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 change my Promise library. 🤫

API & information construction

To start with we’ll want some sort of API to attach, as traditional I’ll use my favourite JSONPlaceholder service with the next information 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 up to now, just a few primary Codable components, and a easy error, as a result of hell yeah, we need to present some error if one thing fails. ❌

The standard means

Doing an HTTP request in Swift is fairly straightforward, you need to use the built-in shared URLSession with a easy information job, and voilá there’s your response. After all you may need to examine for legitimate standing code and if every part is okay, 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 job = 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 = strive decoder.decode([Post].self, from: information)
        print(posts.map { $0.title })
    }
    catch {
        print("Error: (error.localizedDescription)")
    }
}
job.resume()

Do not forget to renew your information job or the request will not hearth in any respect. 🔥

Knowledge duties and the Mix framework

Now as you may see the standard “block-based” method is sweet, however can we do possibly one thing higher right here? , like describing the entire thing as a sequence, like we used to do that with Guarantees? Starting from iOS13 with the assistance of the wonderful Mix framework you really 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:

non-public var cancellable: AnyCancellable?

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

self.cancellable?.cancel()

I really like how the code “explains itself”:

• First we make a cancellable storage on your Writer
• Then we create a model new information job writer object
• Map the response, we solely care concerning the information half (ignore errors)
• Decode the content material of the information utilizing a JSONDecoder
• If something goes mistaken, simply go together with an empty array
• Erase the underlying complexity to a easy AnyPublisher
• Use sink to show some information concerning the last worth
• Non-compulsory: 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 concept of hiding errors. It is so significantly 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(sort: [Post].self, decoder: JSONDecoder())
.eraseToAnyPublisher()
.sink(receiveCompletion: { completion in
    change completion {
    case .completed:
        break
    case .failure(let error):
        fatalError(error.localizedDescription)
    }
}, receiveValue: { posts in
    print(posts.rely)
})

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 may end up in an error state, sink has one other variant, the place you may examine the end result of your entire operation so you are able to do your personal error thingy there, like displaying an alert. 🚨

Assign end result 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 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.rely)")
        }
    }

    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(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 adjustments.

Group a number of requests

Sending a number of requests was a painful course of previously. Now we now 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.information }
.decode(sort: [Post].self, decoder: JSONDecoder())

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

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

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

Request dependency

Generally you must 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 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.information }
    .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.information }
        .decode(sort: Put up.self, decoder: JSONDecoder())
        .eraseToAnyPublisher()
}

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

Conclusion

Mix is an incredible framework, it might probably do so much, nevertheless it positively has some studying curve. Sadly you may solely use it if you’re concentrating on iOS13 or above (because of this you will have one complete 12 months to be taught each single little bit of the framework) so suppose twice earlier than adopting this new know-how.

You must also word that at present there isn’t any add and obtain job 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 am unable to look ahead to Mix to reach as an open supply package deal with linux assist as properly. ❤️