In Xcode 11 beta 5 or 6 my existing code that relied on URLSession.DataTaskPublisher stopped working. It seems like DataTaskPublisher is never publishing any values but I can't work out why.
I've tried with .sink and .handleEvents as subscribers. I've tested .sink with a Just publisher and confirmed it receives a value there.
I've also tried both giving the DataTaskPublisher a URL and giving it a URLRequest. I've tried a request to an API including an authorization header, as well as basic requests to google.com and apple.com. I've tried using URLSession.shared and creating a new instance of URLSession. I've also tried with and without map and decode operators.
I've used XCTest expectations to confirm that the test times out every single time, even if I give it a 4-minute timeout.
I just made a new example project and replicated the problem with the following code in the root view controller:
override func viewDidLoad() {
super.viewDidLoad()
print("view did load")
URLSession.shared.dataTaskPublisher(for: URL(string: "http://apple.com")!)
.handleEvents(receiveSubscription: { (sub) in
print(sub)
}, receiveOutput: { (response) in
print(response)
}, receiveCompletion: { (completion) in
print(completion)
}, receiveCancel: {
print("cancel")
}, receiveRequest: { (demand) in
print(demand)
})
}
The project prints "view did load" but nothing else ever prints. Any ideas about where I'm going wrong here? Thanks!
I think that there are two problems with your code, firstly you only have a publisher (handleEvent returns a publisher) and secondly that publisher goes out of scope and disappears. This works although it isn't exactly elegant.
import Combine
import SwiftUI
var pub: AnyPublisher<(data: Data, response: URLResponse), URLError>? = nil
var sub: Cancellable? = nil
var data: Data? = nil
var response: URLResponse? = nil
func combineTest() {
guard let url = URL(string: "https://apple.com") else {
return
}
pub = URLSession.shared.dataTaskPublisher(for: url)
.print("Test")
.eraseToAnyPublisher()
sub = pub?.sink(
receiveCompletion: { completion in
switch completion {
case .finished:
break
case .failure(let error):
fatalError(error.localizedDescription)
}
},
receiveValue: { data = $0.data; response = $0.response }
)
}
struct ContentView: View {
var body: some View {
Button(
action: { combineTest() },
label: { Text("Do It").font(.largeTitle) }
)
}
}
I did it in SwiftUI so that I would have less to worry about and I used 3 variables so that I could follow better. You need to use the 2 parameter sink as the publisher's error isn't Never. Finally the print() is just for test and works really well.
Related
I know this question is asked a lot, but I can't figure out how to apply any answers to my program. Sorry in advance this async stuff makes absolutely zero sense to me.
Basically, I have a button in SwiftUI that, when pressed, calls a function that makes two API calls to Google Sheets using Alamofire and GoogleSignIn.
Button("Search") {
if fullName != "" {
print(SheetsAPI.nameSearch(name: fullName, user: vm.getUser()) ?? "Error")
}
}
This function should return the values of some cells on success or nil on an error. However, it only ever prints out "Error". Here is the function code.
static func nameSearch<S: StringProtocol>(name: S, advisory: S = "", user: GIDGoogleUser?) -> [String]? {
let name = String(name)
let advisory = String(advisory)
let writeRange = "'App Control'!A2:C2"
let readRange = "'App Control'!A4:V4"
// This function can only ever run when user is logged in, ! should be fine?
let user = user!
let parameters: [String: Any] = [
"range": writeRange,
"values": [
[
name,
nil,
advisory
]
]
]
// What I want to be returned
var data: [String]?
// Google Identity said use this wrapper so that the OAuth tokens refresh
user.authentication.do { authentication, error in
guard error == nil else { return }
guard let authentication = authentication else { return }
// Get the access token to attach it to a REST or gRPC request.
let token = authentication.accessToken
let headers: HTTPHeaders = ["Authorization": "Bearer \(token)"]
AF.request("url", method: .put, parameters: parameters, encoding: JSONEncoding.default, headers: headers).responseString { response in
switch response.result {
case .success:
// I assume there is a better way to make two API calls...
AF.request("anotherURL", headers: headers).responseDecodable(of: NameResponseModel.self) { response2 in
switch response2.result {
case .success:
guard let responseData = response2.value else { return }
data = responseData.values[0]
// print(responseData.values[0]) works fine
case .failure:
print(response2.error ?? "Unknown error.")
data = nil
}
}
case .failure:
print(response.error ?? "Unknown error.")
data = nil
}
}
}
// Always returns nil, "Unknown error." never printed
return data
}
The model struct for my second AF request:
struct NameResponseModel: Decodable { let values: [[String]] }
An example API response for the second AF request:
{
"range": "'App Control'!A4:V4",
"majorDimension": "ROWS",
"values": [
[
"Bob Jones",
"A1234",
"Cathy Jones",
"1234 N. Street St. City, State 12345"
]
]
}
I saw stuff about your own callback function as a function parameter (or something along those lines) to handle this, but I was completely lost. I also looked at Swift async/await, but I don't know how that works with callback functions. Xcode had the option to refactor user.authentication.do { authentication, error in to let authentication = try await user.authentication.do(), but it threw a missing parameter error (the closure it previously had).
EDIT: user.authentication.do also returns void--another reason the refactor didn't work (I think).
There is probably a much more elegant way to do all of this so excuse the possibly atrocious way I did it.
Here is the link to Google Identity Wrapper info.
Thanks in advance for your help.
Solved my own problem.
It appears (according to Apple's async/await intro video) that when you have an unsupported callback that you need to run asynchronously, you wrap it in something called a Continuation, which allows you to manually resume the function on the thread, whether throwing or returning.
So using that code allows you to run the Google Identity token refresh with async/await.
private static func auth(_ user: GIDGoogleUser) async throws -> GIDAuthentication? {
typealias AuthContinuation = CheckedContinuation<GIDAuthentication?, Error>
return try await withCheckedThrowingContinuation { (continuation: AuthContinuation) in
user.authentication.do { authentication, error in
if let error = error {
continuation.resume(throwing: error)
} else {
continuation.resume(returning: authentication)
}
}
}
}
static func search(user: GIDGoogleUser) async throws {
// some code
guard let authentication = try await auth(user) else { ... }
// some code
}
I then ran that before using Alamofire's built-in async/await functionality for each request (here's one).
let dataTask = AF.request(...).serializingDecodable(NameResponseModel.self)
let response = try await dataTask.value
return response.values[0]
I created a simple Publisher from an array of paths I want to fetch from the internet. I am setting the failure type to match the DataTaskPublisher, and then I flatMap to get the new Publisher with the DataTask results. However, when I subscribe to the stream with sink, nothing gets called.
Here is my code:
import Combine
import Foundation
class NetworkManager {
var tasks = Set<AnyCancellable>()
init() {
getData()
}
func getData() {
let baseUrl = URL(string: "https://fmi.unibuc.ro")!
["/prezentare", "/cazare"].publisher
.setFailureType(to: URLError.self)
.flatMap { path -> URLSession.DataTaskPublisher in
let url = baseUrl.appendingPathComponent(path)
return URLSession.shared.dataTaskPublisher(for: url)
}
.sink(receiveCompletion: { completion in
print(completion)
}, receiveValue: { value in
print(value)
})
.store(in: &tasks)
}
}
let manager = NetworkManager()
What am I doing wrong? 🤔
Swift playgrounds finish execution when all synchronous code in them returned. However, you are executing a network request asynchronously, so you need to tell the playground to wait for the async result.
Call this before starting the network request:
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true
And in the sink, you can finish execution by calling
PlaygroundPage.current.finishExecution()
I'm trying to make a fairly simple API call in Swift but, for some reason, my dataTask code is not running. I've made sure that the .resume() is there. This code has worked in the past but, something has changed recently and I don't know what it is. The only thing I can think of is the url. I've changed the ingredients but, when putting the url into a browser, it returns JSON data normally. When running this function, I get two "Outside URLSession.shared.dataTask....." messages in a row with nothing in between, indicating that the URLSession block of code isn't running. I'm a little new to APIs so, any help would be greatly appreciated. Please let me know if there's any more information I can provide. Also, I'm on an older MacBook and am using Swift5 if that makes a difference. Thanks!
let url: URL! = URL(string: "https://api.spoonacular.com/recipes/findByIngredients?ingredients=" + ingredientString + "&apiKey=aaabbbccc111222333")
print("URL: " + url.absoluteString)
let request = URLRequest(url: url)
// Make the API call
print("Outide URLSession.shared.dataTask.....")
let session = URLSession.shared.dataTask(with: request) { data, response, error in
print("Inside URLSession.shared.dataTask.....")
DispatchQueue.main.async {
print("Inside DispatchQueue.main.async....")
if data == nil {
print("No data recieved.")
}
print("data != nil.... Moving on to JSONDecoder....")
self.model = try! JSONDecoder().decode([RecipeSearchElement].self, from: data!)
}
}
session.resume()
print("Outside URLSession.shared.dataTask.....")
Unrelated to your immediate question at hand (which I answered elsewhere), I would advise a few changes to the routine:
One should not build a URL through string interpolation. Use URLComponents. If, for example, the query parameter included a space or other character not permitted in a URL, URLComponents will percent-encode it for you. If do not percent-encode it properly, the building of the URL will fail.
I would avoid try!, which will crash the app if the server response was not what you expected. One should use try within a do-catch block, so it handles errors gracefully and will tell you what is wrong if it failed.
I would recommend renaming the URLSessionDataTask to be task, or something like that, to avoid conflating “sessions” with the “tasks” running on that session.
I would not advise updating the model from the background queue of the URLSession. Fetch and parse the response in the background queue and update the model on the main queue.
Thus:
var components = URLComponents(string: "https://api.spoonacular.com/recipes/findByIngredients")
components?.queryItems = [
URLQueryItem(name: "ingredients", value: ingredientString),
URLQueryItem(name: "apiKey", value: "aaabbbccc111222333")
]
guard let url = components?.url else {
print("Unable to build URL")
return
}
// Make the API call
let task = URLSession.shared.dataTask(with: url) { data, _, error in
DispatchQueue.main.async {
guard error == nil, let data = data else {
print("No data received:", error ?? URLError(.badServerResponse))
return
}
do {
let model = try JSONDecoder().decode([RecipeSearchElement].self, from: data)
DispatchQueue.main.async {
self.model = model
}
} catch let parseError {
print("Parsing error:", parseError, String(describing: String(data: data, encoding: .utf8)))
}
}
}
task.resume()
In a more advanced observation, I would never have a network call update the model directly. I would leave that to the caller. For example, you could use a completion handler pattern:
#discardableResult
func fetchIngredients(
_ ingredientString: String,
completion: #escaping (Result<[RecipeSearchElement], Error>) -> Void
) -> URLSessionTask? {
var components = URLComponents(string: "https://api.spoonacular.com/recipes/findByIngredients")
components?.queryItems = [
URLQueryItem(name: "ingredients", value: ingredientString),
URLQueryItem(name: "apiKey", value: "aaabbbccc111222333")
]
guard let url = components?.url else {
completion(.failure(URLError(.badURL)))
return nil
}
// Make the API call
let task = URLSession.shared.dataTask(with: url) { data, _, error in
print("Inside URLSession.shared.dataTask.....")
DispatchQueue.main.async {
guard error == nil, let data = data else {
DispatchQueue.main.async {
completion(.failure(error ?? URLError(.badServerResponse)))
}
return
}
do {
let model = try JSONDecoder().decode([RecipeSearchElement].self, from: data)
DispatchQueue.main.async {
completion(.success(model))
}
} catch let parseError {
DispatchQueue.main.async {
completion(.failure(parseError))
}
}
}
}
task.resume()
return task
}
And then the caller could do:
fetchIngredients(ingredientString) { [weak self] result in
switch result {
case .failure(let error): print(error)
case .success(let elements): self?.model = elements
}
}
This has two benefits:
The caller now knows when the model is updated, so you can update your UI at the appropriate point in time (if you want).
It maintains a better separation of responsibilities, architecturally avoiding the tight coupling of the network layer with that of the view or view model (or presenter or controller) layers.
Note, I am also returning the URLSessionTask object in case the caller would like to cancel it at a later time, but I made it an #discardableResult so that you do not have to worry about that if you are not tackling cancelation at this point.
If you (a) are reaching the “outside” message, but not seeing the “inside” message; and (b) are absolutely positive that you are reaching the resume statement, it is one of a few possibilities:
The app may be terminating before the asynchronous request has time to finish. This can happen, for example, if this is a command-line app and you are allowing the app to quit before the asynchronous request has a chance to finish. If you want a command-line app to wait for a network request to finish, you might run a RunLoop that does not exit until the network request is done.
It can also happen if you use a playground and neglect to set needsIndefiniteExecution:
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true
For the sake of completeness, there are a few other, less common, possibilities:
You have some other network request whose completion handler is blocked/deadlocked, thereby preventing anything else from running on the URLSession dedicated, serial, queue.
You have thread explosion somewhere else in your code, exhausting the limited pool of worker threads, preventing other tasks/operations from being able to get an available worker thread.
I have a lot (~200) urls for images, and I need to download each one, then process (resize) it, then update the cache. The thing is - I only want to have at max 3 requests at once, and since the images are heavy, I also don't want a lot of responses "hanging" waiting to be processed (and taking memory...).
TLDR I want to call the next (4th) network request only after the receiveValue in the sink is called on one of the first 3 requests... (ie after the network response & processing are both done...).
Will this flow work, and will it hold on to the waiting urls and not drop them on the floor?
Also do I need that buffer() call? I use it after seeing this answer: https://stackoverflow.com/a/67011837/2242359
wayTooManyURLsToHandleAtOnce // this is a `[URL]`
.publisher
.buffer(size: .max, prefetch: .byRequest, whenFull: .dropNewest) // NEEDED?
.flatMap(maxPublishers: .max(3)) { url in
URLSession.shared
.dataTaskPublisher(for: url)
.map { (data: Data, _) -> Picture in
Picture(from: data)
}
}
.tryCompactMap {
resizeImage(picture: $0) // takes a while and might fail
}
.receive(on: DispatchQueue.main)
.sink { completion
// handling completion...
} receiveValue: { resizedImage
self.cache.append(resizedImage)
}
.store(...)
I would use a subject. This not an optimal solution but it looks working and maybe will trigger other ideas
var cancellable: AnyCancellable?
var urls: [String] = (0...6).map { _ in "http://httpbin.org/delay/" + String((0...2).randomElement()!) }
var subject: PassthroughSubject<[String], Never> = .init()
let maxConcurrentRequests = 3
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
print(urls)
cancellable = subject
.flatMap({ urls -> AnyPublisher<[URLSession.DataTaskPublisher.Output], URLError> in
let requests = urls.map { URLSession.shared.dataTaskPublisher(for: URL.init(string: $0)!) }
return Publishers.MergeMany(requests)
.collect().eraseToAnyPublisher()
})
.print()
.sink(receiveCompletion: { completion in
print(completion)
}, receiveValue: { value in
print(value)
if self.urls.count <= self.maxConcurrentRequests {
self.urls.removeAll()
self.subject.send(completion: .finished)
} else {
self.urls.removeLast(self.maxConcurrentRequests)
self.subject.send(self.urls.suffix(self.maxConcurrentRequests))
}
})
subject.send(urls.suffix(maxConcurrentRequests))
}
I'm kind of new to programming in general, so I have this maybe simple question. Actually, writing helps me to identify the problem faster.
Anyway, I have an app with multiple asynchronous calls, they are nested like this:
InstagramUnoficialAPI.shared.getUserId(from: username, success: { (userId) in
InstagramUnoficialAPI.shared.fetchRecentMedia(from: userId, success: { (data) in
InstagramUnoficialAPI.shared.parseMediaJSON(from: data, success: { (media) in
guard let items = media.items else { return }
self.sortMediaToCategories(media: items, success: {
print("success")
// Error Handlers
Looks horrible, but that's not the point. I will investigate the Promise Kit once I get this working.
I need the sortMediaToCategories to wait for completion and then reload my collection view. However, in the sortMediaToCategories I have another nested function, which is async too and has a for in loop.
func sortMediaToCategories(media items: [StoryData.Items],
success: #escaping (() -> Swift.Void),
failure: #escaping (() -> Swift.Void)) {
let group = DispatchGroup()
group.enter()
for item in items {
if item.media_type == 1 {
guard let url = URL(string: (item.image_versions2?.candidates?.first!.url)!) else {return}
mediaToStorageDistribution(withImageUrl: url,
videoUrl: nil,
mediaType: .jpg,
takenAt: item.taken_at,
success: { group.notify(queue: .global(), execute: {
self.collectionView.reloadData()
group.leave()
}) },
failure: { print("error") })
//....
I can't afford the collection view to reload every time obviously, so I need to wait for loop to finish and then reload.
I'm trying to use Dispatch Groups, but struggling with it. Could you please help me with this? Any simple examples and any advice will be very appreciated.
The problem you face is a common one: having multiple asynchronous tasks and wait until all are completed.
There are a few solutions. The most simple one is utilising DispatchGroup:
func loadUrls(urls: [URL], completion: #escaping ()->()) {
let grp = DispatchGroup()
urls.forEach { (url) in
grp.enter()
URLSession.shared.dataTask(with: url) { data, response, error in
// handle error
// handle response
grp.leave()
}.resume()
}
grp.notify(queue: DispatchQueue.main) {
completion()
}
}
The function loadUrls is asynchronous and expects an array of URLs as input and a completion handler that will be called when all tasks have been completed. This will be accomplished with the DispatchGroup as demonstrated.
The key is, to ensure that grp.enter() will be called before invoking a task and grp.leave is called when the task has been completed. enter and leave shall be balanced.
grp.notify finally registers a closure which will be called on the specified dispatch queue (here: main) when the DispatchGroup grp balances out (that is, its internal counter reaches zero).
There are a few caveats with this solution, though:
All tasks will be started nearly at the same time and run concurrently
Reporting the final result of all tasks via the completion handler is not shown here. Its implementation will require proper synchronisation.
For all of these caveats there are nice solutions which should be implemented utilising suitable third party libraries. For example, you can submit the tasks to some sort of "executer" which controls how many tasks run concurrently (match like OperationQueue and async Operations).
Many of the "Promise" or "Future" libraries simplify error handling and also help you to solve such problems with just one function call.
You can reloadData when the last item calls the success block in this way.
let lastItemIndex = items.count - 1
for(index, item) in items.enumerated() {
if item.media_type == 1 {
guard let url = URL(string: (item.image_versions2?.candidates?.first!.url)!) else {return}
mediaToStorageDistribution(withImageUrl: url,
videoUrl: nil,
mediaType: .jpg,
takenAt: item.taken_at,
success: {
if index == lastItemIndex {
DispatchQueue.global().async {
self.collectionView.reloadData()
}
}
},
failure: { print("error") })
}
You have to move the group.enter() call inside your loop. Calls to enter and leave have to be balanced. If your callbacks of the mediaToStorageDistribution function for success and failure are exclusive you also need to leave the group on failure. When all blocks that called enter leave the group notify will be called. And you probably want to replace the return in your guard statement with a break, to just skip items with missing URLs. Right now you are returning from the whole sortMediaToCatgories function.
func sortMediaToCategories(media items: [StoryData.Items], success: #escaping (() -> Void), failure: #escaping (() -> Void)) {
let group = DispatchGroup()
for item in items {
if item.media_type == 1 {
guard let url = URL(string: (item.image_versions2?.candidates?.first!.url)!) else { break }
group.enter()
mediaToStorageDistribution(withImageUrl: url,
videoUrl: nil,
mediaType: .jpg,
takenAt: item.taken_at,
success: { group.leave() },
failure: {
print("error")
group.leave()
})
}
}
group.notify(queue: .main) {
self.collectionView.reloadData()
}
}