struct Banner: Decodable {
    let bannerImageUrl: String

    // coding keys
}

protocol ImageBannerService {
    func load(completion: @escaping (([Banner]?, Error?) -> Void))
    func load(_ image: URL, completion: @escaping ((UIImageView?, Error?) -> Void))
}

class ImageBannerServiceImpl {
    func load(completion: @escaping (([Banner]?, Error?) -> Void)) {
        DispatchQueue.global(qos: .background).async {
            let banners: [Banner] = []
            completion(banners, nil)
        }
    }

    func load(_ image: URL, completion: @escaping ((UIImageView?, Error?) -> Void)) {
        DispatchQueue.global(qos: .background).async {
            // .. код загрузки изображения из сети
            let data = Data(contentsOf: image)
            let loadedImage = UIImage(data: data)
            let imageView = UIImageView(image: loadedImage)
            completion(imageView, nil)
        }
    }
}

class BannerCollectionViewController: UIViewController, UICollectionViewDataSource {

    let service: ImageBannerService = ImageBannerServiceImpl()

    lazy var collectionView: UICollectionView = {
        let collectionView = UICollectionView()
        collectionView.dataSource = self
        return collectionView
    }()

    var banners = [Banner]()

    override func viewDidLoad() {
        view.backgroundColor = .white
        view.addSubview(collectionView)
        NSLayoutConstraint.activate([
        ])


        service.load { [weak self] banners, error in
            DispatchQueue.main.async {
                self?.banners = banners!
                self?.collectionView.reloadData()
            }
        }
    }

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return banners.count
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        guard let cell = collectionView.dequeueReusable(for: id, indexPath: indexPath) as? BannerCell else { return UICollectionViewCell() }

        let banner = banners[indexPath.row]
        let imageUrlString = banner.banner_image_url
        guard let image = banners[indexPath.row].banner_image_url  else { return cell }

        cell.currentUrlString = imageUrlString
        service.load(image) { imageView, error in
            if imageUrlString == cell.currentUrlString,
               let imageView {
                    DispatchQueue.main.async {
                        cell.imageView = imageView
                    }
                }
        }

        return cell
    }
}



class BannerCell: UICollectionViewCell {
    var imageView: UIImageView
    var currentUrlString: String?

    override func prepareForReuse() {
        closure = nil
        currentUrlString = nil
    }
}