network-image-operation.swift

class NetworkOperation: Operation {
    var targetURL: String?
    var resultData: Data?
    var _finished = false
    override var isFinished: Bool {
        get {
            return _finished
        }
        set (newValue) {
            willChangeValue(for: \.isFinished)
            _finished = newValue
            didChangeValue(for: \.isFinished)
        }
    }
    
    convenience init(url: String) {
        self.init()
        targetURL = url
    }
    
    override func start() {
        if isCancelled {
            isFinished = true
            return
        }
        
        guard let url = URL(string: targetURL!) else {
            fatalError("Failed URL")
        }
        
        URLSession.shared.dataTask(with: url) { data, _, error in
            if error != nil {
                print (error)
                return
            }
            
            self.resultData = data
            
            self.isFinished = true
        }.resume
    }
}

let operationQueue = //...
let backendOperation = NetworkOperation(url: <Backend-URL>)
let imageOperation = NetworkOperation()

let adapter = BlockOperation() { [unowned backendOperation, unowned imageOperation] in
    imageOperation.targetUrl = backendOperation.data // let's imagine this is already converted to string
}

backendOperation => adapter => imageOperation => adapter2 => setImageOperation // we skip the setImage part, as you can see the gist of it