取消下载NSData contentsOfUrl

Question

I've got this class for downloading images. But I have a situation where I have multiple views that use this class to download images to them. The views are dynamic and may disappear at anytime before the download has completed.

To prevent errors how would I modify this class to cancel the download should the view using it disappear?

class ImageDownloader{
    var spinner = UIActivityIndicatorView()

    func fetchImage(imageView: UIImageView, url:NSURL, completed: (success: Bool) ->()) {
        self.spinner.frame = CGRectMake((imageView.frame.width-35)/2, (imageView.frame.height-35)/2, 35, 35)
        imageView.addSubview(self.spinner)
        self.spinner.startAnimating()
        let qos = Int(QOS_CLASS_USER_INITIATED.rawValue)
        dispatch_async(dispatch_get_global_queue(qos, 0)) { () -> Void in
            let imageData = NSData(contentsOfURL: url) // this blocks the thread it is on
            dispatch_async(dispatch_get_main_queue()) {
                self.spinner.removeFromSuperview()
                if imageData != nil {
                    imageView.image = UIImage(data: imageData!)
                    completed(success: true)
                }else{
                    completed(success: false)
                }
            }
        }
    }
}

Answer 1

After much research on this it seems NSURLSession is indeed the way to go for what I want to achieve. I found it amazing that I struggled to find any decent tutorials or examples on this. I ended up compiling from snippets of swift and objective-c code from here and there.

class imageDownloader:NSURLSession, NSURLSessionDelegate, NSURLSessionDownloadDelegate{
    var session:NSURLSession!
    var imageView:UIImageView!
    var task = NSURLSessionDownloadTask()
    var spinner = UIActivityIndicatorView()

    override init() {
        super.init()
        let configuration = NSURLSessionConfiguration.defaultSessionConfiguration()
        self.session = NSURLSession(configuration: configuration, delegate: self, delegateQueue: nil)
    }

    func getImage(url:NSURL,imageView:UIImageView){
        self.imageView = imageView
        self.task = session.downloadTaskWithURL(url)
        self.spinner.frame = CGRectMake((imageView.frame.width-35)/2, (imageView.frame.height-35)/2, 35, 35)
        imageView.addSubview(self.spinner)
        self.spinner.startAnimating()
        self.task.resume()
    }


    func URLSession(session: NSURLSession, downloadTask: NSURLSessionDownloadTask, didFinishDownloadingToURL location: NSURL) {
        let data = NSData(contentsOfURL: location)
        dispatch_async(dispatch_get_main_queue()) { () -> Void in
            self.spinner.removeFromSuperview()
            self.imageView.image = UIImage(data: data! )
        }
    }

    func cancel(){
        self.task.cancel()
    }
}

As you can see there isn't much in the way of error handling. Can anyone suggest how to improve it to include this?