Combine 2 PublishSubject's and emit with Observable.combineLatest

Question

I have 2 separate collections of data in my service.

Featured and Standard content.

I have 2 api's calls I make to return these items. They can be consumed separately, however I also have use case when I would like to take both sets of data, provide some enrichment based on a condition and then return them to a consumer.

I was hoping I could do something like this:


class ContentService: ContentServiceType {

    let featured = PublishSubject<[Content]>()
    let standard = PublishSubject<[Content]>()

    let content: Observable<(featured: [Content], standard: [Content])>

    private let client: Client<ContentAPI>
    private let disposeBag = DisposeBag()

    init(client: Client<ContentAPI>) {
        self.client = client

        content = Observable
            .combineLatest(featured, standard)
            .map { (featured, standard) -> (featured: [Content], standard: [Content]) in

        /*
            Do some enrichment and create then return new, updated versions
        */

                return (featured: updatedFeatured, standard: updatedStandard)
        }.share()
    }

    func fetchStandardContent(page: Int = 0, size: Int = 100) -> Single<Void> {

        let params = ["page": page, "size": size]
        let request: Single<Content> = client.request(.getStandardContent(params))

        return request.map { [unowned self] launchers in
            self.standard.onNext(content.props)
            return ()
        }
    }

    func fetchFeaturedContent(page: Int = 0, size: Int = 100) -> Single<Void> {

        let params = ["page": page, "size": size]
        let request: Single<Content> = client.request(.getFeaturedContent(params))

        return request.map { [unowned self] content in
            self.featured.onNext(content.props)
            return ()
        }
    }
 }

Elsewhere in my apps I was then hoping I could do something like

        contentSvc.content
            .observeOn(MainScheduler.instance)
            .subscribeOn(ConcurrentDispatchQueueScheduler(qos: .background))
            .subscribe(onNext: { content in
               /* do something w/ content */
            }).disposed(by: disposeBag)

And then whenever contentSvc.fetchFeaturedContent or contentSvc.fetchStandardContent is called, the contentSvc.content subscriber above gets new data.

Instead content does not appear to be emitting any values.

score 3 · Accepted Answer · answered Oct 21 '19 at 14:39

3

combineLatest requires both sources to emit before it will emit itself I believe.

I would perhaps look at using a BehaviorSubject or BehaviorRelay instead of a PublishSubject.

answered Oct 21 '19 at 14:39

nodediggity

2,458
1
9
12

Or he can use `startWith` method. – Mateusz Chechliński Oct 22 '19 at 11:35

score 0 · Answer 2 · answered Oct 24 '19 at 10:23

I'm using BehaviorRelay instead of PublishSubject because when binding from multiple streams to PublishSubject (which is shared across apps) if any one of those streams sends a complete, it’s possible that PublishSubject may terminate. Relay classes never produce an error or never completes.

let featured = BehaviorRelay(value: [Content]())
let standard = BehaviorRelay(value: [Content]())

func getContent() -> Observable<(featured: [Content], standard: [Content])> {
     return Observable
       .combineLatest(
         featured.asObservable(),
         standard.asObservable(),
         resultSelector: { (featured, standard) -> (featured: [Content], standard: [Content]) in
           return (featured: featured, standard: standard)
       }
     )
  }

func addElemToFeatured() {
    featured.accept([Content(name: "abc")])
  }

  func addElemToStandard() {
    standard.accept([Content(name: "xyz")])
  }

Call the getContent() method from different classes in the initializer method. Also call addElemToFeatured, addElemToStandard from different places like button action.

listener!.getContent()
      .subscribe(onNext: { (featured, standard) in
        print(featured)
        print(standard)
      }).disposed(by: disposeBag)

Combine 2 PublishSubject's and emit with Observable.combineLatest

2 Answers2