So I have here an async method defined on an actor type. The issue im having is that people isnt being mutated at all; mainly due to what from what I understand concurrency issues. From what I think I know the completion handler is executed concurrently on the same thread and so cannot mutate people.
Nonetheless my knowledge in this area is pretty foggy, so any suggestions to solve it and maybe a better explanation as to why this is happening would be great! Ive thought of a few things but im still new to concurrency.
func getAllPeople() async -> [PersonModelProtocol] {
let decoder = JSONDecoder()
var people: [Person] = []
let dataTask = session.dataTask(with: request!) { data, response, error in
do {
let newPeople = try! decoder.decode([Person].self, from: data!)
people = newPeople
} catch {
print(error)
}
}
dataTask.resume()
return people
}
If you do want to use async/await you have to use the appropriate API.
func getAllPeople() async throws -> [Person] {
let (data, _ ) = try await session.data(for: request!)
return try JSONDecoder().decode([Person].self, from: data)
}
In a synchronous context you cannot return data from a completion handler anyway.
vadian is right but it's better-expressed as a computed property nowadays.
var allPeople: [PersonModelProtocol] {
get async throws {
try JSONDecoder().decode(
[Person].self,
from: await session.data(for: request!).0
)
}
}
And also, your code does mutate people, but returning its empty value before it gets mutated.
Related
Picture an image loading function with a closure completion. Let's say it returns a token ID that you can use to cancel the asynchronous operation if needed.
#discardableResult
func loadImage(url: URL, completion: #escaping (Result<UIImage, Error>) -> Void) -> UUID? {
if let image = loadedImages[url] {
completion(.success(image))
return nil
}
let id = UUID()
let task = URLSession.shared.dataTask(with: url) { data, response, error in
defer {
self.requests.removeValue(forKey: id)
}
if let data, let image = UIImage(data: data) {
DispatchQueue.main.async {
self.loadedImages[url] = image
completion(.success(image))
}
return
}
if let error = error as? NSError, error.code == NSURLErrorCancelled {
return
}
//TODO: Handle response errors
print(response as Any)
completion(.failure(.loadingError))
}
task.resume()
requests[id] = task
return id
}
func cancelRequest(id: UUID) {
requests[id]?.cancel()
requests.removeValue(forKey: id)
print("ImageLoader: cancelling request")
}
How would we accomplish this (elegantly) with swift concurrency? Is it even possible or practical?
I haven't done much testing on this, but I believe this is what you're looking for. It allows you to simply await an image load, but you can cancel using the URL from somewhere else. It also merges near-simultaneous requests for the same URL so you don't re-download something you're in the middle of.
actor Loader {
private var tasks: [URL: Task<UIImage, Error>] = [:]
func loadImage(url: URL) async throws -> UIImage {
if let imageTask = tasks[url] {
return try await imageTask.value
}
let task = Task {
// Rather than removing here, you could skip that and this would become a
// cache of results. Making that correct would take more work than the
// question asks for, so I won't go into it
defer { tasks.removeValue(forKey: url) }
let data = try await URLSession.shared.data(from: url).0
guard let image = UIImage(data: data) else {
throw DecodingError.dataCorrupted(.init(codingPath: [],
debugDescription: "Invalid image"))
}
return image
}
tasks[url] = task
return try await task.value
}
func cancelRequest(url: URL) {
// Remove, and cancel if it's removed
tasks.removeValue(forKey: url)?.cancel()
print("ImageLoader: cancelling request")
}
}
Calling it looks like:
let image = try await loader.loadImage(url: url)
And you can cancel a request if it's still pending using:
loader.cancelRequest(url: url)
A key lesson here is that it is very natural to access task.value multiple times. If the task has already completed, then it will just return immediately.
Return a task in a tuple or other structure.
In the cases where you don't care about the ID, do this:
try await imageTask(url: url).task.value
private var requests: [UUID: Task<UIImage, Swift.Error>] = [:]
func imageTask(url: URL) -> (id: UUID?, task: Task<UIImage, Swift.Error>) {
switch loadedImages[url] {
case let image?: return (id: nil, task: .init { image } )
case nil:
let id = UUID()
let task = Task {
defer { requests[id] = nil }
guard let image = UIImage(data: try await URLSession.shared.data(from: url).0)
else { throw Error.loadingError }
try Task.checkCancellation()
Task { #MainActor in loadedImages[url] = image }
return image
}
requests[id] = task
return (id: id, task: task)
}
}
Is it even possible or practical?
Yes to both.
As I say in a comment, I think you may be missing the fact that a Task is an object you can retain and later cancel. Thus, if you create an architecture where you apply an ID to a task as you ask for the task to start, you can use that same ID to cancel that task before it has returned.
Here's a simple demonstration. I've deliberately written it as Playground code (though I actually developed it in an iOS project).
First, here is a general TimeConsumer class that wraps a single time-consuming Task. We can ask for the task to be created and started, but because we retain the task, we can also cancel it midstream. It happens that my task doesn't return a value, but that's neither here nor there; it could if we wanted.
class TimeConsumer {
var current: Task<(), Error>?
func consume(seconds: Int) async throws {
let task = Task {
try await Task.sleep(for: .seconds(seconds))
}
current = task
_ = await task.result
}
func cancel() {
current?.cancel()
}
}
Now then. In front of my TimeConsumer I'll put a TaskVendor actor. A TimeConsumer represents just one time-consuming task, but a TaskVendor has the ability to maintain multiple time-consuming tasks, identifying each task with an identifier.
actor TaskVendor {
private var tasks = [UUID: TimeConsumer?]()
func giveMeATokenPlease() -> UUID {
let uuid = UUID()
tasks[uuid] = nil
return uuid
}
func beginTheTask(uuid: UUID) async throws {
let consumer = TimeConsumer()
tasks[uuid] = consumer
try await consumer.consume(seconds: 10)
tasks[uuid] = nil
}
func cancel(uuid: UUID) {
tasks[uuid]??.cancel()
tasks[uuid] = nil
}
}
That's all there is to it! Observe how TaskVendor is configured. I can do three things: I can ask for a token (really my actual TaskVendor needn't bother doing this, but I wanted to centralize everything for generality); I can start the task with that token; and, optionally, I can cancel the task with that token.
So here's a simple test harness. Here we go!
let vendor = TaskVendor()
func test() async throws {
let uuid = await vendor.giveMeATokenPlease()
print("start")
Task {
try await Task.sleep(for: .seconds(2))
print("cancel?")
// await vendor.cancel(uuid: uuid)
}
try await vendor.beginTheTask(uuid: uuid)
print("finish")
}
Task {
try await test()
}
What you will see in the console is:
start
[two seconds later] cancel?
[eight seconds after that] finish
We didn't cancel anything; the word "cancel?" signals the place where our test might cancel, but we didn't, because I wanted to prove to you that this is working as we expect: it takes a total of 10 seconds between "start" and "finish", so sure enough, we are consuming the expected time fully.
Now uncomment the await vendor.cancel line. What you will see now is:
start
[two seconds later] cancel?
[immediately!] finish
We did it! We made a cancellable task vendor.
I'm including one possible answer to the question, for the benefit of others. I'll leave the question in place in case someone has another take on it.
The only way that I know of having a 'one-shot' async method that would return a token before returning the async result is by adding an inout argument:
func loadImage(url: URL, token: inout UUID?) async -> Result<UIImage, Error> {
token = UUID()
//...
}
Which we may call like this:
var requestToken: UUID? = nil
let result = await imageLoader.loadImage(url: url, token: &requestToken)
However, this approach and the two-shot solution by #matt both seem fussy, from the api design standpoint. Of course, as he suggests, this leads to a bigger question: How do we implement cancellation with swift concurrency (ideally without too much overhead)? And indeed, using tasks and wrapper objects seems unavoidable, but it certainly seems like a lot of legwork for a fairly simple pattern.
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.
This question already has answers here:
Returning data from async call in Swift function
(13 answers)
Closed last year.
I'm new at Swift and that's why i need your help. So I have a function which should send request and return a value
func getAnswer() -> String? {
var answer: String?
guard let url = URL(string: "https://8ball.delegator.com/magic/JSON/_") else { return nil }
URLSession.shared.dataTask(with: url) { data, response, error in
guard let data = data, error == nil else {
return
}
guard let response = response as? HTTPURLResponse else { return }
guard response.statusCode == 200 else { return }
do {
let model = try JSONDecoder().decode(Answer.self, from: data)
DispatchQueue.main.async {
answer = model.magic.answer
}
} catch let error {
fatalError(error.localizedDescription)
}
}.resume()
return answer
}
but it always returns nil.
I suppose problem is here
DispatchQueue.main.async {
answer = model.magic.answer
}
How can I fix it?
In order to know what is happening here, you need to learn about #escaping functions in swift, here is some link1 together with taking function as another functions parameter link2 written in part "Function Types as Parameter Types" , closures in Swift link3 and
Here is what is happening simplified and explained step by step :
you call getAnswer()
variable answer gets initialized with value nil by declaring answer: String?
URLSession.shared.dataTask is called and it is taking as an argument another function - closure (Data?, URLResponse?, Error?) -> Void . Also URLSession.shared.dataTask is executed on different thread and is not returning yet, but will return right after it receives response from server, which can take any time (but usually milliseconds) and will basically happen after your getAnswer() function is returning value.
your getAnswer() immediately returns value of answer which is currently nil
if you get any data from server, or server could not be reached, your URLSession.shared.dataTask function executes your code in closure. This is the code it will execute:
guard let data = data, error == nil else {
return
}
guard let response = response as? HTTPURLResponse else { return }
guard response.statusCode == 200 else { return }
do {
let model = try JSONDecoder().decode(Answer.self, from: data)
DispatchQueue.main.async {
answer = model.magic.answer
}
} catch let error {
fatalError(error.localizedDescription)
}
Your problem lies in how swift executes closures. When you call
URLSession.shared.dataTask(with: url) {
// Closure code here
}
return answer
Your "Closure code here" doesn't get called until the endpoint "https://8ball.delegator.com/magic/JSON/_" actually gives a response. However, you've promised swift that your function will return an optional string immediately after the serial code of your function has completed. For this reason, by the time your "Closure code here" has run, and your "answer" variable has been updated with the correct value, your function is long gone, and has already returned a value (which in this case is whatever you've set it to at the beginning - nil).
You can fix this issue in one of two ways.
Swift's new concurrency system
By defining your own closure.
Swift's new concurrency system
You can define your function as async, meaning that the function won't have to return a value in serial, as follows.
enum GetAnswerError: Error {
case invalidURL
}
func getAnswer() async throws -> String {
var answer: String?
guard let url = URL(string: "https://8ball.delegator.com/magic/JSON/_") else {
throw GetAnswerError.invalidURL
}
// Your function will suspend here and probably be moved to a different thread. It will resume once a response has been received from the endpoint.
let (data, _) = try await URLSession.shared.dataTask(with: url)
let parsedData = try JSONDecoder().decode(Answer.self, from: data)
return parsedData.magic.answer
}
When you call this function, you'll have to do so from an environment which swift can suspend. This means you'll call the function from either another async function like so
func anotherFunction() async throws -> Bool {
let answer = try await getAnswer()
// Run some code here
return answer == "YES" // Return some useful value
}
or from a Task object like so
Task {
// Note that because the function getAnswer() can throw errors, you'll have to handle them when you call the function. In this case, I'm handling them by using try?, which will simply set answer to nil if an error is thrown.
let answer = try? await getAnswer()
}
Note that when you call code in a task, you must be using the return value's from within the scope of the task. If you try to do something like this
func getAnswerTheSecond() -> String? {
var answer: String? = nil
Task {
let receivedAnswer = try? await getAnswer()
answer = receivedAnswer
}
return answer
}
You'll just end up back where you started, where swift immediately returns the nil value because your code is ran in serial. To fix this, run the relevant code on the "answer" from wherever it is needed within the task. If you are using the "answer" to update a SwiftUI view that might look like this.
struct ContentView: View {
#State var answer: String = ""
// This is the function that I've written earlier
func getAnswer() async throws -> String {
// Make URL Request
// Return the value
}
var body: some View {
Text(self.answer)
.onAppear{
Task{
let result = try? await self.getAnswer()
self.answer = result
}
}
}
}
Defining your own closure
You can define your own closure to handle the URL response; however, because of swift's new concurrency framework, this is probably not the right way to go.
If you'd like to go this way, do a google search for "Swift closures", and you'll find what you need.
I'm coming from JS and learning Swift to build an iOS native version of an app.
In JS, I use the following pattern all the time:
...
async function doAyncFunction(item) {
try {
// do async call to fetch data using item
return Promise.resolve(data);
} catch (error) {
return Promise.reject(error);
}
}
const promises = items.map((item) => doAyncFunction(item));
const results = await Promise.all(promises);
...
I've started looking at PromiseKit, but I'm wondering what's are the Swift ways of doing this?
Thanks.
The forthcoming Swift 5.5 in Xcode 13 (still in beta at this point in time) uses a very similar async-await pattern. See The Swift Programming Language: Concurrency.
In the interim, there are a unfortunately dizzying number of alternatives. For example, there are a variety of third-party promise/future frameworks. Or there is the declarative Combine framework, which was launched a few years agar with the advent of the non-imperative patterns of SwiftUI.
All of that having been said, the most common pattern you’ll see in Swift code is the use of escaping “closures” which are effectively units of code that are passed as a parameter to a function, and which the function invokes when the asynchronous task completes. In that pattern you don’t await, but rather just specify what you want to do when the asynchronous task finishes. For example, in this function, it has a parameter called completion which is a closure that is called when the asynchronous task completes:
func fetch(using value: Int, completion: #escaping (Result<Foo, Error>) -> Void) {
let url = …
let task = URLSession.shared.dataTask(with: url) { data, response, error in
// handle errors, if any, e.g.:
if let error == error else {
completion(.failure(error))
return
}
// parse data into `Foo` here, and when done, call the `completion closure:
…
completion(.success(foo))
}
task.resume()
}
And then you would call it like so:
fetch(using: 42, completion: { result in
// this is called when the fetch finishes
switch result {
case .failure(let error): // do something with `error`
case .success(let foo): // do something with `foo`
}
})
// note, execution will continue here, and the above closure will
// be called later, so do not try to use `foo` here
Or, using a more concise “trailing closure” syntax:
fetch(using: 42) { result in
// this is called when the fetch finishes
switch result {
case .failure(let error): // do something with `error`
case .success(let foo): // do something with `foo`
}
}
// note, execution will continue here, and the above closure will
// be called later, so do not try to use `foo` here
And if you wanted to be notified when a series of calls was done, you could use a DispatchGroup, e.g.
let group = DispatchGroup()
for value in values {
group.enter()
fetch(using: value) { result in
// do something with result
group.leave()
}
}
group.notify(queue: .main) {
// this is called when every `enter` call is matched up with a `leave` Call
}
It is up to you whether you stick to the beta version of Swift 5.5 with a very familiar async-await pattern, use a third-party future/promise library, use Combine, or use the traditional closure-based pattern, shown above.
At the very least, I would suggest familiarizing yourself with this latter pattern as it is the predominant technique in Swift right now. But rest assured that the familiar async-await pattern is coming soon, so if you are willing to wait for it to finish going through the beta process (or join that beta process), then check that out.
Using the aforementioned builtin Combine framework, you have several options. The one that you probably want is Publishers.Merge:
let publishers = ... // multiple functions that implement the Publisher protocol
let combined = Publishers.MergeMany(publishers)
Alternatives to MergeMany are Merge, Merge3, Merge4 up to Merge8 when the amount of publishers is set. If the number of outputs is variable, use MergeMany.
Other options include merge on the publishers themselves:
let publisher1 = ...
let publisher2 = ...
publisher1.merge(publisher2)
CombineLatest or, in the case of a publisher that immediately completes, Zip can be used to receive a tuple when everything is done:
let publisher1 = ...
let publisher2 = ...
Publishers.CombineLatest(publisher1, publisher2)
For the moment there is a great framework that is closest to async/await, it's SwiftCoroutine https://github.com/belozierov/SwiftCoroutine (much better than promiseKit, I tested the 2..)
Swift coroutine with your example:
func doFutureFunction() -> CoFuture<Int> {
CoFuture { promise in
myRequest { error, data in
if let error = error {
promise(.failure(error))
} else {
promise(.success(data))
}
}
}
}
let futures = items.map { item in doFutureFunction(item) } // [CoFuture<Int>]
DispatchQueue.main.startCoroutine {
let results = promises.compactMap { try? $0.await() } // [Int]
}
The equivalent of
consts value = await future.value
consts value1 = await future.value
consts value2 = await future.value
console.log("Value " + value + ", value1 " + value1 + ", value2 " + value2)
is
DispatchQueue.main.startCoroutine {
do {
let value = try future.await()
let value1 = try future.await()
let value2 = try future.await()
print("Value \(value), value1 \(value1), value2 \(value2)")
} catch {
print(error.localizedDescription)
}
}
While waiting for swift 5.5 and official async / await from Apple
You can look at PromiseQ it's javascript style promises for Swift. It implements all javascript's Promise features: resolve/reject, then, finally, fetch etc. and appends some additional ones: suspend/resume, cancel, retry, timeout etc.
It also supports all, race, any e.g.:
// Fetch avatars of first 30 GitHub users.
struct User : Codable {
let login: String
let avatar_url: String
}
async {
let response = try fetch("https://api.github.com/users").await()
guard response.ok else {
throw response.statusCodeDescription
}
guard let data = response.data else {
throw "No data"
}
let users = try JSONDecoder().decode([User].self, from: data)
let images =
try Promise.all(
users
.map { $0.avatar_url }
.map { fetch($0) }
).await()
.compactMap { $0.data }
.compactMap { UIImage(data: $0) }
async(.main) {
print(images.count)
}
}
.catch { error in
print(error.localizedDescription)
}
Swift's concurrency such as Dispatch queues, Combine and the newest async\await (Swift 5.5) are different from javascript Promises and you can not find many convenient approaches that you used before.
I'm answering myself here with a solution, using PromiseKit, in case it might help someone.
The below is obviously not a full implementation, but it shows how the pattern can be implemented.
func doManyAsyncRequests(userIds: [String], accessToken: String) -> Promise<Void> {
Promise { seal in
let promises = spotifyUserIds.map {
doSingleAsyncRequest(userId: $0.id, accessToken: accessToken) // this function returns a promise
}
when(fulfilled: promises).done { results in
print("Results: \(results)")
// process results
}.catch { error in
print("\(error)")
// handle error
}
}
}
I am in the process of implementing a REST API with Swift. Of course, part of this API is using HTTP requests to retrieve and send data.
Full disclosure, I am inexperienced with Swift and am using this as a learning project to get my feet wet, so to speak. But it's turned into much more of a difficult project than I anticipated.
In implementing the first get method, I have (finally) gotten rid of all the compilation errors. However, when I call the function which utilizes the URLRequest, URLSession, dataTask, etc, it is never entered.
Upon debugging the program, I can watch the program execution reach the CompletionHandler, and skip over it right to "task.resume()."
A similar construction works in a Swift Playground, but does not work in the actual project proper.
So far I have tried a few things, namely making the function access a class instance variable, in hopes that that would force it to execute. But it does not.
I think the issue may be dealing with synchronicity, and perhaps I need to use a Semaphore, but I want to make sure I'm not missing anything obvious first.
import Foundation
/**
A class to wrap all GET and POST requests, to avoid the necessity of repeatedly writing request code in each API method.
*/
class BasicRequest {
private var url: URL
private var header: [String: String]
private var responseType: String
private var jsonResponse: Any?
init(url: URL, header: [String: String], responseType: String) {
self.url = url
self.header = header
self.responseType = responseType
} //END INIT
public func requestJSON() -> Any {
// Create the URLRequest object, and fill the header with the header fields as provided.
var urlRequest = URLRequest(url: self.url)
for (value, key) in self.header {
urlRequest.addValue(value, forHTTPHeaderField: key)
}
let task = URLSession.shared.dataTask(with: urlRequest) { (data, response, error) in
print("Entered the completion handler")
if error != nil {
return
}
guard let httpResponse = response as? HTTPURLResponse, 200 == httpResponse.statusCode else {
print("HTTP Request unsuccessful")
return
}
guard let mime = response?.mimeType, mime == "application/json" else {
print("Not a JSON response")
return
}
do {
let json = try JSONSerialization.jsonObject(with: data!, options: [])
print(json)
self.jsonResponse = json
} catch {
print("Could not transform to JSON")
return
}
}
task.resume()
return "Function has returned"
} //END REQUESTJSON
}
The expected result would be returning a JSON object, however that does not seem to be the case.
With respect to error messages, I get none. The only log I get in the debugger is the boilerplate "process exited with code 0."
To be truthful, I'm at a loss with what is causing this not to work.
It appears you're writing this in a command-line app. In that case the program is terminating before the URLRequest completes.
I think the issue may be dealing with synchronicity, and perhaps I need to use a Semaphore, but I want to make sure I'm not missing anything obvious first.
Exactly.
The typical tool in Swift is DispatchGroup, which is just a higher-level kind of semaphore. Call dispatchGroup.enter() before starting the request, and all dispatchGroup.leave() at the end of the completion handler. In your calling code, include dispatchGroup.wait() to wait for it. (If that's not clear, I can add code for it, but there are also a lot of SO answers you can find that will demonstrate it.)