重启NSOperation的正确方法是什么？

Question

我正在尝试将文件上传到AWS S3存储桶。如果上传因任何原因失败，我希望能够重试几次。

我有一个自定义的NSOperation，其主方法创建一个AWSS3TransferManagerUploadRequest（）实例。在AWSS3TransferManager的帮助下，我开始上传。

如果失败了，我应该在task.completionBlock中调用main（）吗？或者我应该使用我的NSOperation的completionBlock，并尝试从那里重新启动操作？ 这里是一些清晰的代码

class PhotoUploadOperation: NSOperation {
var photoURL: NSURL!
var fileName: String!
var festival: PFObject!
var sleepTime: NSTimeInterval = 2.0
init(photoURL: NSURL, withFileName fileName:String, forFestival festival: PFObject) {
    super.init()
    self.photoURL = photoURL
    self.fileName = fileName
    self.festival = festival
}

override func main() {
    let uploadRequest = AWSS3TransferManagerUploadRequest()
    uploadRequest.bucket = "festivals123";
    uploadRequest.key = fileName
    uploadRequest.contentType = "image/jpeg"
    uploadRequest.body = photoURL
    uploadRequest.ACL = .PublicRead
    AWSS3TransferManager.registerS3TransferManagerWithConfiguration(DataManager.shared.awsConfiguration, forKey: "ID1")
    let transferManager = AWSS3TransferManager.S3TransferManagerForKey("ID1")

    let task = transferManager.upload(uploadRequest)
    task.continueWithBlock { (task: AWSTask!) -> AnyObject! in
        if (task.faulted) {
            print("now we need to restart upload for \(self.fileName)")
            NSThread.sleepForTimeInterval(self.sleepTime)
            if (self.sleepTime <= 2 ) {
                self.sleepTime = self.sleepTime*2
            }
            self.main()
        }

        return task
    }
}
}