URLSession and the Mix framework

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

API & information construction

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

enum HTTPError: LocalizedError {
    case statusCode
    case submit
}

struct Submit: 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 up to now, 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 standard manner

Doing an HTTP request in Swift is fairly simple, you need to use the built-in shared URLSession with a easy information activity, and voilá there’s your response. After all you may wish to examine for legitimate standing code and if all the things is ok, you may parse your response JSON through the use of the JSONDecoder object from Basis.

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

let activity = URLSession.shared.dataTask(with: url) { information, 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 information = information else {
        fatalError("Error: lacking response information")
    }

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

Remember to renew your information activity or the request will not hearth in any respect. 🔥

Information duties and the Mix framework

Now as you may see the normal “block-based” strategy is sweet, however can we do possibly one thing higher right here? You realize, 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 really can go far past! 😃

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

Information activity with Mix

So the most typical instance is often the next one:

non-public var cancellable: AnyCancellable?

self.cancellable = URLSession.shared.dataTaskPublisher(for: url)
.map { $0.information }
.decode(kind: [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 in your Writer
• Then we create a model new information activity writer object
• Map the response, we solely care in regards to the information half (ignore errors)
• Decode the content material of the info utilizing a JSONDecoder
• If something goes mistaken, simply go along with an empty array
• Erase the underlying complexity to a easy AnyPublisher
• Use sink to show some data in regards to the remaining worth
• Optionally available: you may 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 thought 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.information
}
.decode(kind: [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 examine the response code and if one thing goes mistaken we throw an error. Now as a result of the writer can lead to an error state, sink has one other variant, the place you may examine the end result of all the operation so you are able to do your personal error thingy there, like displaying an alert. 🚨

Assign consequence to property

One other frequent sample is to retailer the response in an inside variable someplace within the view controller. You possibly can merely do that through the use of the assign operate.

class ViewController: UIViewController {

    non-public var cancellable: AnyCancellable?

    non-public 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.information }
        .decode(kind: [Post].self, decoder: JSONDecoder())
        .replaceError(with: [])
        .eraseToAnyPublisher()
        .assign(to: .posts, on: self)
    }
}

Very simple, you can even 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 previously. Now now we have Compose and this activity is simply ridiculously simple 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.information }
.decode(kind: [Post].self, decoder: JSONDecoder())

let publisher2 = URLSession.shared.dataTaskPublisher(for: url2)
.map { $0.information }
.decode(kind: [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 a must to load a useful resource from a given URL, after which use one other one to increase the thing with one thing else. I am speaking about request dependency, which was fairly problematic with out Mix, however now you may chain two HTTP calls along with only a few strains 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.information }
    .decode(kind: [Post].self, decoder: JSONDecoder())
    .tryMap { posts in
        guard let id = posts.first?.id else {
            throw HTTPError.submit
        }
        return id
    }
    .flatMap { id in
        return self.particulars(for: id)
    }
    .sink(receiveCompletion: { completion in

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

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

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

Conclusion

Mix is a tremendous framework, it will probably do so much, nevertheless it undoubtedly has some studying curve. Sadly you may solely use it if you’re focusing on iOS13 or above (because of this you may have one entire 12 months to be taught each single little bit of the framework) so assume twice earlier than adopting this new expertise.

You must also word that presently there isn’t any add and obtain activity writer, however you can also make your very personal resolution till Apple formally releases one thing. Fingers crossed. 🤞

I actually love how Apple applied some ideas of reactive programming, I can not look forward to Mix to reach as an open supply bundle with linux assist as nicely. ❤️