How to combine multi Observable in RxSwift

Question

Let's say I create an Observable for each image download

for _ in 0...N {
    let s : Single<UIImage?> = fetchImage(from url)
}

How can I:

• create a new Observable to emit an event when a download is completed
• create a new Observable to emit only completed event when all download is completed

Thanks!

Answer 1

create a new Observable to emit an event when a download is completed

Look at the method imageObservable for a possible implementation.

create a new Observable to emit only completed event when all download is completed

The zip (documentation) operator might be what you are looking for.

import RxSwift
import UIKit

enum CustomError: Error {
    case someError
}

class Executer {

    let disposeBag = DisposeBag()

    func execute() {
        let imageURLs = [
            URL(string: "http://via.placeholder.com/350x150")!, 
            URL(string: "http://via.placeholder.com/350x150")!
        ]
        let imageObservables = imageURLs.map { self.imageObservable(url: $0) }
        Observable
            .zip(imageObservables) // wait for all image requests to finish
            .subscribe(onNext: { images in
                // here you have every single image in the 'images' array
                images.forEach { print($0) }
            })
            .disposed(by: disposeBag)
     }

     // wrap 'URLSession' datatask into an observable
     func imageObservable(url: URL) -> Observable<UIImage> {
        return Observable.create { observer in
            URLSession
                .shared
                .dataTask(with: url, completionHandler: { (data, response, error) -> Void in
                    if let error = error {
                        observer.onError(error)
                        return
                    }
                    guard let data = data, let image = UIImage(data: data) else {
                        observer.onError(CustomError.someError)
                        return
                    }
                    observer.onNext(image)
                    observer.onCompleted()
                })
                .resume()
            return Disposables.create()
        }
    }
}

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