Concurrent image processing task with photo taking in iOS - ios

Question

i need your help with GCD in swift, i just started using this tool and i'm little confused. So let's say i have a function foo() that taking photo and saving it to var, inside this function there is another function that checking if there are faces on that photo and returning true or false. Next, when user taps usePhotoFromPreview button it will save photo through custom delegate whenever there aren't any faces on that photo like this:
var image: UIImage?
var checkingPhotoForFaces: Bool?
func foo(){
let videoConnection = imageOutput?.connection(withMediaType: AVMediaTypeVideo)
imageOutput?.captureStillImageAsynchronously(from: videoConnection, completionHandler: { (imageDataSampleBuffer, error) -> Void in
if let imageData = AVCaptureStillImageOutput.jpegStillImageNSDataRepresentation(imageDataSampleBuffer) {
image = UIImage(data: imageData)
self.checkingPhotoForFaces = self.detect(image: image)
}
})
}
func detect(image: UIImage) -> Bool{
.....
}
func usePhotoFromPreview(){
self.dismiss(animated: true, completion: {
if self.checkingPhotoForFaces == false{
self.delegate?.takenPhoto(photo: self.image!)
print("photo taken")
}else{
self.delegate?.takenPhoto(photo: nil)
print("no photo taken")
}
})
}
So now the tricky part, i'd like to make detect function to be executed asynchronously and only when it's done execute usePhotoFromPreview. detect func i suppose should be on main thread because of CoreImage implementation. I simply made something like this:
func foo(){
...
DispatchQueue.global().async {
self.checkingPhotoForFaces = self.detect(image: stillImage)
}
...
}
but the problem is when user taps too quick on that button it won't work, cuz detect func is still processing, so i think i need some kind of queue but i'm confused which one.
btw. please, do not cling to the lack of some parameters above, i'm writing it on the fly and that's not the point
Thank you for your help

Answer 1

Change detect as commented above:
func detect(image: UIImage, completion: ()->(detectedFaces: Bool)){
//.....
completion(true|false)
}
Then
if let imageData = AVCaptureStillImageOutput.jpegStillImageNSDataRepresentation(imageDataSampleBuffer) {
image = UIImage(data: imageData)
self.detect(image) { detectedFaces in
self.checkingPhotoForFaces = detectedFaces
//Enable button?
}
}