`PropertyType` for easily fetching images in ReactiveCocoa 4.0.1

ImageProperty.swift

import ReactiveCocoa

#if os(iOS)
    public typealias ImageType = UIImage
#else
#if os(OSX)
    public typealias ImageType = NSImage
#endif
#endif

// MARK: -
// MARK: Image Cacher Protocol
// MARK: -
public protocol ImageCacher {
    func cachedImage(forURL url: String) -> ImageType?
    func cache(image: ImageType, atURL url: String)
}

// MARK: -
// MARK: Image Resource
// MARK: -
public struct ImageProperty {
    public typealias Value = ImageType?

    // MARK: Properties (Private)
    private let _buffer = SignalProducer<ImageType?, NSError>.buffer(1)
    private let _image: MutableProperty<ImageType?> = MutableProperty(nil)
    private var _fetch: Action<String, ImageType?, NSError>! = Action { _ in SignalProducer.empty }

    // MARK: Initialization
    public init(_ imageURL: String, imageCacher: ImageCacher? = nil) {
        _fetch = Action { urlString in
            SignalProducer { _ in
                guard let url = NSURL(string: urlString) else {
                    self._buffer.1.sendNext(nil)
                    self._buffer.1.sendCompleted()
                    return
                }

                let request = NSURLRequest(URL: url)
                let session = NSURLSession.sharedSession()

                self._buffer.1.sendNext(imageCacher?.cachedImage(forURL: imageURL))
                session
                    .rac_dataWithRequest(request)
                    .map { ImageType(data: $0.0) }
                    .takeWhile { $0 != nil }
                    .on(next: {
                        if let image = $0 {
                            imageCacher?.cache(image, atURL: imageURL)
                        }
                    })
                    .start(self._buffer.1)
            }
        }

        _image <~ _buffer.0
            .flatMapError { _ in SignalProducer<ImageType?, NoError>.empty }

        _fetch.apply(imageURL).start()
    }
}

// MARK: -
// MARK: Extension, Public Properties
// MARK: -
extension ImageProperty: PropertyType {
    public var value: Value {
        return _image.value
    }

    public var signal: Signal<Value, NoError> {
        return _image.signal
    }

    public var producer: SignalProducer<Value, NoError> {
        return _image.producer
    }

    public var isFetching: AnyProperty<Bool> {
        return _fetch.executing
    }
}