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Combine: Asynchronous Programming with Swift

Second Edition · iOS 14 · Swift 5.3 · Xcode 12

Before You Begin

Section 0: 3 chapters
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Section I: Introduction to Combine

Section 1: 2 chapters
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Section II: Operators

Section 2: 6 chapters
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Section III: Combine in Action

Section 3: 6 chapters
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Section IV: Advanced Combine

Section 4: 5 chapters
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Section V: Building a Complete App

Section 5: 2 chapters
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14. In Practice: Project "News"
Written by Marin Todorov

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In the past few chapters, you learned about quite a few practical applications of the Combine integration in Foundation types. You learned how to use URLSession‘s data task publisher to make network calls, you saw how to observe KVO-compatible objects with Combine and more.

In this chapter, you will combine your solid knowledge about operators with some of the Foundation integrations you just discovered and will work through a series of tasks like in the previous “In Practice” chapter. This time around, you will work on building a Hacker News API client.

“Hacker News,” whose API you are going to be using in this chapter, is a social news website focused on computers and entrepreneurship. If you haven‘t already, you can check them out at: https://news.ycombinator.com.

In this chapter, you will work in an Xcode playground focusing only on the API client itself.

In Chapter 15, “In Practice: Combine & SwiftUI,” you will take the completed API and use it to build a real Hacker News reader app by plugging the network layer into a SwiftUI-based user interface. Along the way, you will learn the basics of SwiftUI and how to make your Combine code work with the new declarative Apple framework for building amazing, reactive app UIs.

Without further ado, let‘s get started!

Getting started with the Hacker News API

Open the included starter playground API.playground in projects/starter and peek inside. You will find some simple starter code included to help you hit the ground running and let you focus on Combine code only:

Inside the API type, you will find two nested helper types:

  • An enum called Error which features two custom errors your API will throw in case it cannot reach the server or it cannot decode the server response.
  • A second enum called EndPoint which contains the URLs of the two API endpoints your type is going to be connecting to.

Further down, you will find the maxStories property. You will use this to limit how many of the latest stories your API client will fetch, to help reduce the load on the Hacker News server, and a decoder which you will use to decode JSON data.

Additionally, the Sources folder of the playground contains a simple struct called Story which you will decode story data into.

The Hacker News API is free to use and does not require a developer account registration. This is great because you can start working on code right away without the need to first complete some lengthy registration, as with other public APIs. The Hacker News team wins a ton of karma points!

Getting a single story

Your first task is to add a method to API which will contact the server using the EndPoint type to get the correct endpoint URL and will fetch the data about a single story. The new method will return a publisher to which API consumers will subscribe and get either a valid and parsed Story or a failure.

func story(id: Int) -> AnyPublisher<Story, Error> {
  return Empty().eraseToAnyPublisher()
}

URLSession.shared
  .dataTaskPublisher(for: EndPoint.story(id).url)

private let apiQueue = DispatchQueue(label: "API",
                                     qos: .default,
                                     attributes: .concurrent)

.receive(on: apiQueue)

.map(\.data)

.decode(type: Story.self, decoder: decoder)

.catch { _ in Empty<Story, Error>() }
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.eraseToAnyPublisher()

return Empty().eraseToAnyPublisher()

func story(id: Int) -> AnyPublisher<Story, Error> {
  URLSession.shared
    .dataTaskPublisher(for: EndPoint.story(id).url)
    .receive(on: apiQueue)
    .map(\.data)
    .decode(type: Story.self, decoder: decoder)
    .catch { _ in Empty<Story, Error>() }
    .eraseToAnyPublisher()
}

let api = API()
var subscriptions = [AnyCancellable]()

api.story(id: 1000)
   .sink(receiveCompletion: { print($0) },
         receiveValue: { print($0) })
   .store(in: &subscriptions)


{
  "by":"python_kiss",
  "descendants":0,
  "id":1000,
  "score":4,
  "time":1172394646,
  "title":"How Important is the .com TLD?",
  "type":"story",
  "url":"http://www.netbusinessblog.com/2007/02/19/how-important-is-the-dot-com/"
}

How Important is the .com TLD?
by python_kiss
http://www.netbusinessblog.com/2007/02/19/how-important-is-the-dot-com/
-----
finished

Multiple stories via merging publishers

Getting a single story out of the API server was a relatively straight forward task. Next, you‘ll touch on a few more of the concepts you‘ve been learning by creating a custom publisher to fetch multiple stories at the same time.

func mergedStories(ids storyIDs: [Int]) -> AnyPublisher<Story, Error> {

}

let storyIDs = Array(storyIDs.prefix(maxStories))

precondition(!storyIDs.isEmpty)

let initialPublisher = story(id: storyIDs[0])
let remainder = Array(storyIDs.dropFirst())
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return remainder.reduce(initialPublisher) { combined, id in

}

return combined
  .merge(with: story(id: id))
  .eraseToAnyPublisher()

api.story(id: -5)
   .sink(receiveCompletion: { print($0) },
         receiveValue: { print($0) })
   .store(in: &subscriptions)

api.mergedStories(ids: [1000, 1001, 1002])
   .sink(receiveCompletion: { print($0) },
         receiveValue: { print($0) })
   .store(in: &subscriptions)

How Important is the .com TLD?
by python_kiss
http://www.netbusinessblog.com/2007/02/19/how-important-is-the-dot-com/
-----

Wireless: India's Hot, China's Not
by python_kiss
http://www.redherring.com/Article.aspx?a=21355
-----

The Battle for Mobile Search
by python_kiss
http://www.businessweek.com/technology/content/feb2007/tc20070220_828216.htm?campaign_id=rss_daily
-----
finished

Getting the latest stories

In this final chapter section, you will work on creating an API method that fetches the list of latest Hacker News stories.

func stories() -> AnyPublisher<[Story], Error> {
  return Empty().eraseToAnyPublisher()
}

URLSession.shared
  .dataTaskPublisher(for: EndPoint.stories.url)

.map(\.data)

[1000, 1001, 1002, 1003]

.decode(type: [Int].self, decoder: decoder)

.mapError { error -> API.Error in
  switch error {
  case is URLError:
    return Error.addressUnreachable(EndPoint.stories.url)
  default:
    return Error.invalidResponse
  }
}

.filter { !$0.isEmpty }

.flatMap { storyIDs in
  return self.mergedStories(ids: storyIDs)
}
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.scan([]) { stories, story -> [Story] in
  return stories + [story]
}
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.map { $0.sorted() }

.eraseToAnyPublisher()

return Empty().eraseToAnyPublisher()

api.mergedStories(ids: [1000, 1001, 1002])
   .sink(receiveCompletion: { print($0) },
         receiveValue: { print($0) })
   .store(in: &subscriptions)

api.stories()
   .sink(receiveCompletion: { print($0) },
         receiveValue: { print($0) })
   .store(in: &subscriptions)

[
More than 70% of America’s packaged food supply is ultra-processed
by xbeta
https://news.northwestern.edu/stories/2019/07/us-packaged-food-supply-is-ultra-processed/
-----]

[
More than 70% of America’s packaged food supply is ultra-processed
by xbeta
https://news.northwestern.edu/stories/2019/07/us-packaged-food-supply-is-ultra-processed/
-----, 
New AI project expects to map all the word’s reefs by end of next year
by Biba89
https://www.independent.co.uk/news/science/coral-bleaching-ai-reef-paul-allen-climate-a9022876.html
-----]

[
More than 70% of America’s packaged food supply is ultra-processed
by xbeta
https://news.northwestern.edu/stories/2019/07/us-packaged-food-supply-is-ultra-processed/
-----, 
New AI project expects to map all the word’s reefs by end of next year
by Biba89
https://www.independent.co.uk/news/science/coral-bleaching-ai-reef-paul-allen-climate-a9022876.html
-----, 
People forged judges’ signatures to trick Google into changing results
by lnguyen
https://arstechnica.com/tech-policy/2019/07/people-forged-judges-signatures-to-trick-google-into-changing-results/
-----]

Challenges

There is nothing to add per se to the API client but you can still play around a little if you‘d like to put some more work into this chapter‘s project.

Challenge 1: Integrating the API client with UIKit

As already mentioned, in the next chapter, you will learn about SwiftUI and how to integrate it with your Combine code.

Key points

  • Foundation includes several publishers that mirror counterpart methods in the Swift standard library and you can even use them interchangeably as you did with reduce in this chapter.
  • Many of the pre-existing APIs, such as Decodable, have also integrated Combine support. This lets you use one standard approach across all of your code.
  • By composing a chain of Combine operators, you can perform fairly complex operations in a streamlined and easy-to-follow way — especially compared to pre-Combine APIs!

Where to go from here?

Congratulations on completing the “Combine in Action” section! What a ride this was, wasn‘t it?

13. Resource Management
15. In Practice: Combine & SwiftUI
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