UIView fields not update in main thread in Swift - ios

Question

I'm making a download App and to keep the user inform about the progress I use an UIView on top of the main View.
I've got trouble with updating my labels and progressBars.
I try different way to do it like:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {
// do some task
dispatch_async(dispatch_get_main_queue()) {
// update some UI
}
}
And also:
self.lblSpeedDownload.setNeedsDisplay()
But it does not update the UI every time it's call
So I tried this https://stackoverflow.com/a/26744884/2410264
For now it's look like it is the best solution I made but it's still not enough quickly update: it's look like the more I call update function, the more it's take time to update UI (1-2-10-20 seconds...)
Here is my code:
A class variable:
let source = dispatch_source_create(DISPATCH_SOURCE_TYPE_DATA_ADD, 0, 0, dispatch_get_main_queue());
Call when I start download:
dispatch_source_set_event_handler(source) {
[unowned self] in
self.updateDownloadView(self.mediaIndex, numberOfMedia: self.numberOfMedia)
}
dispatch_resume(source)
Call when I want to update UI:
dispatch_source_merge_data(source, 1);
And the update function:
func updateDownloadView(mediaIndex : Float, numberOfMedia:Int32){
if downloadPopup != nil {
var tempText:String
if self.selectedDirectories.count == 1 {
tempText = "Folder "
} else {
tempText = "Folders "
}
self.lblFolders.text = tempText + (self.downloadIndex+1).description + "/" + self.selectedDirectories.count.description + " :"
if numberOfMedia == 1 {
tempText = "Image "
} else {
tempText = "Images "
}
self.lblImages.text = tempText + Int(floor(mediaIndex)+1).description + "/" + numberOfMedia.description + " :"
let images_progress = mediaIndex/Float(numberOfMedia)
let folders_progress = Float(self.downloadIndex)+images_progress/Float(self.selectedDirectories.count)
self.progressFolders.setProgress(folders_progress, animated: true)
self.progressImages.setProgress(images_progress, animated: true)
self.lblSpeedDownload.text = Util.getByteString(self.downloadPopup.getDownloadSpeedBySecond())+"/s"
}
}
Do you find something wrong with my code?
Do you have other way to update UI?
Thank you for reading me!