AsynchronousOperations.swift

class CustomOperation: Operation {
    
    // MARK: - Types
    
    //Step 1: define oppration states
    
    enum State {
        case isReady, isExecuting, isFinished
        
        var keyPath: String {
            switch self {
            case .isReady: return "isReady"
            case .isExecuting: return "isExecuting"
            case .isFinished: return "isFinished"
            }
        }
    }
    
    //Step 2: setup state observers
    var state: State = .isReady {
        
        willSet {
            willChangeValue(forKey: newValue.keyPath) //new value
            willChangeValue(forKey: state.keyPath)    //current value
        }
        didSet {
            didChangeValue(forKey: oldValue.keyPath) //old value
            didChangeValue(forKey: state.keyPath)    //current value
        }
    }
    
    
    //Step 3: override state properties
    
    override var isReady: Bool {
        return super.isReady && state == .isReady
    }
    
    override var isExecuting: Bool {
        return state == .isExecuting
    }
    
    override var isFinished: Bool {
        return state == .isFinished
    }
    
    override var isAsynchronous: Bool {
        return true
    }
}


class ImageDownloadHelper: CustomOperation {
    
    let imageRecord: ImageRecord
    
    init(_ imageRecord: ImageRecord) {
        self.imageRecord = imageRecord
    }
    
    
    override func start() {
        if isCancelled {
            state = .isFinished
        } else {
            state = .isReady
            main()
        }
    }
    
    override func main() {
        if isCancelled {
            state = .isFinished
            return
        }
        
        state = .isExecuting
        
        guard let imageData = try? Data(contentsOf: imageRecord.url) else { return }
        
        
        if !imageData.isEmpty {
            imageRecord.image = UIImage(data:imageData)
            imageRecord.state = .downloaded
        } else {
            imageRecord.state = .failed
            imageRecord.image = UIImage(named: "Failed")
        }
    }
}