Loading a UIImage into a UIImageView from a NSURL using Forbind

RemoteImage.swift

import UIKit
import Forbind
import ForbindExtensions

enum LoadingState {
    case NotSet
    case Loading
    case Loaded
}

class RemoteImageView: UIImageView {
    private(set) var URL: NSURL?
    private(set) var state: LoadingState = .NotSet
    
    private var imagePromise: Promise<Result<UIImage>>?
    private var placeholderImage: UIImage?
    
    init() {
        super.init(image: nil)
    }
    
    convenience init(URL: NSURL, placeholderImage: UIImage) {
        self.init()
        
        loadImage(URL, placeholderImage: placeholderImage)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    func cancelImage() {
        self.imagePromise = nil
        if let placeholder = self.placeholderImage {
            self.image = placeholder
        }
    }
    
    func loadImage(URL: NSURL, placeholderImage: UIImage? = nil) {
        self.placeholderImage = placeholderImage
        self.image = placeholderImage
        self.URL = URL
        
        sendImageRequest()
    }
    
    private func sendImageRequest() {
        guard let URL = self.URL else { return }
        
        let session = NSURLSession.sharedSession()
        let dataTask = session.dataTask(URL)
        
        self.imagePromise = dispatchAsync(dataTask => { UIImage(data: $0.0) }, queue: dispatch_get_main_queue())
        
        self.imagePromise?.getValueWeak { [weak self] result in
            if let image = result.okValue {
                self?.image = image
            }
        }
    }
}