如何从 URL 数组下载图像并将其一张一张附加到数组 Swift

Question

我尝试从 URL 数组下载图像并将图像附加到我的 collection 视图的图像数组。但它不会将其一张一张地下载并附加到图像数组中。所以我尝试使用调度组，但没有用。正确的添加调度组的方法是什么？

func downloadPhoto(){
    progressImgArray.removeAll() // this is the image array
    for i in 0..<progressArray.count{
        let urls = URL(string: progressArray[i].userProgressImage) // this is the URL array
        let group = DispatchGroup()

        print(urls ?? "nil image")
        print("-------GROUP ENTER-------")

        group.enter()
        URLSession.shared.dataTask(with: urls!, completionHandler: { data, response, error in

            print(response?.suggestedFilename ?? urls?.lastPathComponent ?? "nil image")

            if let imgData = data {
                if UIImage(data: imgData) != nil {
                    DispatchQueue.main.async() {
                        self.progressImgArray.append(UIImage(data: data!)!)
                    }
                }
            }

            group.leave()
        }).resume()

        group.notify(queue: .main) {
            self.collectionView.reloadData()
        }
    }
}

打印：顺序错误

http://sweat.asia/app/users_progresses/users_progresses_111_761319316286fcb38a0b.png
-------GROUP ENTER-------
http://sweat.asia/app/users_progresses/users_progresses_111_263983427965d44f5021.png
-------GROUP ENTER-------
http://sweat.asia/app/users_progresses/users_progresses_111_3361893721fa8f84015a.png
-------GROUP ENTER-------
users_progresses_111_263983427965d44f5021.png
users_progresses_111_761319316286fcb38a0b.png
users_progresses_111_3361893721fa8f84015a.png

怎么做成这样

http://sweat.asia/app/users_progresses/users_progresses_111_761319316286fcb38a0b.png
    -------GROUP ENTER-------
users_progresses_111_761319316286fcb38a0b.png

http://sweat.asia/app/users_progresses/users_progresses_111_263983427965d44f5021.png
    -------GROUP ENTER-------
users_progresses_111_263983427965d44f5021.png

...

Answer 1

您的问题是 group.notify 将阻塞，直到调度组为空。但是你在任何 "leaves" 被调用之前用三个 "enters" 填充它所以 "notify" 不会发生直到第三个 "leave" 发生。

相反 "notify" 您需要使用 "wait"。要使用 "wait"，您需要将整个内容放在一个新的后台线程中。

更新后的代码应该如下所示：

func downloadPhoto(){
    DispatchQueue.global().async {
        progressImgArray.removeAll() // this is the image array

        for i in 0..<progressArray.count {
            guard let url = URL(string: progressArray[i].userProgressImage) else {
                continue
            }

            let group = DispatchGroup()

            print(url)
            print("-------GROUP ENTER-------")

            group.enter()
            URLSession.shared.dataTask(with: url, completionHandler: { data, response, error in
                print(response?.suggestedFilename ?? url.lastPathComponent)

                if let imgData = data, let image = UIImage(data: imgData) {
                    DispatchQueue.main.async() {
                        self.progressImgArray.append(image)
                        self.collectionView.reloadData()
                    }
                } else if let error = error {
                    print(error)
                }

                group.leave()
            }).resume()

            group.wait()
        }
    }
}

我还进行了其他几项改进，包括更好地处理可选值。

Answer 2

rmaddy 回答的小更新：

Get Notified when array is full with downloaded images.

func downloadImageFromUrl()
{
    DispatchQueue.global().async
        {
            self.imageArray.removeAll()
            let dispatchGroup = DispatchGroup()

            for i in 0..<(self.imageUrlArray?.count)!
            {
                guard let url = URL(string: (self.imageUrlArray?![i])!)
                    else{ continue }

                dispatchGroup.enter()
                Alamofire.request(url).responseData(completionHandler: { response in

                    guard
                        let data = response.result.value,
                        let image = UIImage(data:data)
                        else {
                            print("Error in downlaoding images : ",(response.error?.localizedDescription)!)
                            return
                    }
                    DispatchQueue.main.async {
                        self.imageArray.append(image)
                    }
                    dispatchGroup.leave()
                })
                dispatchGroup.wait()
            }
            dispatchGroup.notify(queue: DispatchQueue.main,execute: {
                print("are we done.... :",self.imageArray.count)
                self.yourCollection.reloadData()
            })
    }
}

I mean for me it is really working like a charm, I've a tableview in which one of the cell of tableview contains Collection View

如何从 URL 数组下载图像并将其一张一张附加到数组 Swift

How to download the image from URL array and append it to an array one by one Swift

grand-central-dispatch

ios

swift