[swift]画像を非同期でロードするextensions

UIImageをURLから非同期でロードしたい時のextensions。エラー処理とか全く入れてないけど...

UIImageExtension.swift

extension UIImage {
    class func loadAsyncFromURL(urlString: String, callback: (UIImage?) -> ()) {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), {
            let url = NSURL(string: urlString)
            var err: NSError?
            let imageData = NSData(contentsOfURL: url!, options: NSDataReadingOptions.DataReadingMappedIfSafe, error: &err)
            var image: UIImage?
            if let _data = imageData {
                image = UIImage(data: _data)

                dispatch_async(dispatch_get_main_queue(), {
                    callback(image)
                })
            }
        })
    }
}

使い方

UIImage.loadAsyncFromURL(path, {
  (image: UIImage?) in
  self.imageView.image = image
})