Not Executing Function at The Right Time, But Executed After Completion Block - xcode

Question

Need help in figuring out why my function is not executing when I thought it should but it executed after the completion block in the code. I am fairly new to Xcode so please excuse me if things sound confusing here. Below is my code.
class ImageDownloader{
typealias completionHandler = (Result<Set<ARReferenceImage>, Error>) -> ()
typealias ImageData = (image: UIImage, orientation: CGImagePropertyOrientation, physicalWidth: CGFloat, name: String)
static let URLDic = [ReferenceImagePayload]()
class func getDocumentData(completion:#escaping ([ReferenceImagePayload]) -> ()) {
var documentCollection: [ReferenceImagePayload] = []
db.collection("Users").getDocuments {(snapshot, error) in
if error == nil && snapshot != nil {
var index = 0
for document in snapshot!.documents {
let loadData = document.data()
index += 1
if loadData["Target URL"] != nil {
let url = loadData["Target URL"]
let urlString = URL(string: "\(String(describing: url ?? ""))")
let urlName = loadData["Target Image"]
documentCollection.append(ReferenceImagePayload(name: urlName as! String, url: urlString!))
if snapshot!.documents.count == index {
// After finished, send back the loaded data
completion(documentCollection)
}
}
}
}
}
}
static var receivedImageData = [ImageData]()
class func downloadImagesFromPaths(_ completion: #escaping completionHandler) {
// THE LINE BELOW WHERE I CALL THE FUNCTION IS NOT EXECUTED WHEN THIS CLASS IS INITIALLY CALLED. BUT AS THE CODE RUNS, THIS LINE BELOW IS EXECUTED AFTER THE COMPLETIONOPERATION = BLOCKOPERATION IS COMPLETED.
let loadedDataDic: () = getDocumentData { (URLDic) in
print(URLDic.self, "Got it")
}
let operationQueue = OperationQueue()
operationQueue.maxConcurrentOperationCount = 6
let completionOperation = BlockOperation {
OperationQueue.main.addOperation({
completion(.success(referenceImageFrom(receivedImageData)))
// LINE "let loadedDataDic: () = getDocumentData" ONLY GOT EXECUTED AT THIS POINT
})
}
URLDic.forEach { (loadData) in
let urlstring = loadData.url
let operation = BlockOperation(block: {
do{
let imageData = try Data(contentsOf: loadData.url)
print(imageData, "Image Data")
if let image = UIImage(data: imageData){
receivedImageData.append(ImageData(image, .up, 0.1, loadData.name))
}
}catch{
completion(.failure(error))
}
})
completionOperation.addDependency(operation)
}
operationQueue.addOperations(completionOperation.dependencies, waitUntilFinished: false)
operationQueue.addOperation(completionOperation)
}
}