Batch image download with AFNetworking using dispatch group

BatchImageDownload.swift

    /**
     Batch request an array of images for background download task. This method fires completion block when ALL images are downloaded and returns an array of UIImage objects.
     - Parameter imageURLs: Array of strings representing location of images as URL's
     - Parameter completion: Block that provides a dictionary of downloaded images as argument. The dictionary contains images at the index they were passed in the original array and NSNull objects for images that failed to download.
     - Parameter imageScale: Scale to interpret image data for UIImage. A scale of 1.0 will keep full size of image. Default (imageScale = nil) is scale factor of UIScreen resolution.
    */
    @objc
    static func batchImageDownloadWithImageUrlArray(imageURLs: [String], imageScale: NSNumber?, completion: ((images: [Int : AnyObject]) -> Void)?) {
        var imageDict = [Int : AnyObject]() // This should be UIImage? but Objective-C can't have nil in dictionaries
        let manager = AFHTTPSessionManager()
        manager.responseSerializer = AFImageResponseSerializer()
        if let scale = imageScale {
            (manager.responseSerializer as? AFImageResponseSerializer)?.imageScale = CGFloat(scale)
        }
        var contentTypes = manager.responseSerializer.acceptableContentTypes
        contentTypes?.insert("image/png")
        manager.responseSerializer.acceptableContentTypes = contentTypes
        let dispatchGroup = dispatch_group_create()
        for index in 0..<imageURLs.count {
            dispatch_group_enter(dispatchGroup)
            let url = imageURLs[index]
            // Fire request
            manager.GET(url, parameters: nil, progress: nil, success: { (task, responseObject) in
                defer { dispatch_group_leave(dispatchGroup) }
                if let im = responseObject as? UIImage {
                    imageDict[index] = im
                } else {
                    imageDict[index] = NSNull()
                }
                }, failure: { (task, error) in
                    imageDict[index] = NSNull()
                    dispatch_group_leave(dispatchGroup)
            })
        }

        dispatch_group_notify(dispatchGroup, dispatch_get_main_queue()) {
            completion?(images: imageDict)
        }
    }