Displaying Activity indicator with OCR and using threads - swift

Question

I am trying to display an activity indicator whilst some text recognition happens. If i just start and stop[ the indicator around the recognition code it never shows. The issue i have is that if use:
activityIndicator.startAnimating()
DispatchQueue.main.async( execute: {
self.performTextRecognition()
})
activityIndicator.stopAnimating()
self.performSegue(withIdentifier: "segueToMyBills", sender: self)
The indicator never shows as it performs the segue and the table view in the next view controller shows no information because the text recognition hasn't completed. I've never touched on threads until now so a little insight on what to do would be much appreciated. Thanks!

Answer 1

Well your problem is that your OCR happens on the main thread. That blocks the thread thus never having time to draw the activity indicator. Try to modify your code into this:
activityIndicator.startAnimating()
DispatchQueue.global(qos: .background).async { [weak weaKSelf = self] in
// Use a weak reference to self here to make sure no retain cycle is created
weakSelf?.performTextRecognition()
DispatchQueue.main.async {
// Make sure weakSelf is still around
guard let weakSelf = weakSelf else { return }
weakSelf.activityIndicator.stopAnimating()
weakSelf.performSegue(withIdentifier: "segueToMyBills", sender: weakSelf)
}
}

Answer 2

Try to add a completion handler to your self.performTextRecognition() function in this way
function performTextRecognition(completion: ((Error?) -> Void)? = .none) {
//you can replace Error with Any type or leave it nil
//do your logic here
completion?(error)
}
and then call the function like this :
performTextRecognition(completion: { error in
activityIndicator.stopAnimating()
self.performSegue(withIdentifier: "segueToMyBills", sender: self)
})