A easy picture downloader
Downloading a useful resource from an URL looks as if a trivial activity, however is it actually that straightforward? Properly, it relies upon. If it’s important to obtain and parse a JSON file which is only a few KB, then you may go together with the classical method or you should utilize the brand new dataTaskPublisher methodology 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 important to deal loads with threads and queues. Luckily utilizing the Dispatch framework helps loads, so you may flip your blocking features 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.international().async { [weak self] in
do {
let content material = strive String(contentsOf: url)
DispatchQueue.major.async {
print(content material)
}
}
catch {
print(error.localizedDescription)
}
}Apple made an necessary word on their official Information documentation, that it is best to NOT use these strategies for downloading non-file URLs, however nonetheless individuals are instructing / utilizing these unhealthy practices, however why? 😥
Do not use this synchronous methodology to request network-based URLs.
My recommendation right here: at all times use the URLSession to carry out community associated data-transfers. Creating an information activity is straightforward, it is an asynchronous operation by default, the callback runs on a background thread, so nothing will likely 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.major.async {
}
}.resume()It is also value to say if it’s essential use a distinct HTTP methodology (aside from GET), ship particular headers (credentials, settle for insurance policies, and so forth.) or present further information within the physique, it’s essential assemble an URLRequest object first. You possibly can 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) it’s important to disable App Transport Safety.
The issue with information duties
What about large recordsdata, akin to photos? Let me present you just a few tutorials earlier than we dive in:
With all due respect, I believe all of those hyperlinks above are actually unhealthy examples of loading distant photos. Certain 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 information into reminiscence (versus utilizing the URLSessionDownloadTask class, which shops the info 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 appropriate candidate for downloading large belongings. That class is designed to request solely smaller objects, because the underlying information goes to be loaded into reminiscence. However the obtain activity saves the content material of the response on the disk (as a substitute of reminiscence) and you’ll obtain an area file URL as a substitute 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 info activity right into a obtain activity solely elevated the bottom reminiscence dimension by ~4MB (to a complete ~56MB), which is a big enchancment.
let url = URL(string: "https://photos.unsplash.com/photo-1554773228-1f38662139db")!
URLSession.shared.dataTask(with: url) { [weak self] information, response, error in
guard let information = information else {
return
}
DispatchQueue.major.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.major.async {
self?.imageView.picture = UIImage(contentsOfFile: file.path)
}
}
catch {
print(error.localizedDescription)
}
}.resume()After I rendered the picture utilizing an UIImageView the reminiscence footprint was ~118MB (whole: ~170MB) for the info 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 remember that the Basis networking layer comes with 4 forms of session duties. It is best to at all times use the appropriate one that matches the job. We are able to say that the distinction between URLSessionDataTask vs URLSessionDownloadTask is: quite a lot of reminiscence (on this case about 25MB of RAM).
You should utilize Kingfisher or SDWebImage to obtain & manipulate distant photos..
You would possibly say that that is an edge case since a lot of the photos (even HD ones) are most just a few hundred kilobytes. Nonetheless, my takeaway right here is that we will do higher, and we should always at all times accomplish that if attainable. 🤓
Downloading photos 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 acquainted with the brand new SDK that is good, but when you do not know what the heck is that this declarative useful reactive insanity, it is best to learn my complete tutorial concerning the Mix framework.
The primary model of Mix shipped with a pleasant dataTaskPublisher extension methodology 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, it is best to undoubtedly verify this one, I extremely suggest it. 🤘🏻
Now let’s begin creating our personal DownloadTaskPublisher. In case you command + click on on the dataTaskPublisher methodology in Xcode, you may see the corresponding interface. There’s additionally a DataTaskPublisher struct, proper beneath. Based mostly on that template we will create our personal extension. There are two variants of the identical information activity methodology, 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 connected subscriber. You need 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 important to implement the obtain<S>(subscriber: S) generic methodology. This methodology is accountable 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 plenty of values from the subscription with a purpose to start receiving these values.
A Writer and a Subscriber is related via a Subscription. The Writer solely creates the Subscription and passes it to the subscriber. The Subscription comprises 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
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 {
ultimate class DownloadTaskSubscription<SubscriberType: Subscriber>: Subscription the place
SubscriberType.Enter == (url: URL, response: URLResponse),
SubscriberType.Failure == URLError
{
non-public var subscriber: SubscriberType?
non-public weak var session: URLSession!
non-public var request: URLRequest!
non-public 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 should be an identical with the Subscription Enter and Failure sorts. This time we won’t go together with an associatedType, however now we have to create a generic worth that has a constraint on these necessities by utilizing 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 to move just a few properties within the init methodology, retailer them as occasion variables (watch out with lessons, it is best to use weak if relevant). Lastly we implement the worth request methodology, 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 possibly can 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 utterly lose it. This time I will 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() methodology. In our case, we will name the identical methodology 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 methodology to make a brand new subscription, there’s additionally a factor known as Topic, which might be subscribed for writer occasions or you may construct your very personal Subscriber object. In case you select this path you should utilize the .subscribe methodology to affiliate the writer and the subscriber. You may also subscribe a topic.
ultimate 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 to be extraordinarily cautious with reminiscence administration. The acquired subscription will likely be saved as a robust 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 sorts written in Swift 5.1:
class ViewController: UIViewController {
@IBOutlet weak var imageView: UIImageView!
static let url = URL(string: "https://photos.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 are able to obtain a file utilizing our customized Mix based mostly URLSession extension.
Do not forget to retailer the AnyCancellable pointer in any other case the complete Mix operation will likely be deallocated method earlier than you can obtain something from the chain / stream.
Placing every thing collectively
I promised a working picture downloader, so let me clarify the entire circulate. We’ve a customized obtain activity writer that’ll save our take away picture file regionally and returns a tuple with the file URL and the response. ✅
Subsequent I will merely assume that there was a sound picture behind the URL, and the server returned a sound 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 software, it is best to at all times do some further 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 principle thread, so now we have to make use of the obtain(on:) operation to change context. If you wish to be taught extra about schedulers within the Mix framework it is best to 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 is best to verify these hyperlinks: link1, link2
Anyway, this is the ultimate Swift code for a attainable 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://photos.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.major)
.assign(to: .picture, on: self.imageView)
}
}Lastly, we will 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 wish to use the picture in an inventory, should not I cancel the obtain duties when the consumer scrolls? 😳
Perhaps 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 at liberty to mess around with my answer and please share your concepts & ideas with me on Twitter.
