FetchImageOperationExample.swift

final class FetchImageOperation: AsyncOperation {
    
    private(set) var result: Result<UIImage, Error>?
    
    private lazy var imageProvider = ImageProvider()
    private let imageID: String
    
    init(imageID: String) {
        self.imageID = imageID
        super.init()
    }
    
    override func main() {
        imageProvider.fetchImage(byID: imageID) { [self] url in
            if let data = try? Data(contentsOf: url),
               let image = UIImage(data: data) {
                result = .success(image)
            } else {
                result = .failure(NSError(domain: "", code: 0, userInfo: nil))
            }
            state = .finished
        }
    }
    
}

class Interactor {
    
    private(set) var images = [UIImage]()
    private let imageIDs = [String]()
    private lazy var operationQueue: OperationQueue = {
        let opQueue = OperationQueue()
        opQueue.maxConcurrentOperationCount = 10
        return opQueue
    }()
    
    func loadImages(completion: (() -> Void)?) {
        let operations = imageIDs.map { FetchImageOperation(imageID: $0) }
        
        let completedOperation = Operation()
        completedOperation.completionBlock = { [self] in
            images = operations.compactMap { try? $0.result?.get() }
            DispatchQueue.main.async {
                completion?()
            }
        }
        
        operations.forEach { completedOperation.addDependency($0) }
        
        let allOperations: [Operation] = [completedOperation] + operations
        operationQueue.addOperations(allOperations, waitUntilFinished: false)
    }
    
}