ImageCache.swift

import Foundation
import RxSwift
import RxCocoa

class ImageCache {

    static let sharedInstance = ImageCache()

    private let observableCache = NSCache()
    private let imageCache = NSCache()

    func loadImage(url: NSURL) -> Observable<UIImage?> {
        return Observable.deferred {
            if let image = self.imageCache[url] as? UIImage {
                return Observable.just(image).observeOn(MainScheduler.instance).debug("Just")
            }

            if url.absoluteString == "" {
                return Observable.just(nil)
            }

            if let observable = self.observableCache[url] as? Observable<UIImage?> {
                return observable
            }

            let observable: Observable<UIImage?> = NSURLSession.sharedSession()
                .rx_data(NSURLRequest(URL: url))
                .map { data in
                    return UIImage(data: data)
                }
                .debug("NSURLRequest")
                .shareReplay(1)
                .observeOn(MainScheduler.instance)
                .doOnNext { (image) in
                    guard let image = image else { return }
                    self.imageCache[url] = image
            }

            self.observableCache[url] = observable

            return observable
        }
    }
}

private extension NSCache {
    subscript(key: AnyObject) -> AnyObject? {
        get {
            return objectForKey(key)
        }
        set {
            if let value: AnyObject = newValue {
                setObject(value, forKey: key)
            } else {
                removeObjectForKey(key)
            }
        }
    }
}

output.log

2016-08-31 13:45:12.437: NSURLRequest -> subscribed
2016-08-31 13:45:13.259: NSURLRequest -> Event Next(Optional(<UIIma...2791bb0>, {80, 80}))
2016-08-31 13:45:13.259: NSURLRequest -> Event Completed
2016-08-31 13:45:13.259: NSURLRequest -> disposed
2016-08-31 13:45:14.663: Just -> subscribed
2016-08-31 13:45:14.664: Just -> Event Next(Optional(<UIIma...2791bb0>, {80, 80}))
2016-08-31 13:45:14.664: Just -> Event Completed
2016-08-31 13:45:14.664: Just -> disposed
2016-08-31 13:45:14.722: Just -> subscribed
2016-08-31 13:45:14.722: Just -> Event Next(Optional(<UIIma...2791bb0>, {80, 80}))
2016-08-31 13:45:14.722: Just -> Event Completed
2016-08-31 13:45:14.722: Just -> disposed