class OperationViewController: UIViewController {
    
    let imageOperationQueue = ImageOperationQueue()
    
    let taskNumber = ["1", "2", "3", "4", "5", "6", "7", "8"]
    let taskString = ["a", "b", "c", "d", "e", "f", "g", "h"]
    
    let imageDataArray: [ImageData] = {
        var _imageDataArray = [ImageData]()
        for _ in 0..<10 {
            _imageDataArray.append(ImageData(state: .new, url: URL(string: "http://leehonghwa.github.io/")!))
        }
        return _imageDataArray
    }()
    
    @IBAction func didTapOperationTriggerButton(_ sender: UIButton) {
        /*
            add operation 할 때마다 thread를 만들어 준다.
            operation이 끝나면 thread를 없애준다.
         */
        
        //************************
        //***** 내 마음대로 예제 ******
        //************************
        
        /*
         closure
        operationQueue.addOperation {
            for numberString in self.taskNumber {
                print("\(Thread.current): \(numberString)")
            }
        }
        operationQueue.addOperation {
            for string in self.taskString {
                print("\(Thread.current): \(string)")
            }
        }
         */
        
        /*
        Operation subClass
        operationQueue.addOperation(CustomOperation(taskElement: taskNumber))
        operationQueue.addOperation(CustomOperation(taskElement: taskString))
         */
        
        /*
         Start
         현재 메서드를 호출하는 thread에서 실행됨
         CustomOperation(taskElement: taskNumber).start()
         */
        
        //**********************
        //***** 실용적인 예제 ******
        //***********************
        
        //다운로드
        imageDataArray.enumerated().forEach { (offset, imageData) in
            let operation = ImageOperation(imageData: imageData)
            //dependency 추가
            ResizeImageOperation().addDependency(operation)
            imageOperationQueue.downloadImage(id: offset, operation: operation)
        }
        //취소
        imageOperationQueue.cancelDownloadImage(id: 1)

        //일시정지도 가능하다
        imageOperationQueue.suspend()
        
        //시작
        imageOperationQueue.resume()
    }
}

class CustomOperation: Operation {
    let taskElement: [String]
    init(taskElement: [String]) {
        self.taskElement = taskElement
    }
    
    override func main() {
        guard !isCancelled else { return }
        task()
    }
    
    func task() {
        for element in self.taskElement {
            print("\(Thread.current): \(element)")
        }
        //cancel을 한다고 현재 진행중인 메서드가 끝나는것은 아니므로 이렇게 상태값을 확인한뒤 return 시켜줘야한다.
        guard !isCancelled else { return }
    }
}

// MARK: - Image
class ImageData {
    enum State {
        case new
        case downloading
        case downloaded
    }
    
    var state: State
    let url: URL
    var image: UIImage?
    
    init(state: State, url: URL, image: UIImage? = nil) {
        self.state = state
        self.url = url
        self.image = image
    }
}

//이런식으로도 사용할 수 있을것 같아요
class ImageOperation: Operation {
    let imageData: ImageData
    
    init(imageData: ImageData) {
        self.imageData = imageData
    }
    
    override func main() {
        if imageData.state == .new {
            //시작하기전 검사
            guard !isCancelled else { return }
            imageData.state = .downloading
            imageData.image = ImageService.downloadImage(url: imageData.url)
            //오래걸리는 작업 후 검사
            guard !isCancelled else { return }
            imageData.state = .downloaded
        }
    }
}

class ImageOperationQueue {
    private var currentOperation = [Int: ImageOperation]()
    
    let operationQueue: OperationQueue = {
        var queue = OperationQueue()
        queue.name = "operation-queue"
        //한번에 몇개 실행할건가?
        queue.maxConcurrentOperationCount = 1
        return queue
    }()
    
    func cancelDownloadImage(id: Int) {
        guard let operation = currentOperation[id], operation.imageData.state != .downloaded else { return }
        currentOperation.removeValue(forKey: id)
        operation.cancel()
    }
    
    func downloadImage(id: Int, operation: ImageOperation) {
        if currentOperation[id] == nil {
            currentOperation[id] = operation
        }
        operation.completionBlock = { [weak self] in
            self?.currentOperation.removeValue(forKey: id)
        }
        operationQueue.addOperation(operation)
    }

    func suspend() {
        operationQueue.isSuspended = true
    }
    
    func resume() {
        operationQueue.isSuspended = false
    }
}