A easy picture downloader
Downloading a useful resource from an URL looks as if a trivial activity, however is it actually that straightforward? Nicely, it relies upon. If you need to obtain and parse a JSON file which is only a few KB, then you’ll be able to go together 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 knowledge from the web. The issue with these strategies is that you need to deal quite a bit with threads and queues. Luckily utilizing the Dispatch framework helps quite a bit, so you’ll be able to 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 knowledge = strive Information(contentsOf: url)
}
catch {
print(error.localizedDescription)
}
DispatchQueue.international().async { [weak self] in
do {
let content material = strive String(contentsOf: url)
DispatchQueue.most important.async {
print(content material)
}
}
catch {
print(error.localizedDescription)
}
}Apple made an essential word on their official Information documentation, that you must NOT use these strategies for downloading non-file URLs, however nonetheless individuals are instructing / utilizing these dangerous practices, however why? 😥
Do not use this synchronous technique to request network-based URLs.
My recommendation right here: at all times use the URLSession to carry out community associated data-transfers. Creating a knowledge activity is straightforward, it is an asynchronous operation by default, the callback runs on a background thread, so nothing might be blocked by default. Fashionable networking APIs are actual good on iOS, in 99% of the circumstances you will not want Alamofire anymore for these form of duties. Say no to dependencies! 🚫
URLSession.shared.dataTask(with: url) { knowledge, response, error in
DispatchQueue.most important.async {
}
}.resume()It is also value to say if you’ll want to use a unique HTTP technique (aside from GET), ship particular headers (credentials, settle for insurance policies, and so forth.) or present additional knowledge within the physique, you’ll want to assemble an URLRequest object first. You’ll be able to solely ship these customized requests utilizing the URLSession APIs.
On Apple platforms you aren’t allowed to make use of the insecure HTTP protocol anymore. If you wish to attain a URL with out the safe layer (HTTPS) you need to disable App Transport Safety.
The issue with knowledge duties
What about huge information, corresponding to photographs? Let me present you a couple of tutorials earlier than we dive in:
With all due respect, I believe all of those hyperlinks above are actually dangerous examples of loading distant photographs. Positive they do the job, they’re additionally very straightforward to implement, however perhaps we should always cowl the entire story… 🤐
For small interactions with distant servers, you should utilize the URLSessionDataTask class to obtain response knowledge into reminiscence (versus utilizing the URLSessionDownloadTask class, which shops the information on to the file system). An information activity is right 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 knowledge activity is NOT the proper candidate for downloading huge property. That class is designed to request solely smaller objects, for the reason that underlying knowledge goes to be loaded into reminiscence. However 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 shifting from knowledge 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 venture. 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 measurement 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] knowledge, response, error in
guard let knowledge = knowledge else {
return
}
DispatchQueue.most important.async {
self?.imageView.picture = UIImage(knowledge: knowledge)
}
}.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.most important.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. Here 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 do not forget that the Basis networking layer comes with 4 varieties of session duties. It’s best to at all times use the proper one that matches the job. We will say that the distinction between URLSessionDataTask vs URLSessionDownloadTask is: lots of reminiscence (on this case about 25MB of RAM).
You should utilize Kingfisher or SDWebImage to obtain & manipulate distant photographs..
You would possibly say that that is an edge case since a lot of the photographs (even HD ones) are most a couple of hundred kilobytes. Nonetheless, my takeaway right here is that we are able to do higher, and we should always at all times achieve this if potential. 🤓
Downloading photographs utilizing Mix
WWDC19, Apple introduced the Mix framework, which brings us a couple of new extensions for some Basis objects. Fashionable instances require trendy APIs, proper? If you’re already conversant in the brand new SDK that is good, but when you do not know what the heck is that this declarative useful 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? 🤔
Learn how to write a customized Writer?
SwiftLee has a pleasant tutorial about Mix that may assist you numerous 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. In the event you command + click on on the dataTaskPublisher technique in Xcode, you’ll be able to see the corresponding interface. There may be additionally a DataTaskPublisher struct, proper beneath. Primarily based on that template we are able to create our personal extension. There are two variants of the identical knowledge activity technique, we’ll replicate this conduct. The opposite factor we’d like is a DownloadTaskPublisher struct, I am going to 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. You need to create a brand new typealias for every kind, 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 you need to implement the obtain<S>(subscriber: S) generic technique. This technique is chargeable for attaching a brand new subscriber via 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 a lot of values from the subscription with a view to start receiving these values.
A Writer and a Subscriber is linked via a Subscription. The Writer solely creates the Subscription and passes it to the subscriber. The Subscription comprises the logic that’ll fetch new knowledge 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 provides 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
Learn how to make a customized Subscription?
Okay, time to create our subscription for our little Mix based mostly downloader, I believe that you’ll perceive the connection between these three objects if we put collectively the ultimate items of the code. 🧩
extension URLSession {
remaining 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 kind. A subscriber can solely subscribe to a writer with the identical varieties. The Writer’s Output & Failure varieties need to be an identical with the Subscription Enter and Failure varieties. This time we will not go together with an associatedType, however now we have to create a generic worth that has a constraint on these necessities through the use of a the place clause. The explanation 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… 🤷♂️
We’ve got to cross a couple of properties within the init technique, retailer them as occasion variables (watch out with courses, 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’ll be able to ship messages to the subscriber by calling numerous obtain strategies on it.
You need to manually ship the completion occasion with the .completed or the .failure(T) worth. Additionally now we have to maneuver the downloaded non permanent file earlier than the completion block returns in any other case we’ll fully lose it. This time I’ll merely transfer the file to the applying cache listing. As a free of charge cancellation is an effective way to finish battery draining operations. You simply must 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?
Learn how to create a customized Subscriber?
As an instance that there are 4 sorts of subscriptions. You should utilize the .sink or the .assign technique to make a brand new subscription, there’s additionally a factor referred to as Topic, which will be subscribed for writer occasions or you’ll be able to construct your very personal Subscriber object. In the event you select this path you should utilize the .subscribe technique to affiliate the writer and the subscriber. You can even subscribe a topic.
remaining 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. We’ve got to be extraordinarily cautious with reminiscence administration. The obtained subscription might be saved as a powerful property, however when the writer sends a completion occasion we should always cancel the subscription and take away the reference.
When a worth arrives now we have 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 varieties 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)
}()
@Printed 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.
Do not forget to retailer the AnyCancellable pointer in any other case the whole Mix operation might be deallocated means earlier than you could possibly obtain something from the chain / stream.
Placing every thing collectively
I promised a working picture downloader, so let me clarify the entire circulation. We’ve got 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’ll merely assume that there was a legitimate picture behind the URL, and the server returned a legitimate response, so I’ll map the writer’s output to an UIImage object. I am additionally going to interchange any form of error with a fallback picture worth. In a real-world software, you must at all times do some additional checkings on the URLResponse object, however for the sake of simplicity I am going to 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 primary thread, so now we have to make use of the obtain(on:) operation to modify 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. It’s best to verify these hyperlinks: link1, link2
Anyway, here is the ultimate Swift code for a potential 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.most important)
.assign(to: .picture, on: self.imageView)
}
}Lastly, we are able to show our picture. Ouch, however wait… there’s 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 need to use the picture in an inventory, should not I cancel the obtain duties when the person scrolls? 😳
Possibly I am going to write about these points in an upcoming tutorial, however I believe that is the purpose the place I ought to finish this text. Be happy to mess around with my answer and please share your concepts & ideas with me on Twitter.