I have a Swift function with a completion handler that I want to be called when a certain network request is done. The network request can be repeated based on if the server response says there is more data (offset is provided).
I initially call it like this:
func myMainFunction(){
getTasks(project: project, paginationToken: nil) {
print("This never fires...")
}
}
And my getTasks() function is like this:
func getTasks(project: Project, paginationToken: String?, completion: #escaping () -> Void){
var url = "..."
if let token = paginationToken{
url += "&offset=\(token)"
}
Alamofire.request(url).responseJSON { response in
switch response.result {
case .success(let value):
let json = JSON(value)
//...
//Check for pagination
let offset = json["next_page"]["offset"].stringValue
if !offset.isEmpty{
//Theres a pagination token available, go again!
getTasks(project: project, paginationToken: offset){}
}else{
print("This prints to the log on the last run through")
completion()
}
case .failure(let error):
print("Task error: \(error)")
}
}
}
If there is never an offset then the completion() handler works fine. But if getTasks() has to be called again, the completion handler is somehow invalidated and never returns to my original myMainFunction().
Any ideas?
You are passing empty closure if offset is not empty. You need to forward the completion param to getTasks again. Try this.
//...
if !offset.isEmpty {
getTasks(project: project, paginationToken: offset, completion: completion)
}
//...
Related
I'm struggling with making badge requests with Alamofire and I need help.
I have some ids and with them I need to struct parameters (Dictionary String) and send a GET request with Alamofire. Everything is fine, but I need to cover the case when ids are above 200, because when they are more than 200, API returns 414 code status (too long URL). So when ids are more than 200 they are separated in chunks. With each chunk I'm making a new request to API. The problem is that I return only the first 200 ids when I call my method. Here is an example:
func request (_ idsDict: [String: [String]], _ idSchema: String, _ completion: #escaping Result<SomeModel, Error>) -> Void {
let chunks = transformEntitiesIdsToChunks(idsDict)
let decoder = JSONDecoder()
chunks.forEach {chunk in
let parameters = constructQueryParams(idsDict, chunk, idSchema, apiKey, clientId)
AF.request(baseURL, parameters: parameters).response { response in
switch response.result {
case .success(let data):
// some error handling for decoding and no data
completion(.success(data.data))
case .failure(let error):
return completion(.failure(error.localizedDescription))
}
}
}
}
// Method wraps AF request in a continuation block and makes sure that the closure from request method returned data or throwed error.
// That way fetching from API becomes async/await and can be used in do/try/catch block.
func getIdsEntities (_ idsDict: [String: [String]], _ idSchema: String) async throws -> [SomeModel] {
return try await withUnsafeThrowingContinuation { continuation in
request(idsDict, idSchema) { result in
switch result {
case .success(let data):
continuation.resume(returning: data)
return
case .failure(let error):
continuation.resume(throwing: error)
return
}
}
}
}
I have tried with recursive functions and with DispatchGroup but none of them worked. Any help will be appriciated. Thank you in advance.
Thanks to Larme's comment I was able to find my mistake. When making request to API I was passing the decoded response to the completion closure. To fix this I had to declare an array of model let responses:[SomeModel] = [] and append the decoded result to it. I used let group = DispatchGroup() so I can wait the requests to execute and have my final array of results and then I used group.notify(queue: .main, execute: {completion(.success(responses))}) to return to the main queue and have my array of completed fetched data. This is now how my code looks like:
private func request (_ idsDict: [String: [String]], _ idSchema: String, _ completion: #escaping APIListResponseClosure<SomeModel>) -> Void {
var responses: [SomeModel] = []
let group = DispatchGroup()
let chunks = transformEntitiesIdsToChunks(idsDict)
let decoder = JSONDecoder()
chunks.forEach {chunk in
group.enter()
let parameters = constructQueryParams(idsDict, chunk, idSchema, apiKey, clientId)
AF.request(baseURL, parameters: parameters).response { response in
switch response.result {
case .success(let data):
// some error handling for decoding and no data
responses.append(data.data)
group.leave()
case .failure(let error):
return completion(.failure(.APIError(error.localizedDescription)))
}
}
}
group.notify(queue: .main, execute: {
print("Ids are fetched")
completion(.success(responses))
})
}
Thanks again to Larme and I hope I helped someone else with this case.
I am trying to Apollo framework and a graphql api to obtain the data then return it. Once I have the data in another swift file, I want to call on certain parts of the data and assign it to a variable. The errors I get is variable used before it is initialized. and if try to return the variable from within the closure I get "Unexpected Non-Void Return Value In Void Function ". I heard of ways to get around that error but I don't completely understand it and how it works with my code. If you need more code or context you can message me and I can share my GitHub repo. Sorry if the code is bad, please don't roast me. I am still a beginner.
import Foundation
import Apollo
struct AniListAPI {
let aniListUrl = "https://graphql.anilist.co"
func ObtainData(AnimeID: Int)-> QueryQuery.Data{
var theData: QueryQuery.Data
let theInfo = QueryQuery(id: AnimeID)
GraphClient.fetch(query: theInfo) { result in
switch result {
case .failure(let error):
print("A big No no happened \(error)")
case .success(let GraphQLResult):
guard let Info = GraphQLResult.data else {return}
theData = Info
}
}
return theData
}
}
Unexpected Non-Void Return Value In Void Function.
The reason you're getting this warning is because you can't return value from inside the closure. Use closure instead of returning value.
func ObtainData(AnimeID: Int, completion: #escaping (Data) -> Void) {
var TheData: QueryQuery.Data
let TheInfo = QueryQuery(id: AnimeID)
GraphClient.fetch(query: TheInfo) { result in
switch result {
case .failure(let error):
print("A big no no happened retard \(error)")
case .success(let GraphQLResult):
guard let Info = GraphQLResult.data else {return}
TheData = Info
completion(TheData)
}
}
}
and call it like..
ObtainData(AnimeID: 123) { (anyData) in
print (anyData)
// continue your logic
}
I tried using DispatchQueue and DispatchGroup but its still asynchronous, I also tried both dispatchQueue.Async and dispathQueue.sync and neither have worked.
myFunc is a function called in the init() which itself calls 2 functions, getArrOneData() annd getArrTwoData([ArrOneType]).
The first function downloads ArrOneType data from firestore and returns an array which is initialised to a field.
The second function uses the downloaded data/field from the first function to initialise a field in ArrTwoType while simultaneously downloading other relevent ArrTwoType data from firestore to return as an array.
So the Problem is its still asynchronous. How do I use DispatchQueue and DispatchGroup correctly in this scenario?
thanks
note: get arrTwoData is mainly psuedocode
func myFunc(){
let group = DispatchGroup()
let dispatchQueue = DispatchQueue.global(qos: .default)
group.enter()
dispatchQueue.sync {
self.arrOne = self.getArrOneData()//getArrOneData gets data from firestore
group.leave()
}
dispatchQueue.sync {
group.enter()
self.arrTwo = self.getArrTwoData(inputArr: self.arrOne)//getArrTwoData gets data from firestore
group.leave()
}
}
//ArrOneType is an array field in ArrTwoType
func getArrTwoData(inputArr: [ArrOneType]) -> [ArrTwoType]{
var result = [ArrTwoType]()
//retrieving data from firestore, code excluded...
for document in querySnapshot!.documents {
let data = document.data()
name = data["Name"] as? String ?? "Name Unknown"
//returns an array which is a subset of inputArr filtered by name, however
//I dont think the inputArr is populated at this point when I run the program
var field2:[ArrOneType] = someFunc(name, inputArr)
var x = ArrTwoType(name: name, field2: field2)
result.append(x)
}
}
}
return result
}
If you have two requests, one which uses the response from one to prepare the next, the idea is to use #escaping completion handler closures for all asynchronous methods, e.g.
func getArrayOneData(completion: #escaping (Result<[ArrayOneType], Error>) -> Void) {
someAsynchronousMethod {
let values: [ArrayOneType] = ...
completion(.success(values))
}
}
func getArrayTwoData(for typeOneValues: [ArrayOneType], completion: #escaping (Result<[ArrayTwoType], Error>) -> Void) {
someAsynchronousMethod(for: typeOneValues) {
let values: [ArrayTwoType] = ...
completion(.success(values))
}
}
Then you can do things like:
func getEverything(completion: #escaping (Result<[ArrayTwoType], Error>) -> Void) {
getArrayOneData { result in
switch result {
case .failure(let error):
print(error)
case .success(let typeOneValues):
getArrayTwoData(for: typeOneValues) { result in
switch result {
case .failure(let error):
print(error)
case .success(let typeTwoValues):
completion(.success(typeTwoValues))
}
}
}
}
}
Note, no dispatch groups needed. Having added completion handlers to the asynchronous methods, we can just call the second method from the completion handler of the first.
I'm having a hard time understanding how chaining completion handlers with closures works.
The example I'm working with is the following :
typealias CompletionHandler = (_ result: AnyObject?, _ error: NSError?) -> Void
func getRequestToken(completionHandler: CompletionHandler){
taskForGET(path: "PathToWebsite.com") { (result, error) in
if let error = error {
print(error)
} else {
print(result)
}
}
}
func taskForGET(path: String, completionHandler: CompletionHandler) {
//URLSESSIONCODE with completion handler
(data, response, error) {
if let error = error {
CompletionHandler(result: nil, error: error)
} else {
let data = data
parseJSON(data: data, completionHandler: completionHandler)
}
}
}
func parseJSON(data: NSData, completionHandler: CompletionHandler) {
//SerializingJSON with do catch block
do {
completionHandler(result: theResult, error: nil)
} catch {
completionHandler(result: nil, error: error)
}
}
So basically what this code does is it kicks off a GET request to a server. If the GET request sends back data, then it parses it into JSON. If at any point along the way something fails, it returns an error.
I understand basically what is going on here, but I don't understand how the completion handlers are being fired off.
First taskForGET is called which has a completion handler as a parameter that can return a result or an error, I've got that.
The next step is calling parseJSON, where the data from taskForGET is passed but then the completionhandler that's being passed is taskForGET's completion handler. I don't understand this at all.
Then down in parseJSON, the completion handler either returns JSON or an error by calling the completion handler from its parameters..which in this case is taskForGET's completion handler?
I don't understand the flow. Even once we've parsed JSON successfully, how does calling taskForGET's result ever get back up to getRequestToken.
Any help with this would be appreciated.
There is only one completion handler which is passed from one method to another.
Lets declare the handler separately, btw. in Swift 3 omit the parameter labels in the type alias:
typealias CompletionHandler = (Any?, Error?) -> Void
let handler : CompletionHandler = { (result, error) in
if let error = error {
print(error)
} else {
print(result)
}
}
This closure is supposed to be executed at the end of the process. Now the getRequestToken method looks like
func getRequestToken(completionHandler: CompletionHandler){
taskForGET(path: "PathToWebsite.com", completionHandler: handler)
}
The handler / closure is passed as a parameter from getRequestToken to taskForGET and then from taskForGET to parseJSON but it's always the same object.
I'm using a lot of async network request (btw any network request in iOS need to by async) and I'm finding way to better handle errors from Apple's dataTaskWithRequest which not supports throws.
I have code like that:
func sendRequest(someData: MyCustomClass?, completion: (response: NSData?) -> ()) {
let request = NSURLRequest(URL: NSURL(string: "http://google.com")!)
if someData == nil {
// throw my custom error
}
let task = NSURLSession.sharedSession().dataTaskWithRequest(request) {
data, response, error in
// here I want to handle Apple's error
}
task.resume()
}
I need to parse my possible custom errors and handle possible connection errors from dataTaskWithRequest. Swift 2 introduced throws, but you can't throw from Apple's closure because they have no throw support and running async.
I see only way to add to my completion block NSError returning, but as I know using NSError is old-style Objective-C way. ErrorType can be used only with throws (afaik).
What's the best and most modern method to handle error when using Apple network closures? There is no way no use throws in any async network functions as I understand?
there are many ways you can solve this, but i would recommend using a completion block which expects a Result Enum. this would probably be the most 'Swift' way.
the result enum has exactly two states, success and error, which a big advantage to the usual two optional return values (data and error) which lead to 4 possible states.
enum Result<T> {
case Success(T)
case Error(String, Int)
}
Using the result enum in a completion block finishes the puzzle.
let InvalidURLCode = 999
let NoDataCode = 998
func getFrom(urlString: String, completion:Result<NSData> -> Void) {
// make sure the URL is valid, if not return custom error
guard let url = NSURL(string: urlString) else { return completion(.Error("Invalid URL", InvalidURLCode)) }
let request = NSURLRequest(URL: url)
NSURLSession.sharedSession().dataTaskWithRequest(request) { data, response, error in
// if error returned, extract message and code then pass as Result enum
guard error == nil else { return completion(.Error(error!.localizedDescription, error!.code)) }
// if no data is returned, return custom error
guard let data = data else { return completion(.Error("No data returned", NoDataCode)) }
// return success
completion(.Success(data))
}.resume()
}
because the return value is a enum, you should switch off of it.
getFrom("http://www.google.com") { result in
switch result {
case .Success(let data):
// handle successful data response here
let responseString = String(data:data, encoding: NSASCIIStringEncoding)
print("got data: \(responseString)");
case .Error(let msg, let code):
// handle error here
print("Error [\(code)]: \(msg)")
}
}
another solution would be to pass two completion blocks, one for success and one for error. something along the lines of:
func getFrom(urlString: String, successHandler:NSData -> Void, errorHandler:(String, Int) -> Void)
It's very similar to Casey's answer,
but with Swift 5, now we have Result (generic enumeration) implementation in standard library,
//Don't add this code to your project, this has already been implemented
//in standard library.
public enum Result<Success, Failure: Error> {
case success(Success), failure(Failure)
}
It's very easy to use,
URLSession.shared.dataTask(with: url) { (result: Result<(response: URLResponse, data: Data), Error>) in
switch result {
case let .success(success):
handleResponse(success.response, data: success.data)
case let .error(error):
handleError(error)
}
}
https://developer.apple.com/documentation/swift/result
https://github.com/apple/swift-evolution/blob/master/proposals/0235-add-result.md
There's an elegant approach utilising a JavaScript-like Promise library or a Scala-like "Future and Promise" library.
Using Scala-style futures and promises, it may look as follows:
Your original function
func sendRequest(someData: MyCustomClass?, completion: (response: NSData?) -> ())
may be implemented as shown below. It also shows, how to create a promise, return early with a failed future and how to fulfill/reject a promise:
func sendRequest(someData: MyCustomClass) -> Future<NSData> {
guard let url = ... else {
return Future.failure(MySessionError.InvalidURL) // bail out early with a completed future
}
let request = ... // setup request
let promise = Promise<NSData>()
NSURLSession.sharedSession().dataTaskWithRequest(request) { data, response, error in
guard let error = error else {
promise.reject(error) // Client error
}
// The following assertions should be true, unless error != nil
assert(data != nil)
assert(response != nil)
// We expect HTTP protocol:
guard let response = response! as NSHTTPURLResponse else {
promise.reject(MySessionError.ProtocolError) // signal that we expected HTTP.
}
// Check status code:
guard myValidStatusCodeArray.contains(response.statusCode) else {
let message: String? = ... // convert the response data to a string, if any and if possible
promise.reject(MySessionError.InvalidStatusCode(statusCode: response.statusCode, message: message ?? ""))
}
// Check MIME type if given:
if let mimeType = response.MIMEType {
guard myValidMIMETypesArray.contains(mimeType) else {
promise.reject(MySessionError.MIMETypeNotAccepted(mimeType: mimeType))
}
} else {
// If we require a MIMEType - reject the promise.
}
// transform data to some other object if desired, can be done in a later, too.
promise.fulfill(data!)
}.resume()
return promise.future!
}
You might expect a JSON as response - if the request succeeds.
Now, you could use it as follows:
sendRequest(myObject).map { data in
return try NSJSONSerialization.dataWithJSONObject(data, options: [])
}
.map { object in
// the object returned from the step above, unless it failed.
// Now, "process" the object:
...
// You may throw an error if something goes wrong:
if failed {
throw MyError.Failed
}
}
.onFailure { error in
// We reach here IFF an error occurred in any of the
// previous tasks.
// error is of type ErrorType.
print("Error: \(error)")
}