Append images in array in sequence

Question

I want to add images in array in sequence after downloading. I am appending images in array after downloading one by one but they are not in sequence. Can any one please tell me what is best way to do this.

var queue: NSOperationQueue = {
    let _queue = NSOperationQueue()
    _queue.maxConcurrentOperationCount = 4
    return _queue
}()

var imageArrayNsData : [NSData] = []

let session = NSURLSession.sharedSession()

@IBAction func didClickOnStart(sender: AnyObject) {

    queue.cancelAllOperations()

    let completionOperation = NSBlockOperation() {
        print("all done")
    }

    for (index, imageURL) in imageURLs.enumerate() {
        let operation = ImageNetworkOperation(session: session, urlString: imageURL) { image, response, error in

            let dtA : NSData = NSData(data: UIImageJPEGRepresentation(image!, 0.75)!)
            self.imageArrayNsData.append(dtA)
            print("JPEG download\(index)")
        }

        completionOperation.addDependency(operation)
        queue.addOperation(operation)
    }

    NSOperationQueue.mainQueue().addOperation(completionOperation)        
}

The resulting output:

JPEG download0
JPEG download2
JPEG download1
JPEG download3
all done

Answer 1

You should change your model such that it doesn't matter what order the images are downloaded. For example, you have your array of image URL strings:

var imageURLs: [String]

So, your NSData should be stored in a dictionary (or NSCache) keyed by that URL string:

var imageData = [String: NSData]()

Then when you download the data, you can update this dictionary:

self.imageData[imageURL] = dtA

Then, when you need to retrieve this data later, you can use the imageURL, e.g.:

let data = imageData[imageURLs[index]]

Or you could define it as a [Int: NSData] and use the number index as the key. But the idea is that you can use a dictionary, and then the order you receive the responses doesn't matter, but you still enjoy the performance benefit of doing concurrent requests.

---

What I'd suggest would be something like:

var imageData = [String: NSData]()

@IBAction func didClickOnStart(sender: AnyObject) {

    queue.cancelAllOperations()

    let completionOperation = NSBlockOperation() {
        print("all done")
    }

    for (index, imageURL) in imageURLs.enumerate() {
        let operation = DataOperation(session: session, urlString: imageURL) { data, response, error in
            guard let data = data where error == nil else { return }
            guard let httpResponse = response as? NSHTTPURLResponse where httpResponse.statusCode == 200 else { return }

            NSOperationQueue.mainQueue().addOperationWithBlock {
                self.imageData[imageURL] = data
            }
            print("JPEG download\(index)")
        }

        completionOperation.addDependency(operation)
        queue.addOperation(operation)
    }

    NSOperationQueue.mainQueue().addOperation(completionOperation)        
}

And then access it like so:

if let data = imageData[imageURLs[index]], let image = UIImage(data: data) {
    // use `image` here
}

Or

var imageData = [Int: NSData]()

@IBAction func didClickOnStart(sender: AnyObject) {

    queue.cancelAllOperations()

    let completionOperation = NSBlockOperation() {
        print("all done")
    }

    for (index, imageURL) in imageURLs.enumerate() {
        let operation = DataOperation(session: session, urlString: imageURL) { data, response, error in
            guard let data = data where error == nil else { return }
            guard let httpResponse = response as? NSHTTPURLResponse where httpResponse.statusCode == 200 else { return }

            NSOperationQueue.mainQueue().addOperationWithBlock {
                self.imageData[index] = data
            }
            print("JPEG download\(index)")
        }

        completionOperation.addDependency(operation)
        queue.addOperation(operation)
    }

    NSOperationQueue.mainQueue().addOperation(completionOperation)        
}

And access it like so:

if let data = imageData[index], let image = UIImage(data: data) {
    // use `image` here
}

Note, ImageNetworkOperation is just calling DataOperation to get the NSData, and then converting it to a UIImage. If you really want the original NSData, I'd suggest bypassing ImageNetworkOperation and just calling DataOperation directly, like shown above.