run a function when another ends, iOS, Swift - ios

Question

Im trying to call a function when another one ends but I keep getting an error that tells me the image is not above 0 pixels high and wide so I presume it hasn't got the image before the function is called. when I add a breakpoint where the function to call the OCR the app doesn't show the image on the screen at this point which is why I came to that conclusion.
here is the error I get from the ocr.
NSAssert( widthOfImage > 0 && heightOfImage > 0, #"Passed image must not be empty - it should be at least 1px tall and wide");
below is my console readout where I placed prints to see the flow.
Tony 1 Requested....
Tony 3 run OCR....
Tony 2 Handle Rectangle....
Tony: Corected image here......
(lldb)
Below is my code. should i have in a completion that makes sure the function is not called until the image is in place?.
func startOCR() {
swiftOCRInstance.recognize(correctedImageView.image!) {recognizedString in
print(recognizedString)
self.classificationLabel.text = recognizedString
}
}
lazy var rectanglesRequest: VNDetectRectanglesRequest = {
print("Tony 1 Requested....")
return VNDetectRectanglesRequest(completionHandler: self.handleRectangles)
}()
func handleRectangles(request: VNRequest, error: Error?) {
guard let observations = request.results as? [VNRectangleObservation]
else { fatalError("unexpected result type from VNDetectRectanglesRequest") }
guard let detectedRectangle = observations.first else {
DispatchQueue.main.async {
self.classificationLabel.text = "No rectangles detected."
}
return
}
let imageSize = inputImage.extent.size
// Verify detected rectangle is valid.
let boundingBox = detectedRectangle.boundingBox.scaled(to: imageSize)
guard inputImage.extent.contains(boundingBox)
else { print("invalid detected rectangle"); return }
// Rectify the detected image and reduce it to inverted grayscale for applying model.
let topLeft = detectedRectangle.topLeft.scaled(to: imageSize)
let topRight = detectedRectangle.topRight.scaled(to: imageSize)
let bottomLeft = detectedRectangle.bottomLeft.scaled(to: imageSize)
let bottomRight = detectedRectangle.bottomRight.scaled(to: imageSize)
let correctedImage = inputImage
.cropped(to: boundingBox)
.applyingFilter("CIPerspectiveCorrection", parameters: [
"inputTopLeft": CIVector(cgPoint: topLeft),
"inputTopRight": CIVector(cgPoint: topRight),
"inputBottomLeft": CIVector(cgPoint: bottomLeft),
"inputBottomRight": CIVector(cgPoint: bottomRight)
])
// .applyingFilter("CIColorControls", parameters: [
// kCIInputSaturationKey: 0,
// kCIInputContrastKey: 32
// ])
// Show the pre-processed image
DispatchQueue.main.async {
self.correctedImageView.image = UIImage(ciImage: correctedImage)
if self.correctedImageView.image != nil {
print("Tony 2 Handle Rectangle....")
print("Tony: Corected image here......")
}else {
print("Tony: No corected image......")
}
}
print("Tony 3 run OCR....")
self.startOCR()
}
I also get a purple error the says the UIImage should be used on the main thread in the pic below...

Answer 1

You need to use the main thread whenever you're working with UIImageView, or any other UIKit class (unless otherwise noted, such as when constructing UIImages on background threads).
You can use GCD to do this on the main thread.
DispatchQueue.main.async {
//Handle UIKit actions here
}
Source: Apple Documentation
Threading Considerations:
Manipulations to your application’s user
interface must occur on the main thread. Thus, you should always call
the methods of the UIView class from code running in the main thread
of your application. The only time this may not be strictly necessary
is when creating the view object itself, but all other manipulations
should occur on the main thread.