创建一个加载图像/活动指示器，直到图像迅速显示在屏幕上

问题描述：

我想在从网站URL下载图像时在屏幕上显示活动指示器，然后在images.image中显示这是我下面的代码.每次下载图像时，它都会立即打印状态，并且活动指示器永远不会出现.一直在网上搜索，但我还是不明白.请帮助

i want to show an activity indicator in the screen while the image is being downloaded from a website url then show in the images.image this is my code below. everytime i download image it print the status right away and the activity indicator never appear. been searching through the web but i still didnt understand. please help

 let mygroup = DispatchGroup()
 var activityIndicator = UIActivityIndicatorView()

func downloadpic(sender:UIButton){
    let catPictureURL = URL(string: addr)!

    // Creating a session object with the default configuration.
    // You can read more about it here https://developer.apple.com/reference/foundation/urlsessionconfiguration
    let session = URLSession(configuration: .default)

    // Define a download task. The download task will download the contents of the URL as a Data object and then you can do what you wish with that data.

    let downloadPicTask = session.dataTask(with: catPictureURL) { (data, response, error) in
        // The download has finished.
        if let e = error {
            print("Error downloading cat picture: \(e)")
        } else {
            // No errors found.
            // It would be weird if we didn't have a response, so check for that too.
            if let res = response as? HTTPURLResponse {
                // put loading screen here
                self.activityIndicator = UIActivityIndicatorView(activityIndicatorStyle: .white)
                self.activityIndicator.frame = CGRect(x: 0, y: 0, width: 46, height: 46)
                self.activityIndicator.startAnimating()
                self.mygroup.enter()
                print("downloading image")
                print("Downloaded cat picture with response code \(res.statusCode)")
                if let imageData = data {
                    // Finally convert that Data into an image and do what you wish with it.
                     self.images.image = UIImage(data: imageData)
                        self.mygroup.leave()
                        print("image already downloaded")
                        self.activityIndicator.stopAnimating()
                        self.activityIndicator.hidesWhenStopped = true


                    // Do something with your image.
                } else {
                    print("Couldn't get image: Image is nil")
                }
            } else {
                print("Couldn't get response code for some reason")
            }
        }
    }

    downloadPicTask.resume()
}

答

您遇到了此问题，因为:

You got this problem, because:

您没有将 activityIndicator 添加到任何视图
您在 completionHandler 块中配置了 activityIndicator

You didn't add the activityIndicator to any views
You configured the activityIndicator in the completionHandler block

让我们在 activityIndicator 中配置 viewDidLoad()方法:

self.activityIndicator = UIActivityIndicatorView(activityIndicatorStyle: .gray)
self.activityIndicator.frame = CGRect(x: 0, y: 0, width: 46, height: 46)
self.activityIndicator.hidesWhenStopped = true

view.addSubview(self.activityIndicator)

并开始对其进行动画处理，然后再恢复 dataTask

And start to animate it before resuming the dataTask

self.activityIndicator.startAnimating()

然后，在 completionHandler

self.activityIndicator.stopAnimating()

最终代码:

let mygroup = DispatchGroup()
var activityIndicator = UIActivityIndicatorView()

override func viewDidLoad() {
    super.viewDidLoad()

    self.activityIndicator = UIActivityIndicatorView(activityIndicatorStyle: .gray)
    self.activityIndicator.frame = CGRect(x: 0, y: 0, width: 46, height: 46)
    self.activityIndicator.hidesWhenStopped = true

    view.addSubview(self.activityIndicator)
}

func downloadpic(sender: UIButton) {
    let catPictureURL = URL(string: addr)!

    // Creating a session object with the default configuration.
    // You can read more about it here https://developer.apple.com/reference/foundation/urlsessionconfiguration
    let session = URLSession(configuration: .default)

    // Define a download task. The download task will download the contents of the URL as a Data object and then you can do what you wish with that data.

    self.activityIndicator.startAnimating()

    let downloadPicTask = session.dataTask(with: catPictureURL) { (data, response, error) in
        // The download has finished.
        if let e = error {
            print("Error downloading cat picture: \(e)")
        } else {
            // No errors found.
            // It would be weird if we didn't have a response, so check for that too.
            if let res = response as? HTTPURLResponse {
                // put loading screen here

                self.mygroup.enter()
                print("downloading image")
                print("Downloaded cat picture with response code \(res.statusCode)")
                if let imageData = data {
                    // Finally convert that Data into an image and do what you wish with it.
                    self.images.image = UIImage(data: imageData)
                    self.mygroup.leave()
                    print("image already downloaded")
                    self.activityIndicator.stopAnimating()

                    // Do something with your image.
                } else {
                    print("Couldn't get image: Image is nil")
                }
            } else {
                print("Couldn't get response code for some reason")
            }
        }
    }

    downloadPicTask.resume()
}

结果！

创建一个加载图像/活动指示器，直到图像迅速显示在屏幕上

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