非同期通信でのUIScrollViewの更新について

アマゾンのAWSのS3に上がっている画像データを読み込み、ScrollViewに表示するというコードを書きました。

UIの更新はdispatch_get_main_queue()の中に明記することを記事で見たので、以下のように記述したのですが、最初の一枚しかscrollviewに追加されません。デバックをしたところ、addImageは画像の枚数回呼び出されているそうです。

複数枚の画像を非同期的に読み込んで、一枚一枚順を追ってscrollviewに追加したいです。複数枚の追加はどうしたらいいでしょうか？

downloadメソッドを追加しました。

class LoadViewController: UIViewController,UIScrollViewDelegate {
    var loadImages:[UIImage] = []
    @IBOutlet weak var scrollView: UIScrollView!
    var uploadRequests = Array<AWSS3TransferManagerUploadRequest?>()
    var uploadFileURLs = Array<NSURL?>()
    var downloadRequests = Array<AWSS3TransferManagerDownloadRequest?>()
    var downloadFileURLs = Array<NSURL?>()

    override func viewDidLoad() {
        super.viewDidLoad()
        self.scrollView.delegate = self

        do {
            try NSFileManager.defaultManager().createDirectoryAtURL(
                NSURL(fileURLWithPath: NSTemporaryDirectory()).URLByAppendingPathComponent("download"),
                withIntermediateDirectories: true,
                attributes: nil)
        } catch let error1 as NSError {
            error.memory = error1
            print("Creating 'download' directory failed. Error: \(error)")
        }

        listObjects()
    }
    func addImage(image:UIImage)
    {
        let sheight = self.scrollView.frame.height
        let swidth = self.scrollView.frame.width

        loadImages.append(image)
        let myImageView = UIImageView()
        myImageView.frame = CGRectMake(swidth * CGFloat(loadImages.count-1), 0, swidth, sheight)
        myImageView.image = image
        scrollView.addSubview(myImageView)
        scrollView.contentSize = CGSizeMake(swidth * CGFloat(loadImages.count), sheight)

    }
    func listObjects() {
        let s3 = AWSS3.defaultS3()

        let listObjectsRequest = AWSS3ListObjectsRequest()
        listObjectsRequest.bucket = S3BucketName
        s3.listObjects(listObjectsRequest).continueWithBlock { (task) -> AnyObject! in
            if let error = task.error {
                print("listObjects failed: [\(error)]")
            }
            if let exception = task.exception {
                print("listObjects failed: [\(exception)]")
            }
            if let listObjectsOutput = task.result as? AWSS3ListObjectsOutput {
                if let contents = listObjectsOutput.contents as [AWSS3Object]? {
                    for s3Object in contents {
                        let downloadingFileURL = NSURL(fileURLWithPath: NSTemporaryDirectory()).URLByAppendingPathComponent("download").URLByAppendingPathComponent(s3Object.key!)
                        let downloadingFilePath = downloadingFileURL.path!

                        if NSFileManager.defaultManager().fileExistsAtPath(downloadingFilePath) {
                            self.downloadRequests.append(nil)
                            self.downloadFileURLs.append(downloadingFileURL)


                        } else {
                            let downloadRequest = AWSS3TransferManagerDownloadRequest()
                            downloadRequest.bucket = S3BucketName
                            downloadRequest.key = s3Object.key
                            downloadRequest.downloadingFileURL = downloadingFileURL

                            self.downloadRequests.append(downloadRequest)
                            self.downloadFileURLs.append(nil)

                        }
                        dispatch_async(dispatch_get_main_queue(), { () -> Void in
                            if let data = NSData(contentsOfURL: downloadingFileURL) {
                                self.addImage(UIImage(data: data)!)
                                self.displayNum++
                            }
                        })

                    }

                }
            }
            return nil
        }
    }
    func download(downloadRequest: AWSS3TransferManagerDownloadRequest) {
        switch (downloadRequest.state) {
        case .NotStarted, .Paused:
            let transferManager = AWSS3TransferManager.defaultS3TransferManager()
            transferManager.download(downloadRequest).continueWithBlock({ (task) -> AnyObject! in
                if let error = task.error {
                    if error.domain == AWSS3TransferManagerErrorDomain as String
                        && AWSS3TransferManagerErrorType(rawValue: error.code) == AWSS3TransferManagerErrorType.Paused {
                            print("Download paused.")
                    } else {
                        print("download failed: [\(error)]")
                    }
                } else if let exception = task.exception {
                    print("download failed: [\(exception)]")
                } else {
                    dispatch_async(dispatch_get_main_queue(), { () -> Void in
                        if let index = self.indexOfDownloadRequest(self.downloadRequests, downloadRequest: downloadRequest) {
                            self.downloadRequests[index] = nil
                            self.downloadFileURLs[index] = downloadRequest.downloadingFileURL

                            if let data = NSData(contentsOfURL: downloadRequest.downloadingFileURL) {
                                self.addImage(UIImage(data: data)!)
                                self.displayNum++
                            }
                        }
                    })
                }
                return nil
            })

            break
        default:
            break
        }
    }
}