如何在Swift中下载数据时不阻止UI

Question

I have question regarding DispatchQueue. In one of my view controllers, I do need to download an image from web and display it on a UIImageView. While the app downloading the image, I am trying not to block the UI using DispatchQueue. The below code does not seem to work. Can anyone explain why? and how to do this properly?

let store_id = self.store!._id

DispatchQueue.global().async {
let imageUrl = URL(string: "www.image.com")
self.storeImageData = imageUrl != nil ? try? Data(contentsOf:imageUrl!) : nil

    if self.storeImageData != nil {
        DispatchQueue.main.async {
            self.topBackgroundImageView.image = UIImage(data:self.storeImageData!)
        }
    }
}

Answer 1

I would not recommend your way of downloading over the network, use either URLSession or Alamofire . Here is some code you can follow:

func downloadImageFrom(urlString:String, completion:@escaping(Data?)->()) {
    guard let url = URL(string:urlString) else { return }
    let request = URLRequest(url: url)
    URLSession.shared.dataTask(with: request) { (data, _, err) in
        if err != nil {
            // handle error if any
        }
        // you should check the reponse status
        // if data is a json object/dictionary so decode it
        // if data is regular data then pass it to your callback
        completion(data)
    }.resume()
}

func doWork() {
    DispatchQueue.global(qos: .background).async { [weak self] in
        guard let strongSelf = self else { return } // to avoid reference cycle
        strongSelf.downloadImageFrom(urlString: "http://www.image.com", completion: { (data) in
            if let _data = data {
                // now you have the data
                DispatchQueue.main.async {
                    // display your imageView with the data
                }
            }
        })
    }
}

Answer 2

Hope will helpful to you download an image from web and display it on a UIImageView.

SDWebImage does very aggressive caching by default. It ignores all kind of caching control header returned by the HTTP server and cache the returned images with no time restriction. It implies your images URLs are static URLs pointing to images that never change. If the pointed image happen to change, some parts of the URL should change accordingly.

import SDWebImage

imageView.sd_setImage(with: URL(string: "http://www.image.com/image.jpg"), placeholderImage: UIImage(named: "placeholder.png"))

// Or using this code
[imageView sd_setImageWithURL:[NSURL URLWithString:@"http://www.image.com/image.jpg"]
         placeholderImage:[UIImage imageNamed:@"avatar-placeholder.png"]
                  options:SDWebImageRefreshCached];

Add a progress indicator

imageView.sd_setShowActivityIndicatorView(true)
imageView.sd_setIndicatorStyle(.Gray)