This repository contains RxSwift extensions for Nuke as well as examples of common use cases solved by Rx.

Use Cases

• Going From Low to High Resolution
• Loading the First Available Image
• Load Multiple Images, Display All at Once
• Showing Stale Image While Validating It
• Auto Retry
• Tracking Activities
• Display Placeholder on Failure
• Table or Collection View

Getting Started

• Installation Guide
• Getting Started with RxSwift

Usage

RxNuke provides a set of reactive extensions for Nuke:

extension Reactive where Base: ImagePipeline {
    public func loadImage(with url: URL) -> Single<ImageResponse>
    public func loadImage(with request: ImageRequest) -> Single<ImageResponse>
}

A Single is a variation of Observable that, instead of emitting a series of elements, is always guaranteed to emit either a single element or an error. The common use case of Single is to wrap HTTP requests. See Traits for more info.

Here's a basic example where we load an image and display the result on success:

ImagePipeline.shared.rx.loadImage(with: url)
    .subscribe(onSuccess: { imageView.image = $0.image })
    .disposed(by: disposeBag)

Going From Low to High Resolution

Suppose you want to show users a high-resolution, slow-to-download image. Rather than let them stare a placeholder for a while, you might want to quickly download a smaller thumbnail first.

You can implement this using concat operator which results in a serial execution. It would first start a thumbnail request, wait until it finishes, and only then start a request for a high-resolution image.

Observable.concat(pipeline.rx.loadImage(with: lowResUrl).orEmpty,
                  pipeline.rx.loadImage(with: highResUtl).orEmpty)
    .subscribe(onNext: { imageView.image = $0.image })
    .disposed(by: disposeBag)

orEmpty is a custom property which ignores errors and completes the sequence instead (equivalent to func catchErrorJustComplete() from RxSwiftExt.

extension RxSwift.PrimitiveSequence {
    public var orEmpty: Observable<Element> {
        return self.asObservable().catchError { _ in .empty() }
    }
}

Loading the First Available Image

Suppose you have multiple URLs for the same image. For instance, you might have uploaded an image taken from the camera. In such case, it would be beneficial to first try to get the local URL, and if that fails, try to get the network URL. It would be a shame to download the image that we may have already locally.

This use case is very similar Going From Low to High Resolution, but an addition of .take(1) guarantees that we stop execution as soon as we receive the first result.

Observable.concat(pipeline.rx.loadImage(with: localUrl).orEmpty,
                  pipeline.rx.loadImage(with: networkUrl).orEmpty)
    .take(1)
    .subscribe(onNext: { imageView.image = $0.image })
    .disposed(by: disposeBag)

Load Multiple Images, Display All at Once

Suppose you want to load two icons for a button, one icon for .normal state and one for .selected state. Only when both icons are loaded you can show the button to the user. This can be done using a combineLatest operator:

Observable.combineLatest(pipeline.rx.loadImage(with: iconUrl).asObservable(),
                         pipeline.rx.loadImage(with: iconSelectedUrl).asObservable())
    .subscribe(onNext: { icon, iconSelected in
        button.isHidden = false
        button.setImage(icon.image, for: .normal)
        button.setImage(iconSelected.image, for: .selected)
    }).disposed(by: disposeBag)

Showing Stale Image While Validating It

Suppose you want to show users a stale image stored in a disk cache (Foundation.URLCache) while you go to the server to validate it. This use case is actually similar to Going From Low to High Resolution.

let cacheRequest = URLRequest(url: imageUrl, cachePolicy: .returnCacheDataDontLoad)
let networkRequest = URLRequest(url: imageUrl, cachePolicy: .useProtocolCachePolicy)

Observable.concat(pipeline.rx.loadImage(with: ImageRequest(urlRequest: cacheRequest).orEmpty,
                  pipeline.rx.loadImage(with: ImageRequest(urlRequest: networkRequest)).orEmpty)
    .subscribe(onNext: { imageView.image = $0.image })
    .disposed(by: disposeBag)

See Image Caching to learn more about HTTP cache

Auto Retry

Auto-retry with an exponential backoff of other delay options (including immediate retry when a network connection is re-established) using smart retry.

pipeline.rx.loadImage(with: request).asObservable()
    .retry(3, delay: .exponential(initial: 3, multiplier: 1, maxDelay: 16))
    .subscribe(onNext: { imageView.image = $0.image })
    .disposed(by: disposeBag)

Tracking Activities

Suppose you want to show an activity indicator while waiting for an image to load. Here's how you can do it using ActivityIndicator class provided by RxSwiftUtilities:

let isBusy = ActivityIndicator()

pipeline.rx.loadImage(with: imageUrl)
    .trackActivity(isBusy)
    .subscribe(onNext: { imageView.image = $0.image })
    .disposed(by: disposeBag)

isBusy.asDriver()
    .drive(activityIndicator.rx.isAnimating)
    .disposed(by: disposeBag)

In a Table or Collection View

Here's how you can integrate the code provided in the previous examples into your table or collection view cells:

final class ImageCell: UICollectionViewCell {
    private var imageView: UIImageView!
    private var disposeBag = DisposeBag()

    // <.. create an image view using your preferred way ..>

    func display(_ image: Single<ImageResponse>) {

        // Create a new dispose bag, previous dispose bag gets deallocated
        // and cancels all previous subscriptions.
        disposeBag = DisposeBag()

        imageView.image = nil

        // Load an image and display the result on success.
        image.subscribe(onSuccess: { [weak self] response in
            self?.imageView.image = response.image
        }).disposed(by: disposeBag)
    }
}

Requirements

License

RxNuke is available under the MIT license. See the LICENSE file for more info.