Loading images asynchronously

There are a few details that need to be paid attention to when loading images from URLs in your Swift code:

• loading and decoding the image on a background thread
• updating the UIImageView on the main thread
• not starting too many parallel downloads
• cancelling downloads when images are not needed anymore

I recommend to use one of the two popular libraries that do the trick in just a few lines of code: SDWebImage or Kingfisher. Both are easy to get via CocoaPods and are quite similar in API and supported features.

SDWebImage

After adding the SDWebImage pod to the project:

pod 'SDWebImage'

Given an image view and a URL to load the image from, loading the image with an activity indicator is as simple as:

import UIKit
import SDWebImage

class ExampleViewController: UIViewController {
    
    @IBOutlet weak var imageView : UIImageView!
    
    // ...
    
    func loadImage() {
        let imageURL = URL(string: "https://upload.wikimedia.org/wikipedia/commons/1/15/Red_Apple.jpg")!
        imageView.sd_setShowActivityIndicatorView(true)
imageView.sd_setIndicatorStyle(.gray)
imageView.sd_setImage(with: imageURL)
    }
    
}

Kingfisher

Pretty much the same for Kingfisher:

pod 'Kingfisher'

Loading an image from an URL with activity indicator and displaying it in the image view:

import UIKit
import Kingfisher

class ExampleViewController: UIViewController {
    
    @IBOutlet weak var imageView : UIImageView!
    
    // ...
    
    func loadImage() {
        let imageURL = URL(string: "https://upload.wikimedia.org/wikipedia/commons/1/15/Red_Apple.jpg")!
        imageView.kf.indicatorType = .activity
imageView.kf.setImage(with: imageURL)
    }
    
}

There is a nice cheat sheet that demonstrates common cases like customizing the indicator or getting notified when downloads complete.

Differences between the libraries: SDWebImage vs. Kingfisher

• SDWebImage supports progressive display of the image while loading is in progress via options: [.progressiveDownload] which is nice when loading larger images. This is not supported in Kingfisher as of now.
• SDWebImage has support for Webp images included as a subspec that works out of the box, while for Kingfisher it’s an external plugin KingfisherWebP that requires extra configuration - for applications that have a lot of resources, webp helps delivering higher quality with shorter download times.
• SDWebImage is implemented in Objective-C, Kingfisher in Swift 4.

Example project

Here is an example project that includes an example for both libraries which are included via CocoaPods: