NSOperation Dependencies

class ResizeImageOperation : Operation {
    let path: String
    let containingSize: CGSize

    init(path: String, containingSize: CGSize) {
        self.path = path
        self.containingSize = containingSize
    }

    override func execute() {
        let sourceImage = UIImage(contentsOfFile: path)!
        println("Source image size: \(sourceImage.size)")
        printSizeOnDisk(UIImageJPEGRepresentation(sourceImage, 1.0))

        let width: CGFloat
        let height: CGFloat
        let ratio = sourceImage.size.width / sourceImage.size.height
        if sourceImage.size.width >= sourceImage.size.height {
            width = containingSize.width
            height = width / ratio
        } else {
            height = containingSize.height
            width = height * ratio
        }

        let imageSize = CGSize(width: width, height: height)
        println("Resized image: \(imageSize)")

        UIGraphicsBeginImageContextWithOptions(imageSize, true, 0.0)
        let rect = CGRect(origin: CGPoint(x: 0, y: 0), size: imageSize)
        sourceImage.drawInRect(rect)
        let resizedImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        let imageData = UIImageJPEGRepresentation(resizedImage, 0.8)
        printSizeOnDisk(imageData)
        imageData.writeToFile(path, atomically: true)

        finish()
    }

    func printSizeOnDisk(data: NSData) {
        let bytes = Int64(data.length)
        let size = NSByteCountFormatter.stringFromByteCount(bytes, countStyle: NSByteCountFormatterCountStyle.File)
        println("Size on disk: \(size)")
    }
}

class FilterImageOperation : Operation {
    let path: String
    var outputImage: CIImage?

    init(path: String) {
        self.path = path
    }

    lazy var inputImage: CIImage = {
        let url = NSURL(fileURLWithPath: self.path)!
        return CIImage(contentsOfURL: url)
    }()

    lazy var filter: CIFilter = {
        return CIFilter(name: "CISepiaTone", withInputParameters: [
            kCIInputImageKey : self.inputImage
        ])
    }()

    override func execute() {
        outputImage = filter.outputImage
        finish()
    }
}

@IBAction func downloadButtonTapped(sender: AnyObject) {

        let url = NSURL(string: "http://imgsrc.hubblesite.org/hu/db/images/hs-2006-10-a-hires_jpg.jpg")!

        let docsDir = NSSearchPathForDirectoriesInDomains(.CachesDirectory, .UserDomainMask, true).first! as! String
        let targetPath = docsDir.stringByAppendingPathComponent("hubble.jpg")

        let downloadOperation = DownloadImageOperation(imageURL: url, targetPath: targetPath)

        downloadOperation.progressBlock = { self.downloadProgressView.progress = $0 }

        var size = CGSize(width: imageView.bounds.size.width * 2, height: imageView.bounds.size.height * 2)
        let resizeOperation = ResizeImageOperation(path: targetPath, containingSize: size)
        resizeOperation.addDependency(downloadOperation)

        let filterOperation = FilterImageOperation(path: targetPath)
        filterOperation.addDependency(resizeOperation)

        // Thanks to Derek in the comments for pointing out the retain cycle here. Updated to use a weak reference.
        filterOperation.completionBlock = { [weak filterOperation] in
            if let outputImage = filterOperation?.outputImage {
                let context = CIContext(options: [:])
                let cgImage = context.createCGImage(outputImage, fromRect: outputImage.extent())
                let image = UIImage(CGImage: cgImage)!
                self.displayImage(image)
            }
        }

        operationQueue.suspended = true
        operationQueue.addOperation(downloadOperation)
        operationQueue.addOperation(resizeOperation)
        operationQueue.addOperation(filterOperation)

        operationQueue.suspended = false
    }