<T>,
completionHandler: completionHandler
)
}
Hi I am trying to understand the following code from Alamofire. How can you initialise a struct with "{}" I know that you can call a closure with Trailing Closures. I know I am totally missing something, but what?
extension Request {
public func responseObject<T: ResponseObjectSerializable>(completionHandler: Response<T, NSError> -> Void) -> Self {
let responseSerializer = ResponseSerializer<T, NSError> { // What is this?
request, response, data, error in
guard error == nil else { return .Failure(error!) }
let JSONResponseSerializer = Request.JSONResponseSerializer(options: .AllowFragments)
let result = JSONResponseSerializer.serializeResponse(request, response, data, error)
switch result {
case .Success(let value):
if let
response = response,
responseObject = T(response: response, representation: value)
{
return .Success(responseObject)
} else {
let failureReason = "JSON could not be serialized into response object: \(value)"
let error = Error.errorWithCode(.JSONSerializationFailed, failureReason: failureReason)
return .Failure(error)
}
case .Failure(let error):
return .Failure(error)
}
}
return response(responseSerializer: responseSerializer, completionHandler: completionHandler)
}
}
The struct ResponseSerializer from Alamofire
public struct ResponseSerializer<Value, Error: ErrorType>: ResponseSerializerType {
/// The type of serialized object to be created by this `ResponseSerializer`.
public typealias SerializedObject = Value
/// The type of error to be created by this `ResponseSerializer` if serialization fails.
public typealias ErrorObject = Error
/**
A closure used by response handlers that takes a request, response, data and error and returns a result.
*/
public var serializeResponse: (NSURLRequest?, NSHTTPURLResponse?, NSData?, NSError?) -> Result<Value, Error>
/**
Initializes the `ResponseSerializer` instance with the given serialize response closure.
- parameter serializeResponse: The closure used to serialize the response.
- returns: The new generic response serializer instance.
*/
public init(serializeResponse: (NSURLRequest?, NSHTTPURLResponse?, NSData?, NSError?) -> Result<Value, Error>) {
self.serializeResponse = serializeResponse
}
}
Your question can be greatly pared down (and should have been). Here is the relevant declaration of ResponseSerializer:
public struct ResponseSerializer<Value, Error: ErrorType>: ResponseSerializerType {
public init(serializeResponse: (NSURLRequest?, NSHTTPURLResponse?, NSData?, NSError?) -> Result<Value, Error>) {
self.serializeResponse = serializeResponse
}
}
So this initializer, init(serializeResponse:), takes one parameter — a function taking four parameters and returning one parameter (of the specified types).
Thus, we can initialize like this:
func f (request:NSURLRequest?, response:NSHTTPURLResponse?, data:NSData?, error:NSError?) -> Result<Value, Error>) {
guard error == nil else { return .Failure(error!)
}
let responseSerializer = ResponseSerializer<T, NSError>(serializeResponse:f)
However, this can be condensed. We don't really need the function f for anything else, so we can supply it as an anonymous function; it doesn't need a name or a full declaration. Moreover, there is a "shortcut" rule for anonymous functions, that if an anonymous function is the last parameter to a function, it can be provided literally after the function's closing parentheses, with the parameter name omitted. And if the function takes no other parameters, its parentheses can be omitted altogether.
Well, this init is exactly such a function — it takes a function as its last (and only) parameter — so that is exactly what the code in question does:
let responseSerializer = ResponseSerializer<T, NSError> {
request, response, data, error in
guard error == nil else { return .Failure(error!)
}
If I read it all correctly the code above has a pattern similar to this:
// just a something
struct Blah {
var stuffs : (message:String) -> Void
init(closure:(message:String) -> Void) {
self.stuffs = closure
}
}
// an extension because the code above is also in an extension, but not needed at all
extension Blah {
// a function with a closure that also returns an instance of Self
func spawnChild(closure:(message:String) -> Void) -> Blah {
return Blah(closure: closure) // closure is passed to spawn
}
}
Tests :
let alpha = Blah { (message) -> Void in
print("alpha",message)
}
let beta = alpha.spawnChild { (message) -> Void in
print("beta", message)
}
alpha.stuffs(message: "parrent") // alpha parent
beta.stuffs(message: "spawn") // beta spawn
Remember that the trailing closure is just syntactic sugar for an input parameter that is a closure. So anything that takes input parameters can have a trailing closure.