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需要帮助来弄清楚为什么我的函数在我认为应该执行但它在代码中的完成块之后执行时没有执行。我对 Xcode 还很陌生，所以如果这里听起来有些混乱，请原谅。下面是我的代码。

类 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) {

// 最初调用这个类时，我调用函数的下面的行没有执行。但是随着代码的运行，下面的这行代码是在 COMPLETIONOPERATION = BLOCKOPERATION 完成后执行的。

    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" 仅在此时执行

        })
    }



    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)

}

}

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