Dispatch After Download is Finished in Swift? - ios

Question

I am downloading files in swift, and the download session is triggered by a button on each table view cell. However, I do not want the next download (if someone presses the download button on another cell) to happen until after the previous one is finished.
Is there a way that I can use something like dispatch_after to accomplish this?
Here is my code where the downloading occur, if it helps at all.
//FUNCTION TO DOWNLOAD THE PDF
//PASS THE ONLINE PDF URL AS NSURL
//ASYNC REQUEST
let defaultSession = NSURLSession(configuration: NSURLSessionConfiguration.defaultSessionConfiguration())
var dataTask: NSURLSessionDataTask?
var temp_name = String()
var temp_index = Int()
var temp_indexPath = NSIndexPath()
lazy var downloadsSession: NSURLSession = {
let configuration = NSURLSessionConfiguration.defaultSessionConfiguration()
let session = NSURLSession(configuration: configuration, delegate: self, delegateQueue: nil)
return session
}()
func getUrl(name: String) -> NSURL?{
let documentsUrl = NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask).first as NSURL!
return documentsUrl.URLByAppendingPathComponent(name)
}
func getIndex() -> Int?{
return temp_index
}
func URLSession(session: NSURLSession, downloadTask: NSURLSessionDownloadTask, didFinishDownloadingToURL location: NSURL) {
if let originalURL = downloadTask.originalRequest?.URL?.absoluteString,
destinationURL = getUrl(temp_name){
let fileManager = NSFileManager.defaultManager()
do {
try fileManager.removeItemAtURL(destinationURL)
} catch {
// Non-fatal: file probably doesn't exist
}
do {
try fileManager.copyItemAtURL(location, toURL: destinationURL)
} catch let error as NSError {
print("Could not copy file to disk: \(error.localizedDescription)")
}
}
if let url = downloadTask.originalRequest?.URL?.absoluteString {
activeDownloads[url] = nil
if let trackIndex = getIndex() {
dispatch_async(dispatch_get_main_queue(), {
defaults.setBool(false, forKey: self.temp_name + "_downloading")
self.tableView.reloadRowsAtIndexPaths([NSIndexPath(forRow: trackIndex, inSection: 0)], withRowAnimation: .None)
})
}
}
}
func URLSession(session: NSURLSession, downloadTask: NSURLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
if let downloadUrl = downloadTask.originalRequest?.URL?.absoluteString,
download = activeDownloads[downloadUrl] {
download.progress = Float(totalBytesWritten)/Float(totalBytesExpectedToWrite)
if let trackIndex = getIndex(), let cell = tableView.cellForRowAtIndexPath(NSIndexPath(forRow: trackIndex, inSection: 0)) as? MainTableViewCell {
dispatch_async(dispatch_get_main_queue(), {
cell.progress.progress = download.progress
if(download.progress < 1.0){
cell.progress.hidden = false
}
else{
cell.progress.hidden = true
}
})
}
}
}
// Action triggered by UIButton (in this case the download button)
//Access tag, which is the IndexPath.row, using sender.tag
#IBAction func downloadFile(sender: UIButton){
let indexPath = NSIndexPath(forRow: sender.tag, inSection: 0)
let cell = tableView.cellForRowAtIndexPath(indexPath) as! MainTableViewCell
cell.downloadButton.hidden = true
cell.progress.progress = 0
cell.progress.hidden = false
let isAvailable = true
let key = names[sender.tag] + "_offline"
defaults.setValue(isAvailable, forKey: key)
let name = (names[sender.tag])
let fileName = name + ".pdf"
let attachment = attachments[sender.tag]
temp_name = fileName
temp_index = sender.tag
temp_indexPath = indexPath
let destinationURL = getUrl(temp_name)!
defaults.setValue(destinationURL.path!, forKey: name + "_path")
defaults.synchronize()
defaults.setBool(true, forKey: name + "_downloading")
let urlString = attachment
let url = NSURL(string: urlString)
let download = PDFDownload(url: urlString)
download.downloadTask = downloadsSession.downloadTaskWithURL(url!)
download.downloadTask!.resume()
download.isDownloading = true
activeDownloads[download.url] = download
}
There is a boolean that stores whether or not a download session is occurring, so maybe there is a way that I can use that? Wait until the boolean is false to execute my code?

Answer 1

#Deepak kumar answer is correct,But adding dependency for each operation is not good idea.
you can do it in a simpler way. only 3 steps required.
create NSOperationQueue object.
then set the property maxConcurrentOpeations to 1.
then add operations to queue , it will perform the operations one by one.

Answer 2

You can use NSOperationQueue to accomplish this. Create one operationqueue and one NSOperation object to store previous operation which was added to operation queue before the current operation. on every click on tableviewcell's button create new NSOperation instance and before adding it to operationqueue do the followings.
1- check if tempoperation is nil. then assign current operation to it and then add to operation queue.
2. else add dependency on tempoperation first then assign current operation to it and then add to operation queue.
This way each task will start after the completion of previous task. Hope this will help you. :)