Swift 3 - Display alert controller asynchronously with long function running in background - ios

Question

I am using Swift 3.
The behavior I am trying to do is: the user clicks on a button, a spinning gear alert controller displays while it kicks off a long-running function. Once that function is done executing, the spinning gear goes away and the view controller dismisses.
The code below kicks off the doProcessing function but doesn't display the spinning gear until about a second before the view dismisses. So this isn't quite right.
func displaySpinningGear() {
print("display spinning gear")
// show the alert window box
let activityAlertController = UIAlertController(title: "Processing", message: "Please wait while the photo is being processed.", preferredStyle: .alert)
//create an activity indicator
let indicator = UIActivityIndicatorView(frame: activityAlertController.view.bounds)
indicator.autoresizingMask = [.flexibleWidth, .flexibleHeight]
indicator.hidesWhenStopped = true
indicator.activityIndicatorViewStyle = UIActivityIndicatorViewStyle.gray
//add the activity indicator as a subview of the alert controller's view
activityAlertController.view.addSubview(indicator)
indicator.isUserInteractionEnabled = false // required otherwise if there buttons in the UIAlertController you will not be able to press them
indicator.startAnimating()
print("start animating")
self.present(activityAlertController, animated: true, completion: nil)
}
func onButtonClick() {
self.displaySpinningGear()
DispatchQueue.main.async {
self.doProcessing() // long running function
}
if let viewController = presentingViewController {
// This block will dismiss both current and a view controller presenting current
viewController.dismiss(animated: true, completion: nil)
}
else {
// This block will dismiss only current view controller
self.dismiss(animated: true, completion: nil)
}
}
The code below kicks off the doProcessing function but the view dismisses immediately and I can tell from the console that my doProcessing function is still running. This is not right either.
function onButtonClick() {
DispatchQueue.global(qos: .background).async {
print("Processing")
self.doProcessing() // run in background
DispatchQueue.main.async {
self.displaySpinningGear()
}
}
if let viewController = presentingViewController {
// This block will dismiss both current and a view controller presenting current
viewController.dismiss(animated: true, completion: nil)
}
else {
// This block will dismiss only current view controller
self.dismiss(animated: true, completion: nil)
}
}
How do I get the background function to kick off while displaying a spinning gear and dismiss the view and alert controller when the background function is done running (not before)?
EDIT
Tried moving the code to spin the gear outside the background block as per #Honey's suggestion in the comment but to no avail. The view immediately dismisses while the process function is still processing (I can tell through print statements).
func onButtonClick() {
DispatchQueue.main.async {
self.displaySpinningGear()
}
DispatchQueue.global(qos: .background).async {
print("Processing")
self.doProcessing() // run in background
}
if let viewController = presentingViewController {
// This block will dismiss both current and a view controller presenting current
viewController.dismiss(animated: true, completion: nil)
}
else {
// This block will dismiss only current view controller
self.dismiss(animated: true, completion: nil)
}
}

Answer 1

Make a Callback from long running function so when it ends returns a value and catch it to disappear the alert.
Try it:
typealias DoProcessingCallback = (_ finished: Bool) -> Void
func onButtonClick() {
self.displaySpinningGear()
self.doProcessing(callback: { (finished) in
if finished {
// Here you DismissViewController
// Here you DismissAlert
}
}) // long running function
}
func doProcessing(callback: DoProcessingCallback) {
// YOUR LONG CODE....
// When you know it already finished
callback(true)
}
Hope it helps you

Answer 2

I had the same issue and tried a bunch of different things and this is what worked:
activityView.alpha = 0.8
DispatchQueue.global(qos: .default).async(execute: {
DispatchQueue.main.async(execute: {
self.performSegue(withIdentifier: "cropToProcessed", sender: self)
})
})
Basically I set the alpha for activity indicator to 0.0 initially and when the button is pressed I set it to 0.8 and I set it back to 0.0 in viewWillDisappear and it works