func handleGetAllPhotoURLs 从下面的行调用，我已经确认代码行只执行一次断点。

_ = FlickrClient.getAllPhotoURLs(currentPin: self.currentPin, fetchCount: fetchCount, completion: self.handleGetAllPhotoURLs(pin:urls:error:))

根据我的 print 语句的输出，该函数运行两次，因为如果 urls.count 不为零，它会打印两行输出。但是，如果 urls.count 为零，那么我只会得到一个打印语句，指出“urls.count ---> 0”

handleGetAllPhotoURLs ---> urls.count ---> 0//这一行总是被打印

handleGetAllPhotoURLs ---> urls.count ---> 21//只有当urls参数不为空时才会打印这一行

func handleGetAllPhotoURLs(pin: Pin, urls: [URL], error: Error?){
    print("handleGetAllPhotoURLs ---> urls.count  ---> \(urls.count)")

    let backgroundContext: NSManagedObjectContext! = dataController.backGroundContext
    if let error = error {
        print("func mapView(_ mapView: MKMapView, didSelect... \n\(error)")
        return
    }

    let pinId = pin.objectID
    backgroundContext.perform {
        let backgroundPin = backgroundContext.object(with: pinId) as! Pin
        backgroundPin.urlCount = Int32(urls.count)
        try? backgroundContext.save()
    }

    for (index, currentURL) in urls.enumerated() {
        URLSession.shared.dataTask(with: currentURL, completionHandler: { (imageData, response, error) in
            guard let imageData = imageData else {return}
            connectPhotoAndPin(dataController: self.dataController, currentPin:  pin , data: imageData, urlString: currentURL.absoluteString, index: index)
        }).resume()
    }
}

此外，我有一个 UILabel，它只在 urls.count 为零时显示自己，而我只想在 urls 为空时显示它。

现在，如果 urls 不为空，应用程序会非常快速地闪烁空消息 UILabel。这对我来说很有意义，因为 print 语句显示 urls 数组暂时为空。

当 urls.count 不为零时，我是否有办法确定避免向用户闪烁空消息 UILabel？

编辑:根据要求添加了下面的代码。调用下面的函数以在完成处理程序中获取 [URL]。然后完成处理程序被送入: func handleGetAllPhotoURLs(pin: Pin, urls: [URL], error: 错误？)

class func getAllPhotoURLs(currentPin: Pin, fetchCount count: Int, completion: @escaping (Pin, [URL], Error?)->Void)-> URLSessionTask?{
    let latitude = currentPin.latitude
    let longitude = currentPin.longitude
    let pageNumber = currentPin.pageNumber


    let url = Endpoints.photosSearch(latitude, longitude, count, pageNumber).url

    var array_photo_URLs = [URL]()
    var array_photoID_secret = [[String: String]]()
    var array_URLString = [String]()
    var array_URLString2 = [String]()
    var count = 0

    let task = URLSession.shared.dataTask(with: url) { data, response, error in
        guard let dataObject = data, error == nil else {
            DispatchQueue.main.async {
                completion(currentPin, [], error)
            }
            return
        }

        do {
            let temp = try JSONDecoder().decode(PhotosSearch.self, from: dataObject)
            temp.photos.photo.forEach{
                let tempDict = [$0.id : $0.secret]
                array_photoID_secret.append(tempDict)

                let photoURL = FlickrClient.Endpoints.getOnePicture($0.id, $0.secret)
                let photoURLString = photoURL.toString
                array_URLString.append(photoURLString)

                getPhotoURL(photoID: $0.id, secret: $0.secret, completion: { (urlString, error) in
                    guard let urlString = urlString else {return}
                    array_URLString2.append(urlString)
                    array_photo_URLs.append(URL(string: urlString)!)
                    count = count + 1
                    if count == temp.photos.photo.count {
                        completion(currentPin, array_photo_URLs, nil)
                    }
                })
            }
            completion(currentPin, [], nil)
            return
        } catch let conversionErr {
            DispatchQueue.main.async {
                completion(currentPin, [], conversionErr)
            }
            return
        }
    }
    task.resume()
    return task
}

最佳答案

在 do block 中，您调用了两次 completion。请看更正，

do {
    let temp = try JSONDecoder().decode(PhotosSearch.self, from: dataObject)
    if temp.photos.photo.isEmpty == false {
       temp.photos.photo.forEach{
        let tempDict = [$0.id : $0.secret]
        array_photoID_secret.append(tempDict)

        let photoURL = FlickrClient.Endpoints.getOnePicture($0.id, $0.secret)
        let photoURLString = photoURL.toString
        array_URLString.append(photoURLString)

        getPhotoURL(photoID: $0.id, secret: $0.secret, completion: { (urlString, error) in
            guard let urlString = urlString else {return}
            array_URLString2.append(urlString)
            array_photo_URLs.append(URL(string: urlString)!)
            count = count + 1
            if count == temp.photos.photo.count {
                completion(currentPin, array_photo_URLs, nil)
            }
        })
      }
    } else {
        completion(currentPin, [], nil)
    }
    return
}

关于swift - 如果执行嵌套异步调用，函数运行两次，否则运行一次。需要帮助预先确定何时会发生，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/55625101/