URLSession and the Mix framework

That is going to be a very brief, however hopefully very helpful tutorial about how I began to make the most of the Mix framework to slowly exchange my Promise library. 🤫

API & knowledge construction

To begin with we will want some form of API to attach, as standard I’ll use my favourite JSONPlaceholder service with the next knowledge fashions:

enum HTTPError: LocalizedError {
    case statusCode
    case publish
}

struct Put up: Codable {

    let id: Int
    let title: String
    let physique: String
    let userId: Int
}

struct Todo: Codable {

    let id: Int
    let title: String
    let accomplished: Bool
    let userId: Int
}

Nothing particular to date, just a few fundamental Codable components, and a easy error, as a result of hell yeah, we wish to present some error if one thing fails. ❌

The normal approach

Doing an HTTP request in Swift is fairly straightforward, you should use the built-in shared URLSession with a easy knowledge job, and voilá there’s your response. After all you would possibly wish to test for legitimate standing code and if all the pieces is ok, you’ll be able to parse your response JSON by utilizing the JSONDecoder object from Basis.

let url = URL(string: "https://jsonplaceholder.typicode.com/posts")!

let job = URLSession.shared.dataTask(with: url) { knowledge, response, error in
    if let error = error {
        fatalError("Error: (error.localizedDescription)")
    }
    guard let response = response as? HTTPURLResponse, response.statusCode == 200 else {
        fatalError("Error: invalid HTTP response code")
    }
    guard let knowledge = knowledge else {
        fatalError("Error: lacking response knowledge")
    }

    do {
        let decoder = JSONDecoder()
        let posts = attempt decoder.decode([Post].self, from: knowledge)
        print(posts.map { $0.title })
    }
    catch {
        print("Error: (error.localizedDescription)")
    }
}
job.resume()

Remember to renew your knowledge job or the request will not hearth in any respect. 🔥

Knowledge duties and the Mix framework

Now as you’ll be able to see the standard “block-based” method is good, however can we do possibly one thing higher right here? You recognize, like describing the entire thing as a series, like we used to do that with Guarantees? Starting from iOS13 with the assistance of the superb Mix framework you truly can go far past! 😃

My favourite a part of Mix is reminiscence administration & cancellation.

Knowledge job with Mix

So the commonest instance is often the next one:

personal var cancellable: AnyCancellable?

self.cancellable = URLSession.shared.dataTaskPublisher(for: url)
.map { $0.knowledge }
.decode(sort: [Post].self, decoder: JSONDecoder())
.replaceError(with: [])
.eraseToAnyPublisher()
.sink(receiveValue: { posts in
    print(posts.depend)
})

self.cancellable?.cancel()

I like how the code “explains itself”:

• First we make a cancellable storage on your Writer
• Then we create a model new knowledge job writer object
• Map the response, we solely care concerning the knowledge half (ignore errors)
• Decode the content material of the info utilizing a JSONDecoder
• If something goes improper, simply go along with an empty array
• Erase the underlying complexity to a easy AnyPublisher
• Use sink to show some information concerning the remaining worth
• Non-compulsory: you’ll be able to cancel your community request any time

Error dealing with

Let’s introduce some error dealing with, as a result of I do not like the concept of hiding errors. It is so a lot better to current an alert with the precise error message, is not it? 🤔

enum HTTPError: LocalizedError {
    case statusCode
}

self.cancellable = URLSession.shared.dataTaskPublisher(for: url)
.tryMap { output in
    guard let response = output.response as? HTTPURLResponse, response.statusCode == 200 else {
        throw HTTPError.statusCode
    }
    return output.knowledge
}
.decode(sort: [Post].self, decoder: JSONDecoder())
.eraseToAnyPublisher()
.sink(receiveCompletion: { completion in
    swap completion {
    case .completed:
        break
    case .failure(let error):
        fatalError(error.localizedDescription)
    }
}, receiveValue: { posts in
    print(posts.depend)
})

In a nutshell, this time we test the response code and if one thing goes improper we throw an error. Now as a result of the writer may end up in an error state, sink has one other variant, the place you’ll be able to test the result of your entire operation so you are able to do your individual error thingy there, like displaying an alert. 🚨

Assign consequence to property

One other frequent sample is to retailer the response in an inner variable someplace within the view controller. You possibly can merely do that by utilizing the assign operate.

class ViewController: UIViewController {

    personal var cancellable: AnyCancellable?

    personal var posts: [Post] = [] {
        didSet {
            print("posts --> (self.posts.depend)")
        }
    }

    override func viewDidLoad() {
        tremendous.viewDidLoad()

        let url = URL(string: "https://jsonplaceholder.typicode.com/posts")!

        self.cancellable = URLSession.shared.dataTaskPublisher(for: url)
        .map { $0.knowledge }
        .decode(sort: [Post].self, decoder: JSONDecoder())
        .replaceError(with: [])
        .eraseToAnyPublisher()
        .assign(to: .posts, on: self)
    }
}

Very straightforward, you may also use the didSet property observer to get notified about modifications.

Group a number of requests

Sending a number of requests was a painful course of prior to now. Now now we have Compose and this job is simply ridiculously straightforward with Publishers.Zip. You possibly can actually mix a number of requests togeter and wait till each of them are completed. 🤐

let url1 = URL(string: "https://jsonplaceholder.typicode.com/posts")!
let url2 = URL(string: "https://jsonplaceholder.typicode.com/todos")!

let publisher1 = URLSession.shared.dataTaskPublisher(for: url1)
.map { $0.knowledge }
.decode(sort: [Post].self, decoder: JSONDecoder())

let publisher2 = URLSession.shared.dataTaskPublisher(for: url2)
.map { $0.knowledge }
.decode(sort: [Todo].self, decoder: JSONDecoder())

self.cancellable = Publishers.Zip(publisher1, publisher2)
.eraseToAnyPublisher()
.catch { _ in
    Simply(([], []))
}
.sink(receiveValue: { posts, todos in
    print(posts.depend)
    print(todos.depend)
})

Identical sample as earlier than, we’re simply zipping collectively two publishers.

Request dependency

Generally it’s important to load a useful resource from a given URL, after which use one other one to increase the article with one thing else. I am speaking about request dependency, which was fairly problematic with out Mix, however now you’ll be able to chain two HTTP calls along with just some traces of Swift code. Let me present you:

override func viewDidLoad() {
    tremendous.viewDidLoad()

    let url1 = URL(string: "https://jsonplaceholder.typicode.com/posts")!

    self.cancellable = URLSession.shared.dataTaskPublisher(for: url1)
    .map { $0.knowledge }
    .decode(sort: [Post].self, decoder: JSONDecoder())
    .tryMap { posts in
        guard let id = posts.first?.id else {
            throw HTTPError.publish
        }
        return id
    }
    .flatMap { id in
        return self.particulars(for: id)
    }
    .sink(receiveCompletion: { completion in

    }) { publish in
        print(publish.title)
    }
}

func particulars(for id: Int) -> AnyPublisher<Put up, Error> {
    let url = URL(string: "https://jsonplaceholder.typicode.com/posts/(id)")!
    return URLSession.shared.dataTaskPublisher(for: url)
        .mapError { $0 as Error }
        .map { $0.knowledge }
        .decode(sort: Put up.self, decoder: JSONDecoder())
        .eraseToAnyPublisher()
}

The trick right here is you could flatMap a writer into one other.

Conclusion

Mix is a tremendous framework, it may possibly do so much, however it undoubtedly has some studying curve. Sadly you’ll be able to solely use it in case you are focusing on iOS13 or above (which means you’ve got one entire 12 months to study each single little bit of the framework) so assume twice earlier than adopting this new expertise.

You must also be aware that at present there isn’t a add and obtain job writer, however you may make your very personal answer till Apple formally releases one thing. Fingers crossed. 🤞

I actually love how Apple applied some ideas of reactive programming, I am unable to anticipate Mix to reach as an open supply bundle with linux assist as nicely. ❤️