Subsequent ordered HTTP calls - swift

Question

I'm building a simple iOS client for HackerNews. I'm using their APIs, according to which I'll be able to get the ordered post IDs (sorted by new, best and top) and a single post item passing the ID to the request. The problem I'm facing is the following: how can I, once I get the IDs array, make an HTTP call for every post in an ordered fashion? With the way I currently implemented it, I'm not having any luck.
E.g. say the IDs array is [3001, 3002, 3003, 3004]. I tried calling the method to get those posts inside a for loop issuing dispatch groups and dispatch semaphores, but I still get them unordered, like the call for item 3003 completes before 3002, and so on.
The methods I'm using:
#Published var posts: [Post] = []
func getPosts(feedType: FeedType){
posts = []
self.getFeedIDs(feedType: feedType).subscribe{ ids in
let firstFifteen = ids[0...15]
let dGroup = DispatchGroup()
let dQueue = DispatchQueue(label: "network-queue")
let dSemaphore = DispatchSemaphore(value: 0)
dQueue.async {
for id in firstFifteen{
dGroup.enter()
self.getPost(id: id).subscribe{ post in
self.posts.append(post)
dSemaphore.signal()
dGroup.leave()
}
dSemaphore.wait()
}
}
}
}
func getFeedIDs(feedType: FeedType) -> Observable<[Int]> {
return self.execute(url: URL(string: "https://hacker-news.firebaseio.com/v0/\(feedType)stories.json")!)
}
func getPost(id: Int) -> Observable<Post>{
return self.execute(url: URL(string: "https://hacker-news.firebaseio.com/v0/item/\(id).json")!)
}
func execute <T: Decodable>(url: URL) -> Observable<T> {
return Observable.create { observer -> Disposable in
let task = URLSession.shared.dataTask(with: url) { res, _, _ in
guard let data = res, let decoded = try? JSONDecoder().decode(T.self, from: data) else {
return
}
observer.onNext(decoded)
observer.onCompleted()
}
task.resume()
return Disposables.create {
task.cancel()
}
}
}
Any help would be greatly appreciated.

Answer 1

The semaphore makes no sense and is inefficient anyway.
Use the same pattern which Apple suggests in conjunction with TaskGroups: Collect the data in a dictionary and after being notified sort the data by the dictionary keys
func getPosts(feedType: FeedType){
var postData = [Int:Post]()
posts = []
self.getFeedIDs(feedType: feedType).subscribe{ ids in
let firstFifteen = ids[0...15]
let dGroup = DispatchGroup()
for (index, element) in firstFifteen.enumerated() {
dGroup.enter()
self.getPost(id: element).subscribe{ post in
postData[index] = post
dGroup.leave()
}
}
dGroup.notify(queue: .main) {
for key in postData.keys.sorted() {
posts.append(postData[key]!)
}
}
}
}