let url = URL(string: "image_url")!
imageView.stx.image(atURL: url)

let url = URL(string: "image_url")!
let placeholder = UIImage(named: "placeholder")
let operation = imageView.stx.image(atURL: url, placeholder: placeholder, progress: { progress in
        // update progressView
    }, completion: { image, _ in
        // do image processing
        return image
    })

import UIKit
import STXImageCache

final class Cell: UITableViewCell {
    var operation: STXImageOperation?
    @IBOutlet weak var progress: UIProgressView!

    override func prepareForReuse() {
        super.prepareForReuse()
        operation?.cancel()
    }

    func configure(withURL url: URL, placeholder: UIImage) {
        imageView?.image = nil
        progress.progress = 0
        self.progress.isHidden = false
        operation = imageView?.stx.image(atURL: url, placeholder: placeholder, progress: { progress in
            DispatchQueue.main.async {
                self.progress.setProgress(progress, animated: true)
            }
        }, completion: { image, _ in
            DispatchQueue.main.async {
                self.progress.isHidden = true
            }
            return image
        })
    }
}