Swift: How to perform concurrent API calls using Combine

Question

I am attempting to perform concurrent API calls using the Combine framework. The API calls are set up like so:

First, call an API to get a list of Posts
For each post, call another API to get Comments

I would like to use Combine to chain these two calls together and concurrently so that it returns an array of Post objects with each post containing the comments array.

My attempt:

struct Post: Decodable { let userId: Int let id: Int let title: String let body: String var comments: [Comment]? } struct Comment: Decodable { let postId: Int let id: Int let name: String let email: String let body: String } class APIClient: ObservableObject { @Published var posts = [Post]() var cancellables = Set<AnyCancellable>() init() { getPosts() } func getPosts() { let urlString = "https://jsonplaceholder.typicode.com/posts" guard let url = URL(string: urlString) else {return} URLSession.shared.dataTaskPublisher(for: url) .receive(on: DispatchQueue.main) .tryMap({ (data, response) -> Data in guard let response = response as? HTTPURLResponse, response.statusCode >= 200 else { throw URLError(.badServerResponse) } return data }) .decode(type: [Post].self, decoder: JSONDecoder()) .sink { (completion) in print("Posts completed: \(completion)") } receiveValue: { (output) in //Is there a way to chain getComments such that receiveValue would contain Comments?? output.forEach { (post) in self.getComments(post: post) } } .store(in: &cancellables) } func getComments(post: Post) { let urlString = "https://jsonplaceholder.typicode.com/posts/\(post.id)/comments" guard let url = URL(string: urlString) else { return } URLSession.shared.dataTaskPublisher(for: url) .receive(on: DispatchQueue.main) .tryMap({ (data, response) -> Data in guard let response = response as? HTTPURLResponse, response.statusCode >= 200 else { throw URLError(.badServerResponse) } return data }) .decode(type: [Comment].self, decoder: JSONDecoder()) .sink { (completion) in print("Comments completed: \(completion)") } receiveValue: { (output) in print("Comment", output) } .store(in: &cancellables) } }

How do I chain getComments to getPosts so that the output of comments can be received in getPosts? Traditionally using UIKit, I would use DispatchGroup for this task.

Note that I would like to receive just a single Publisher event for posts from the APIClient so that the SwiftUI view is refreshed only once.

@DanChase There is a CombineLatest, but I imagine that this method would listen for 2 publishers. Any idea how do I apply it in the above use case? — Koh
– Koh, Commented Jul 10, 2021 at 5:25
@Koh Sorry I misread the use-case. Not an actual answer, but In the past I have solved this by creating a combined structure in the back-end, and just having the one call on the HTTP side. On the back-end the API surface can call the multiple business layer functions and create a structure to return. In a current project I'm working on, it had grown to 8 HTTP Get's, and I started having problems with some returning before others and causing confusion for the user, as well as issues with stalling the browser. HTTP 1.1 I believe has a limit of 6.. I hope this helps more than my prev. comment. — Dan Chase
– Dan Chase, Commented Jul 10, 2021 at 5:48
@Koh thinking more about combineLatest, I think my thought process was about using combineLatest with 2 observables, one for master, and one for detail, for each loop. But the more I thought about it the worse the idea seemed, which led to my comment above. — Dan Chase
– Dan Chase, Commented Jul 10, 2021 at 5:51

Koh · Accepted Answer · 2021-07-10 07:28:38Z

Thanks to @matt's post in the comments above, I've adapted the solution in that SO post for my use case above.

Not too sure if it is the best implementation, but it addresses my problem for now.

 func getPosts() { let urlString = "https://jsonplaceholder.typicode.com/posts" guard let url = URL(string: urlString) else {return} URLSession.shared.dataTaskPublisher(for: url) .receive(on: DispatchQueue.main) .tryMap({ (data, response) -> Data in guard let response = response as? HTTPURLResponse, response.statusCode >= 200 else { throw URLError(.badServerResponse) } return data }) .decode(type: [Post].self, decoder: JSONDecoder()) .flatMap({ (posts) -> AnyPublisher<Post, Error> in //Because we return an array of Post in decode(), we need to convert it into an array of publishers but broadcast as 1 publisher Publishers.Sequence(sequence: posts).eraseToAnyPublisher() }) .compactMap({ post in //Loop over each post and map to a Publisher self.getComments(post: post) }) .flatMap {$0} //Receives the first element, ie the Post .collect() //Consolidates into an array of Posts .sink(receiveCompletion: { (completion) in print("Completion:", completion) }, receiveValue: { (posts) in self.posts = posts }) .store(in: &cancellables) } func getComments(post: Post) -> AnyPublisher<Post, Error>? { let urlString = "https://jsonplaceholder.typicode.com/posts/\(post.id)/comments" guard let url = URL(string: urlString) else { return nil } let publisher = URLSession.shared.dataTaskPublisher(for: url) .receive(on: DispatchQueue.main) .tryMap({ (data, response) -> Data in guard let response = response as? HTTPURLResponse, response.statusCode >= 200 else { throw URLError(.badServerResponse) } return data }) .decode(type: [Comment].self, decoder: JSONDecoder()) .tryMap { (comments) -> Post in var newPost = post newPost.comments = comments return newPost } .eraseToAnyPublisher() return publisher }

Essentially, we will need to return a Publisher from the getComments method so that we can loop over each publisher inside getPosts.

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