The way to obtain information with URLSession utilizing Mix Publishers and Subscribers?

A easy picture downloader

Downloading a useful resource from an URL looks like a trivial activity, however is it actually that simple? Properly, it relies upon. If it’s a must to obtain and parse a JSON file which is just some KB, then you may go along with the classical means or you should utilize the brand new dataTaskPublisher technique on the URLSession object from the Mix framework.

Dangerous practices ⚠️

There are some fast & soiled approaches that you should utilize to get some smaller information from the web. The issue with these strategies is that it’s a must to deal loads with threads and queues. Thankfully utilizing the Dispatch framework helps loads, so you may flip your blocking capabilities into non-blocking ones. 🚧

let url = URL(string: "https://jsonplaceholder.typicode.com/todos/1")!


do {
    
    let content material = strive String(contentsOf: url)
    print(content material)

    
    let information = strive Information(contentsOf: url)
}
catch {
    print(error.localizedDescription)
}



DispatchQueue.world().async { [weak self] in
    
    do {
        let content material = strive String(contentsOf: url)
        DispatchQueue.foremost.async {
            
            print(content material)
        }
    }
    catch {
        print(error.localizedDescription)
    }
}

Apple made an essential notice on their official Information documentation, that you must NOT use these strategies for downloading non-file URLs, however nonetheless individuals are educating / utilizing these dangerous practices, however why? 😥

Do not use this synchronous technique to request network-based URLs.

My recommendation right here: all the time use the URLSession to carry out community associated data-transfers. Creating a knowledge activity is easy, it is an asynchronous operation by default, the callback runs on a background thread, so nothing will probably be blocked by default. Trendy networking APIs are actual good on iOS, in 99% of the instances you will not want Alamofire anymore for these sort of duties. Say no to dependencies! 🚫

URLSession.shared.dataTask(with: url) { information, response, error in
    
    DispatchQueue.foremost.async {
        
    }
}.resume()

It is also price to say if it’s worthwhile to use a unique HTTP technique (aside from GET), ship particular headers (credentials, settle for insurance policies, and so forth.) or present further information within the physique, it’s worthwhile to assemble an URLRequest object first. You may solely ship these customized requests utilizing the URLSession APIs.

on Apple platforms you aren’t allowed to make use of the unsecure HTTP protocol anymore. If you wish to attain a URL with out the safe layer (HTTPS) it’s a must to disable App Transport Safety.

The issue with information duties

What about huge information, equivalent to photographs? Let me present you just a few tutorials earlier than we dive in:

With all due respect, I feel all of those hyperlinks above are actually dangerous examples of loading distant photographs. Certain they do the job, they’re additionally very simple to implement, however perhaps we must always cowl the entire story… 🤐

For small interactions with distant servers, you should utilize the URLSessionDataTask class to obtain response information into reminiscence (versus utilizing the URLSessionDownloadTask class, which shops the information on to the file system). A knowledge activity is good for makes use of like calling an online service endpoint.

What’s distinction between URLSessionDataTask vs URLSessionDownloadTask?

If we learn the docs rigorously, it turns into clear that information activity is NOT the suitable candidate for downloading huge belongings. That class is designed to request solely smaller objects, because the underlying information goes to be loaded into reminiscence. Then again the obtain activity saves the content material of the response on the disk (as an alternative of reminiscence) and you’ll obtain an area file URL as an alternative of a Information object. Seems that transferring from information duties to obtain duties can have a HUGE impression in your reminiscence consumption. I’ve some numbers. 📈

I downloaded the following picture file (6000x4000px 💾 13,1MB) utilizing each strategies. I made a model new storyboard based mostly Swift 5.1 challenge. The essential RAM utilization was ~52MB, once I fetched the picture utilizing the URLSessionDataTask class, the reminiscence utilization jumped to ~82MB. Turning the information activity right into a obtain activity solely elevated the bottom reminiscence dimension by ~4MB (to a complete ~56MB), which is a major enchancment.

let url = URL(string: "https://photographs.unsplash.com/photo-1554773228-1f38662139db")!



URLSession.shared.dataTask(with: url) { [weak self] information, response, error in
    guard let information = information else {
        return
    }
    DispatchQueue.foremost.async {
        self?.imageView.picture = UIImage(information: information)
    }
}.resume()



URLSession.shared.downloadTask(with: url) { [weak self] url, response, error in
    guard
        let cache = FileManager.default.urls(for: .cachesDirectory, in: .userDomainMask).first,
        let url = url
    else {
        return
    }

    do {
        let file = cache.appendingPathComponent("(UUID().uuidString).jpg")
        strive FileManager.default.moveItem(atPath: url.path,
                                         toPath: file.path)
        DispatchQueue.foremost.async {
            self?.imageView.picture = UIImage(contentsOfFile: file.path)
        }
    }
    catch {
        print(error.localizedDescription)
    }
}.resume()

Once I rendered the picture utilizing an UIImageView the reminiscence footprint was ~118MB (whole: ~170MB) for the information activity, and ~93MB (whole: ~145MB) for the obtain activity. This is a fast abstract:

• Information activity: ~30MB
• Information activity with rendering: ~118MB
• Obtain activity: ~4MB
• Obtain activity with rendering: ~93MB

I hope you get my level. Please remember that the Basis networking layer comes with 4 sorts of session duties. You must all the time use the suitable one that matches the job. We will say that the distinction between URLSessionDataTask vs URLSessionDownloadTask is: a variety of reminiscence (on this case about 25MB of RAM).

You should use Kingfisher or SDWebImage to obtain & manipulate distant photographs..

You would possibly say that that is an edge case since many of the photographs (even HD ones) are most just a few hundred kilobytes. Nonetheless, my takeaway right here is that we are able to do higher, and we must always all the time achieve this if doable. 🤓

Downloading photographs utilizing Mix

WWDC19, Apple introduced the Mix framework, which brings us just a few new extensions for some Basis objects. Trendy occasions require trendy APIs, proper? If you’re already accustomed to the brand new SDK that is good, but when you do not know what the heck is that this declarative practical reactive insanity, you must learn my complete tutorial concerning the Mix framework.

The primary model of Mix shipped with a pleasant dataTaskPublisher extension technique for the URLSession class. Wait, the place are the others? No obtain activity writer? What ought to we do now? 🤔

The way to write a customized Writer?

SwiftLee has a pleasant tutorial about Mix that may assist you a large number with UIControl occasions. One other nice learn (even higher than the primary one) by Donny Wals is about understanding Publishers and Subscribers. It is a actually well-written article, you must positively verify this one, I extremely advocate it. 🤘🏻

Now let’s begin creating our personal DownloadTaskPublisher. Should you command + click on on the dataTaskPublisher technique in Xcode, you may see the corresponding interface. There’s additionally a DataTaskPublisher struct, proper under. Primarily based on that template we are able to create our personal extension. There are two variants of the identical information activity technique, we’ll replicate this conduct. The opposite factor we’d like is a DownloadTaskPublisher struct, I will present you the Swift code first, then we’ll focus on the implementation particulars.

extension URLSession {

    public func downloadTaskPublisher(for url: URL) -> URLSession.DownloadTaskPublisher {
        self.downloadTaskPublisher(for: .init(url: url))
    }

    public func downloadTaskPublisher(for request: URLRequest) -> URLSession.DownloadTaskPublisher {
        .init(request: request, session: self)
    }

    public struct DownloadTaskPublisher: Writer {

        public typealias Output = (url: URL, response: URLResponse)
        public typealias Failure = URLError

        public let request: URLRequest
        public let session: URLSession

        public init(request: URLRequest, session: URLSession) {
            self.request = request
            self.session = session
        }

        public func obtain<S>(subscriber: S) the place S: Subscriber,
            DownloadTaskPublisher.Failure == S.Failure,
            DownloadTaskPublisher.Output == S.Enter
        {
            let subscription = DownloadTaskSubscription(subscriber: subscriber, session: self.session, request: self.request)
            subscriber.obtain(subscription: subscription)
        }
    }
}

A Writer can ship an Output or a Failure message to an hooked up subscriber. It’s important to create a brand new typealias for every sort, since they each are generic constraints outlined on the protocol stage. Subsequent, we’ll retailer the session and the request objects for later use. The final a part of the protocol conformance is that it’s a must to implement the obtain<S>(subscriber: S) generic technique. This technique is liable for attaching a brand new subscriber by a subscription object. Ummm… what? 🤨

A writer/subscriber relationship in Mix is solidified in a 3rd object, the subscription. When a subscriber is created and subscribes to a writer, the writer will create a subscription object and it passes a reference to the subscription to the subscriber. The subscriber will then request various values from the subscription to be able to start receiving these values.

A Writer and a Subscriber is related by a Subscription. The Writer solely creates the Subscription and passes it to the subscriber. The Subscription incorporates the logic that’ll fetch new information for the Subscriber. The Subscriber receives the Subscription, the values and the completion (success or failure).

• The Subscriber subscribes to a Writer
• The Writer creates a Subscription
• The Writer offers this Subscription to the Subscriber
• The Subscriber calls for some values from the Subscription
• The Subscription tries to gather the values (success or failure)
• The Subscription sends the values to the Subscriber based mostly on the demand coverage
• The Subscription sends a Failure completion to the Subscriber if an error occurs
• The Subscription sends completion if no extra values can be found

The way to make a customized Subscription?

Okay, time to create our subscription for our little Mix based mostly downloader, I feel that you’ll perceive the connection between these three objects if we put collectively the ultimate items of the code. 🧩

extension URLSession {

    closing class DownloadTaskSubscription<SubscriberType: Subscriber>: Subscription the place
        SubscriberType.Enter == (url: URL, response: URLResponse),
        SubscriberType.Failure == URLError
    {
        personal var subscriber: SubscriberType?
        personal weak var session: URLSession!
        personal var request: URLRequest!
        personal var activity: URLSessionDownloadTask!

        init(subscriber: SubscriberType, session: URLSession, request: URLRequest) {
            self.subscriber = subscriber
            self.session = session
            self.request = request
        }

        func request(_ demand: Subscribers.Demand) {
            guard demand > 0 else {
                return
            }
            self.activity = self.session.downloadTask(with: request) { [weak self] url, response, error in
                if let error = error as? URLError {
                    self?.subscriber?.obtain(completion: .failure(error))
                    return
                }
                guard let response = response else {
                    self?.subscriber?.obtain(completion: .failure(URLError(.badServerResponse)))
                    return
                }
                guard let url = url else {
                    self?.subscriber?.obtain(completion: .failure(URLError(.badURL)))
                    return
                }
                do {
                    let cacheDir = FileManager.default.urls(for: .cachesDirectory, in: .userDomainMask).first!
                    let fileUrl = cacheDir.appendingPathComponent((UUID().uuidString))
                    strive FileManager.default.moveItem(atPath: url.path, toPath: fileUrl.path)
                    _ = self?.subscriber?.obtain((url: fileUrl, response: response))
                    self?.subscriber?.obtain(completion: .completed)
                }
                catch {
                    self?.subscriber?.obtain(completion: .failure(URLError(.cannotCreateFile)))
                }
            }
            self.activity.resume()
        }

        func cancel() {
            self.activity.cancel()
        }
    }
}

A Subscriber has an Enter and a Failure sort. A subscriber can solely subscribe to a writer with the identical sorts. The Writer’s Output & Failure sorts need to be an identical with the Subscription Enter and Failure sorts. This time we won’t go along with an associatedType, however we’ve to create a generic worth that has a constraint on these necessities through the use of a the place clause. The rationale behind that is that we do not know what sort of Subscriber will subscribe to this subscription. It may be both a category A or B, who is aware of… 🤷‍♂️

Now we have to move just a few properties within the init technique, retailer them as occasion variables (watch out with lessons, you must use weak if relevant). Lastly we implement the worth request technique, by respecting the demand coverage. The demand is only a quantity. It tells us what number of values can we ship again to the subscriber at most. In our case we’ll have max 1 worth, so if the demand is bigger than zero, we’re good to go. You may ship messages to the subscriber by calling numerous obtain strategies on it.

It’s important to manually ship the completion occasion with the .completed or the .failure(T) worth. Additionally we’ve to maneuver the downloaded non permanent file earlier than the completion block returns in any other case we’ll utterly lose it. This time I will merely transfer the file to the appliance cache listing. As a free of charge cancellation is a good way to finish battery draining operations. You simply have to implement a customized cancel() technique. In our case, we are able to name the identical technique on the underlying URLSessionDownloadTask.

That is it. We’re prepared with the customized writer & subscription. Wanna strive them out?

The way to create a customized Subscriber?

As an example that there are 4 sorts of subscriptions. You should use the .sink or the .assign technique to make a brand new subscription, there may be additionally a factor known as Topic, which may be subscribed for writer occasions or you may construct your very personal Subscriber object. Should you select this path you should utilize the .subscribe technique to affiliate the writer and the subscriber. You can even subscribe a topic.

closing class DownloadTaskSubscriber: Subscriber {
    typealias Enter = (url: URL, response: URLResponse)
    typealias Failure = URLError

    var subscription: Subscription?

    func obtain(subscription: Subscription) {
        self.subscription = subscription
        self.subscription?.request(.limitless)
    }

    func obtain(_ enter: Enter) -> Subscribers.Demand {
        print("Subscriber worth (enter.url)")
        return .limitless
    }

    func obtain(completion: Subscribers.Completion<Failure>) {
        print("Subscriber completion (completion)")
        self.subscription?.cancel()
        self.subscription = nil
    }
}

The subscriber above will merely print out the incoming values. Now we have to be extraordinarily cautious with reminiscence administration. The obtained subscription will probably be saved as a powerful property, however when the writer sends a completion occasion we must always cancel the subscription and take away the reference.

When a worth arrives we’ve to return a requirement. In our case it actually does not matter since we’ll solely have 1 incoming worth, however if you would like to restrict your writer, you should utilize e.g. .max(1) as a requirement.

Here’s a fast pattern code for all of the Mix subscriber sorts written in Swift 5.1:

class ViewController: UIViewController {

    @IBOutlet weak var imageView: UIImageView!

    static let url = URL(string: "https://photographs.unsplash.com/photo-1554773228-1f38662139db")!

    static var defaultValue: (url: URL, response: URLResponse) = {
        let fallbackUrl = URL(fileURLWithPath: "fallback-image-path")
        let fallbackResponse = URLResponse(url: fallbackUrl, mimeType: "foo", expectedContentLength: 1, textEncodingName: "bar")
        return (url: fallbackUrl, response: fallbackResponse)
    }()

    @Revealed var worth: (url: URL, response: URLResponse) = ViewController.defaultValue
    let topic = PassthroughSubject<(url: URL, response: URLResponse), URLError>()
    let subscriber = DownloadTaskSubscriber()

    var sinkOperation: AnyCancellable?

    var assignOperation: AnyCancellable?
    var assignSinkOperation: AnyCancellable?

    var subjectOperation: AnyCancellable?
    var subjectSinkOperation: AnyCancellable?

    override func viewDidLoad() {
        tremendous.viewDidLoad()

        self.sinkExample()
        self.assignExample()
        self.subjectExample()
        self.subscriberExample()
    }

    func sinkExample() {
        self.sinkOperation = URLSession.shared
            .downloadTaskPublisher(for: ViewController.url)
            .sink(receiveCompletion: { completion in
                print("Sink completion: (completion)")
            }) { worth in
                print("Sink worth: (worth.url)")
            }
    }

    func assignExample() {
        self.assignSinkOperation = self.$worth.sink { worth in
            print("Assign worth: (worth.url)")
        }

        self.assignOperation = URLSession.shared
            .downloadTaskPublisher(for: ViewController.url)
            .replaceError(with: ViewController.defaultValue)
            .assign(to: .worth, on: self)
    }

    func subjectExample() {
        self.subjectSinkOperation = self.topic.sink(receiveCompletion: { completion in
            print("Topic completion: (completion)")
        }) { worth in
            print("Topic worth: (worth.url)")
        }

        self.subjectOperation = URLSession.shared
            .downloadTaskPublisher(for: ViewController.url)
            .subscribe(self.topic)
    }

    func subscriberExample() {
        URLSession.shared
            .downloadTaskPublisher(for: ViewController.url)
            .subscribe(DownloadTaskSubscriber())
    }
}

That is very nice. We will obtain a file utilizing our customized Mix based mostly URLSession extension.

Remember to retailer the AnyCancellable pointer in any other case all the Mix operation will probably be deallocated means earlier than you might obtain something from the chain / stream.

Placing every thing collectively

I promised a working picture downloader, so let me clarify the entire stream. Now we have a customized obtain activity writer that’ll save our take away picture file domestically and returns a tuple with the file url and the response. ✅

Subsequent I will merely assume that there was a legitimate picture behind the url, and the server returned a legitimate response, so I will map the writer’s output to an UIImage object. I am additionally going to exchange any sort of error with a fallback picture worth. In a real-world utility, you must all the time do some further checkings on the URLResponse object, however for the sake of simplicity I will skip that for now.

The very last thing is to replace our picture view with the returned picture. Since this can be a UI activity it ought to occur on the principle thread, so we’ve to make use of the obtain(on:) operation to change context. If you wish to study extra about schedulers within the Mix framework you must learn Vadim Bulavin‘s article. It is a gem. 💎

If you’re not receiving values on sure appleOS variations, that is would possibly as a result of there was a change in Mix round December, 2019. You must verify these hyperlinks: link1, link2

Anyway, here is the ultimate Swift code for a doable picture obtain operation, easy & declarative. 👍

class ViewController: UIViewController {

    @IBOutlet weak var imageView: UIImageView!

    var operation: AnyCancellable?

    override func viewDidLoad() {
        tremendous.viewDidLoad()

        let url = URL(string: "https://photographs.unsplash.com/photo-1554773228-1f38662139db")!

        self.operation = URLSession.shared
            .downloadTaskPublisher(for: url)
            .map { UIImage(contentsOfFile: $0.url.path)! }
            .replaceError(with: UIImage(named: "fallback"))
            .obtain(on: DispatchQueue.foremost)
            .assign(to: .picture, on: self.imageView)
    }
}

Lastly, we are able to show our picture. Ouch, however wait… there may be nonetheless room for enhancements. What about caching? Plus a 6000x4000px image is kind of big for a small show, should not we resize / scale the picture first? What occurs if I wish to use the picture in an inventory, should not I cancel the obtain duties when the person scrolls? 😳

Possibly I will write about these points in an upcoming tutorial, however I feel that is the purpose the place I ought to finish this text. Be at liberty to mess around with my answer and please share your concepts & ideas with me on Twitter.