Assignment of completion handler to closure variable in Swift - ios

Question

I have stumbled across the following piece of code and I can't understand exactly how it works.
There is the following property which is populated when a method of AVCapturePhotoCaptureDelegate is called:
var photoCaptureCompletionBlock: ((UIImage?, Error?) -> Void)?
The delegate method is triggered by the following piece of code:
func captureImage(completion: #escaping (UIImage?, Error?) -> Void) {
let settings = AVCapturePhotoSettings()
self.photoOutput?.capturePhoto(with: settings, delegate: self)
self.photoCaptureCompletionBlock = completion
}
The line that triggers the delegate is:
self.photoOutput?.capturePhoto(with: settings, delegate: self)
and immediately after that the completion variable is assigned to self.photoCaptureCompletionBlock
Conceptually I would understand the opposite, i.e. to assign self.photoCaptureCompletionBlock to completion and not the other way around (which is not possible without an inout variable since completion is a let).
What are the mechanics behind this assignment? How does it work?
EDIT: For context, the delegate method that is called is the following:
func photoOutput(_ output: AVCapturePhotoOutput,
didFinishProcessingPhoto photoSampleBuffer: CMSampleBuffer?,
previewPhoto previewPhotoSampleBuffer: CMSampleBuffer?,
resolvedSettings: AVCaptureResolvedPhotoSettings,
bracketSettings: AVCaptureBracketedStillImageSettings?,
error: Error?) {
if let error = error {
self.photoCaptureCompletionBlock?(nil, error)
} else if let buffer = photoSampleBuffer,
let data = AVCapturePhotoOutput.jpegPhotoDataRepresentation(forJPEGSampleBuffer: buffer, previewPhotoSampleBuffer: nil) {
let image = UIImage(data: data)
self.photoCaptureCompletionBlock?(image, nil)
} else {
self.photoCaptureCompletionBlock?(nil, CameraControllerError.unknown)
}
}

Answer 1

Your method captureImage(completion: #escaping (UIImage?, Error?) -> Void) is not a part of AVCapturePhotoCaptureDelegate protocol. It is a custom method of the object's API which implements this protocol.
So since there is no full code of that object, I can only guess. In this method you start photo capturing and pass the completion block, which will be triggered when photo capturing will finish.
This completion block stored in object's variable and I think some other method of delegate, for ex this one func photoOutput(AVCapturePhotoOutput, didFinishProcessingPhoto: AVCapturePhoto, error: Error?) in object implementation will trigger this completion block after photo capturing will be finished.