UIActivityIndicatorView not showing for duration of async task - ios

Question

I'm working on an app in iOS wherein I need to start spinning a UIActivityIndicatorView, upload an image to a server, and when the upload is completed, stop spinning the activity indicator.
I'm currently using XCode 7 Beta and am testing the app on the iOS simulator as an iPhone 6 and iPhone 5. My issue is that the activity indicator won't end immediately after file upload, but several (~28 seconds) later. Where should I place my calls to cause it to end?
I have an #IBOutlet function attached to the button I use to start the process, which contains the startAnimating() function, and which calls a dispatch_async method that contains the call to uploadImage, which contains the signal, wait, and stopAnimating() functions.
Note that
let semaphore = dispatch_semaphore_create(0)
let priority = DISPATCH_QUEUE_PRIORITY_HIGH
are defined at the top of my class.
#IBAction func uploadButton(sender: AnyObject) {
self.activityIndicatorView.startAnimating()
dispatch_async(dispatch_get_global_queue(priority, 0)) {
self.uploadImage(self.myImageView.image!)
} // end dispatch_async
} // works with startAnimating() and stopAnimating() in async but not with uploadImage() in async
func uploadImage(image: UIImage) {
let request = self.createRequest(image)
let session : NSURLSession = NSURLSession.sharedSession()
let task : NSURLSessionTask = session.dataTaskWithRequest(request, completionHandler: {
(data, response, error) in
if error != nil {
print(error!.description)
} else {
let httpResponse: NSHTTPURLResponse = response as! NSHTTPURLResponse
if httpResponse.statusCode != 200 {
print(httpResponse.description)
} else {
print("Success! Status code == 200.")
dispatch_semaphore_signal(self.semaphore)
}
}
})! // end dataTaskWithResult
task.resume()
dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER)
self.activityIndicatorView.stopAnimating()
} // end uploadImage
This is just one version of my code, I have moved several things around several different ways. I have tried this:
#IBAction func uploadButton(sender: AnyObject) {
self.activityIndicatorView.startAnimating()
dispatch_async(dispatch_get_global_queue(priority, 0)) {
self.uploadImage(self.myImageView.image!)
dispatch_semaphore_signal(self.semaphore)
} // end dispatch_async
dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER)
self.activityIndicatorView.stopAnimating()
}
And several, several other ways of moving my code around to attempt to get the activity indicator to display for the duration of the image upload and then immediately quit. In some cases the spinner doesn't appear at all for the duration of program execution. I read this post and this question and have migrated my dispatch_semaphore_wait and stopAnimating() to the uploadImage() method to circumvent this, but can't find enough information in the UIActivityIndicatorView documentation about the UI updating to know any other way of updating it, though I believe this might be at the core of the problem.
All I need is for the spinner to start before the upload process begins (dataTaskWithRequest) and end once it has succeeded or failed. What am I doing wrong?

Answer 1

Instead of using semaphores, you could just dispatch directly to the main thread in your async task,
func uploadImage(image: UIImage) {
let request = self.createRequest(image)
let session : NSURLSession = NSURLSession.sharedSession()
let task : NSURLSessionTask = session.dataTaskWithRequest(request, completionHandler: {
(data, response, error) in
if error != nil {
print(error!.description)
} else {
let httpResponse: NSHTTPURLResponse = response as! NSHTTPURLResponse
if httpResponse.statusCode != 200 {
print(httpResponse.description)
} else {
print("Success! Status code == 200.")
}
}
// dispatch to main thread to stop activity indicator
dispatch_async(disptach_get_main_queue()) {
self.activityIndicatorView.stopAnimating()
}
})! // end dataTaskWithResult
task.resume()
} // end uploadImage