Why aren't my Swift network requests working? - swift

Question

I've been trying out Swift/SwiftUI for the first time, so I decided to make a small Hacker News client. I seem to be able to get the list of ids of the top stories back fine, but once the dispatchGroup gets involved, nothing works. What am I doing wrong?
Data.swift
import SwiftUI
struct HNStory: Codable, Identifiable {
var id: UInt
var title: String
var score: UInt
}
class Fetch {
func getStory(id: Int, completion: #escaping (HNStory) -> ()) {
let url = URL(string: "https://hacker-news.firebaseio.com/v0/item/\(id).json")
URLSession.shared.dataTask(with: url!) { (data, _, _) in
let story = try!JSONDecoder().decode(HNStory.self, from: data!)
print(id, story)
DispatchQueue.main.async {
completion(story)
}
}
}
func getStories(completion: #escaping ([HNStory]) -> ()) {
let url = URL(string: "https://hacker-news.firebaseio.com/v0/topstories.json")
var stories: [HNStory] = []
print("here")
URLSession.shared.dataTask(with: url!) { (data, _, _) in
var ids = try!JSONDecoder().decode([Int].self, from: data!)
ids = Array(ids[0...10])
print(ids)
let dispatchGroup = DispatchGroup()
for id in ids {
dispatchGroup.enter()
self.getStory(id: id) { (story) in
stories.append(story)
dispatchGroup.leave()
}
}
dispatchGroup.notify(queue: .main) {
print("Completed work")
DispatchQueue.main.async {
completion(stories)
}
}
}.resume()
}
}
ContentView.swift (probably doesn't matter, but just in case)
import SwiftUI
struct ContentView: View {
#State private var stories: [HNStory] = []
var body: some View {
Text("Hacker News").font(.headline)
List(stories) { story in
VStack {
Text(story.title)
Text(story.score.description)
}
}.onAppear{
Fetch().getStories { (stories) in
self.stories = stories
}
}
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
}

Answer 1

One major problem is this line:
Fetch().getStories...
Networking takes time. You make a Fetch instance and immediately let it destroy itself. Thus it does not survive long enough to do any networking! You need to configure a singleton that persists.
Another problem, as OOPer points out in a comment, is that your getStory creates a data task but never tells it to resume — so that method is doing no networking at all, even if it did have time to do so.

Answer 2

FWIW, with Swift UI, I would suggest you consider using Combine publishers for your network requests.
So, the publisher to get a single story:
func storyUrl(for id: Int) -> URL {
URL(string: "https://hacker-news.firebaseio.com/v0/item/\(id).json")!
}
func hackerNewsStoryPublisher(for identifier: Int) -> AnyPublisher<HNStory, Error> {
URLSession.shared.dataTaskPublisher(for: storyUrl(for: identifier))
.map(\.data)
.decode(type: HNStory.self, decoder: decoder)
.eraseToAnyPublisher()
}
And a publisher for a sequence of the above:
func hackerNewsIdsPublisher(for ids: [Int]) -> AnyPublisher<HNStory, Error> {
Publishers.Sequence(sequence: ids.map { hackerNewsStoryPublisher(for: $0) })
.flatMap(maxPublishers: .max(4)) { $0 }
.eraseToAnyPublisher()
}
Note, the above constrains it to four at a time, enjoying the performance gains of concurrency, but limiting it so you do not risk having latter requests time out:
Anyway, here is the first fetch of the array of ids and then launching the above:
let mainUrl = URL(string: "https://hacker-news.firebaseio.com/v0/topstories.json")!
func hackerNewsPublisher() -> AnyPublisher<HNStory, Error> {
URLSession.shared.dataTaskPublisher(for: mainUrl)
.map(\.data)
.decode(type: [Int].self, decoder: decoder)
.flatMap { self.hackerNewsIdsPublisher(for: $0) }
.receive(on: RunLoop.main)
.eraseToAnyPublisher()
}
(Now, you could probably cram all of the above into a single publisher, but I like to keep them small, so each individual publisher is very easy to reason about.)
So, pulling that all together, you have a view model like so:
import Combine
struct HNStory: Codable, Identifiable {
var id: UInt
var title: String
var score: UInt
}
class ViewModel: ObservableObject {
#Published var stories: [HNStory] = []
private let networkManager = NetworkManager()
private var request: AnyCancellable?
func fetch() {
request = networkManager.hackerNewsPublisher().sink { completion in
if case .failure(let error) = completion {
print(error)
}
} receiveValue: {
self.stories.append($0)
}
}
}
class NetworkManager {
private let decoder = JSONDecoder()
let mainUrl = URL(string: "https://hacker-news.firebaseio.com/v0/topstories.json")!
func storyUrl(for id: Int) -> URL {
URL(string: "https://hacker-news.firebaseio.com/v0/item/\(id).json")!
}
func hackerNewsPublisher() -> AnyPublisher<HNStory, Error> {
URLSession.shared.dataTaskPublisher(for: mainUrl)
.map(\.data)
.decode(type: [Int].self, decoder: decoder)
.flatMap { self.hackerNewsIdsPublisher(for: $0) }
.receive(on: RunLoop.main)
.eraseToAnyPublisher()
}
// publisher for array of news stories, processing max of 4 at a time
func hackerNewsIdsPublisher(for ids: [Int]) -> AnyPublisher<HNStory, Error> {
Publishers.Sequence(sequence: ids.map { hackerNewsStoryPublisher(for: $0) })
.flatMap(maxPublishers: .max(4)) { $0 }
.eraseToAnyPublisher()
}
// publisher for single news story
func hackerNewsStoryPublisher(for identifier: Int) -> AnyPublisher<HNStory, Error> {
URLSession.shared.dataTaskPublisher(for: storyUrl(for: identifier))
.map(\.data)
.decode(type: HNStory.self, decoder: decoder)
.eraseToAnyPublisher()
}
}
And your main ContentView is:
struct ContentView: View {
#ObservedObject var viewModel = ViewModel()
var body: some View {
Text("Hacker News").font(.headline)
List(viewModel.stories) { story in
VStack {
Text(story.title)
Text(story.score.description)
}
}.onAppear {
viewModel.fetch()
}
}
}

Answer 3

By calling Fetch().getStories, the Fetch class goes out of scope immediately and isn't retained.
I'd recommend making Fetch an ObservableObject and setting it as a property on your view:
#StateObject var fetcher = Fetch()
Then, call:
fetcher.getStories {
self.stories = stories
}
If you wanted to get even more SwiftUI-ish with it, you may want to look into #Published properties on ObservableObjects and how you can make your view respond to them automatically. By doing this, you could avoid having a #State variable on your view at all, not have to have a callback function, and instead just load the stories into a #Published property on the ObservableObject. Your view will be re-rendered when the #Published property changes. More reading: https://www.hackingwithswift.com/quick-start/swiftui/observable-objects-environment-objects-and-published