ios - [NSBlockOperation addExecutionBlock :]: blocks cannot be added after the operation has started executing or finished

Question

我试图在完成或取消后重新启动 NSBlockOperation，但出现错误？任何人都知道错误在哪里？谢谢

let imageURLs = ["http://www.planetware.com/photos-large/F/france-paris-eiffel-tower.jpg",
    "http://adriatic-lines.com/wp-content/uploads/2015/04/canal-of-Venice.jpg",
    "http://algoos.com/wp-content/uploads/2015/08/ireland-02.jpg",
    "http://bdo.se/wp-content/uploads/2014/01/Stockholm1.jpg"]

class Downloader {

    class func downloadImageWithURL(url:String) -> UIImage! {

        let data = NSData(contentsOfURL: NSURL(string: url)!)
        return UIImage(data: data!)
    }
}

class ViewController: UIViewController {

    @IBOutlet weak var imageView1: UIImageView!
    var indeX = 0

    let operation1 = NSBlockOperation()
    var queue = NSOperationQueue()

    override func viewDidLoad() {
        super.viewDidLoad()
    }
    @IBAction func didClickOnStart(sender: AnyObject) {
        queue = NSOperationQueue()

        operation1.addExecutionBlock { () -> Void in

            for _ in imageURLs {
                if !self.operation1.cancelled {
                    let img1 = Downloader.downloadImageWithURL(imageURLs[self.indeX])
                    NSOperationQueue.mainQueue().addOperationWithBlock({
                        self.imageView1.image = img1

                        print("indeX \(self.indeX)")
                        self.indeX++
                    })

                }
            }
        }
        queue.addOperation(operation1)
    }

    @IBAction func didClickOnCancel(sender: AnyObject) {
        self.queue.cancelAllOperations()
        print(operation1.finished)
    }
}  

Output

indeX 0
false
indeX 1
2016-07-20 02:00:26.157 ConcurrencyDemo[707:15846] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[NSBlockOperation addExecutionBlock:]: blocks cannot be added after the operation has started executing or finished'
*** First throw call stack:
(
    0   CoreFoundation                      0x000000010c94be65 __exceptionPreprocess + 165
    1   libobjc.A.dylib                     0x000000010e68bdeb objc_exception_throw + 48
    2   Foundation                          0x000000010cd369fe -[NSBlockOperation addExecutionBlock:] + 356
    3   ConcurrencyDemo                     0x000000010c766edd _TFC15ConcurrencyDemo14ViewController15didClickOnStartfS0_FPSs9AnyObject_T_ + 253
    4   ConcurrencyDemo                     0x000000010c767086 _TToFC15ConcurrencyDemo14ViewController15didClickOnStartfS0_FPSs9AnyObject_T_ + 54
    5   UIKit                               0x000000010d16a194 -[UIApplication sendAction:to:from:forEvent:] + 92
    6   UIKit                               0x000000010d56b7b7 -[UIBarButtonItem(UIInternal) _sendAction:withEvent:] + 152
    7   UIKit                               0x000000010d16a194 -[UIApplication sendAction:to:from:forEvent:] + 92
    8   UIKit                               0x000000010d2d96fc -[UIControl sendAction:to:forEvent:] + 67
    9   UIKit                               0x000000010d2d99c8 -[UIControl _sendActionsForEvents:withEvent:] + 311
    10  UIKit                               0x000000010d2d9b43 -[UIControl _sendActionsForEvents:withEvent:] + 690
    11  UIKit                               0x000000010d2d8af8 -[UIControl touchesEnded:withEvent:] + 601
    12  UIKit                               0x000000010d1d949b -[UIWindow _sendTouchesForEvent:] + 835
    13  UIKit                               0x000000010d1da1d0 -[UIWindow sendEvent:] + 865
    14  UIKit                               0x000000010d188b66 -[UIApplication sendEvent:] + 263
    15  UIKit                               0x000000010d162d97 _UIApplicationHandleEventQueue + 6844
    16  CoreFoundation                      0x000000010c877a31 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
    17  CoreFoundation                      0x000000010c86d95c __CFRunLoopDoSources0 + 556
    18  CoreFoundation                      0x000000010c86ce13 __CFRunLoopRun + 867
    19  CoreFoundation                      0x000000010c86c828 CFRunLoopRunSpecific + 488
    20  GraphicsServices                    0x0000000110f5ead2 GSEventRunModal + 161
    21  UIKit                               0x000000010d168610 UIApplicationMain + 171
    22  ConcurrencyDemo                     0x000000010c76906d main + 109
    23  libdyld.dylib                       0x000000010f19492d start + 1
    24  ???                                 0x0000000000000001 0x0 + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException
(lldb)

Answer 1

让我概述一系列备选方案。第一个只是解决您问题中的直接战术问题，后两个是进一步改进，越来越复杂。

1. 您不能在操作开始后调用 addExecutionBlock。因此，只需创建一个新的 Operation。

   例如:

   class ViewController: UIViewController {
   
       @IBOutlet weak var imageView1: UIImageView!
   
       weak var downloadOperation: Operation?    // make this weak
   
       var queue = OperationQueue()
   
       let imageURLs: [String] = ...
   
       @IBAction func didClickOnStart(_ sender: Any) {
           downloadOperation?.cancel()             // you might want to stop the previous one if you restart this
   
           let operation = BlockOperation {
               for (index, imageURL) in self.imageURLs.enumerate() {
                   guard let cancelled = self.downloadOperation?.cancelled where !cancelled else  { return }
   
                   let img1 = Downloader.downloadImageWithURL(imageURL)
                   OperationQueue.main.addOperation {
                       self.imageView1.image = img1
   
                       print("index \(index)")
                   }
               }
           }
           queue.addOperation(operation)
   
           downloadOperation = operation
       }
   
       @IBAction func didClickOnCancel(_ sender: Any) {
           downloadOperation?.cancel()
       }
   
   }
   

2. 值得注意的是，连续加载图像会导致不必要的缓慢。您可以同时加载它们，例如:

   class ViewController: UIViewController {
   
       @IBOutlet weak var imageView1: UIImageView!
   
       var queue: OperationQueue = {
           let _queue = OperationQueue()
           _queue.maxConcurrentOperationCount = 4
           return _queue
       }()
   
       let imageURLs: [String] = ...
   
       @IBAction func didClickOnStart(_ sender: Any) {
           queue.cancelAllOperations()
   
           let completionOperation = BlockOperation {
               print("all done")
           }
   
           for (index, imageURL) in self.imageURLs.enumerate() {
               let operation = BlockOperation {
                   let img1 = Downloader.downloadImageWithURL(imageURL)
                   OperationQueue.main.addOperation {
                       self.imageView1.image = img1
   
                       print("index \(index)")
                   }
               }
               completionOperation.addDependency(operation)
               queue.addOperation(operation)
           }
   
           OperationQueue.main.addOperation(completionOperation)
       }
   
       @IBAction func didClickOnCancel(_ sender: Any) {
           queue.cancelAllOperations()
       }
   }
   

3. 即使那样也有问题。另一个问题是，当您“取消”时，它可能会继续尝试下载当前正在下载的资源，因为您没有使用可取消的网络请求。

   更好的方法是将下载(将通过 URLSession 执行)包装在它自己的异步 Operation 子类中，并使其可取消，例如:

   class ViewController: UIViewController {
       var queue: OperationQueue = {
           let _queue = OperationQueue()
           _queue.maxConcurrentOperationCount = 4
           return _queue
       }()
   
       let imageURLs: [URL] = ...
   
       @IBAction func didClickOnStart(_ sender: Any) {
           queue.cancelAllOperations()
   
           let completion = BlockOperation {
               print("done")
           }
   
           for url in imageURLs {
               let operation = ImageDownloadOperation(url: url) { result in
                   switch result {
                   case .failure(let error): 
                       print(url.lastPathComponent, error)
   
                   case .success(let image): 
                       OperationQueue.main.addOperation {
                           self.imageView1.image = img1
   
                           print("index \(index)")
                       }
                   }
               }
               completion.addDependency(operation)
               queue.addOperation(operation)
           }
   
           OperationQueue.main.addOperation(completion)
       }
   
       @IBAction func didClickOnCancel(_ sender: AnyObject) {
           queue.cancelAllOperations()
       }
   }
   

   在哪里

   /// Simple image network operation
   
   class ImageDownloadOperation: DataOperation {
       init(url: URL, session: URLSession = .shared, networkCompletionHandler: @escaping (Result<UIImage, Error>) -> Void) {
           super.init(url: url, session: session) { result in
               switch result {
               case .failure(let error):
                   networkCompletionHandler(.failure(error))
   
               case .success(let data):
                   guard let image = UIImage(data: data) else {
                       networkCompletionHandler(.failure(DownloadError.notImage))
                       return
                   }
   
                   networkCompletionHandler(.success(image))
               }
           }
       }
   }
   
   /// Simple network data operation
   ///
   /// This can be subclassed for image-specific operations, JSON-specific operations, etc.
   
   class DataOperation: AsynchronousOperation {
       var downloadTask: URLSessionTask?
   
       init(url: URL, session: URLSession = .shared, networkCompletionHandler: @escaping (Result<Data, Error>) -> Void) {
           super.init()
   
           downloadTask = session.dataTask(with: url) { data, response, error in
               defer { self.complete() }
   
               guard let data = data, let response = response as? HTTPURLResponse, error == nil else {
                   networkCompletionHandler(.failure(error!))
                   return
               }
   
               guard 200..<300 ~= response.statusCode else {
                   networkCompletionHandler(.failure(DownloadError.invalidStatusCode(response)))
                   return
               }
   
               networkCompletionHandler(.success(data))
           }
       }
   
       override func main() {
           downloadTask?.resume()
       }
   
       override func cancel() {
           super.cancel()
   
           downloadTask?.cancel()
       }
   }
   
   /// Asynchronous Operation base class
   ///
   /// This class performs all of the necessary KVN of `isFinished` and
   /// `isExecuting` for a concurrent `NSOperation` subclass. So, to developer
   /// a concurrent NSOperation subclass, you instead subclass this class which:
   ///
   /// - must override `main()` with the tasks that initiate the asynchronous task;
   ///
   /// - must call `completeOperation()` function when the asynchronous task is done;
   ///
   /// - optionally, periodically check `self.cancelled` status, performing any clean-up
   ///   necessary and then ensuring that `completeOperation()` is called; or
   ///   override `cancel` method, calling `super.cancel()` and then cleaning-up
   ///   and ensuring `completeOperation()` is called.
   
   public class AsynchronousOperation: Operation {
   
       override public var isAsynchronous: Bool { return true }
   
       private let stateLock = NSLock()
   
       private var _executing: Bool = false
       override private(set) public var isExecuting: Bool {
           get {
               stateLock.withCriticalScope { _executing }
           }
           set {
               willChangeValue(forKey: "isExecuting")
               stateLock.withCriticalScope { _executing = newValue }
               didChangeValue(forKey: "isExecuting")
           }
       }
   
       private var _finished: Bool = false
       override private(set) public var isFinished: Bool {
           get {
               stateLock.withCriticalScope { _finished }
           }
           set {
               willChangeValue(forKey: "isFinished")
               stateLock.withCriticalScope { _finished = newValue }
               didChangeValue(forKey: "isFinished")
           }
       }
   
       /// Complete the operation
       ///
       /// This will result in the appropriate KVN of isFinished and isExecuting
   
       public func complete() {
           if isExecuting {
               isExecuting = false
           }
   
           if !isFinished {
               isFinished = true
           }
       }
   
       override public func start() {
           if isCancelled {
               isFinished = true
               return
           }
   
           isExecuting = true
   
           main()
       }
   
       override public func main() {
           fatalError("subclasses must override `main`")
       }
   }
   
   extension NSLock {
   
       /// Perform closure within lock.
       ///
       /// An extension to `NSLock` to simplify executing critical code.
       ///
       /// - parameter block: The closure to be performed.
   
       func withCriticalScope<T>(block: () throws -> T) rethrows -> T {
           lock()
           defer { unlock() }
           return try block()
       }
   }