[NSBlockOperation addExecutionBlock:]:在操作开始执行或完成后无法添加块 [英] [NSBlockOperation addExecutionBlock:]: blocks cannot be added after the operation has started executing or finished
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问题描述
我试图在完成或取消它之后再次启动NSBlockOperation，但是出现错误?任何人都知道哪里出错了吗?谢谢
I am trying to start again NSBlockOperation after completing or canceling it, but getting an error? Any one have any idea where is mistake? Thanks
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)
}
}
输出
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)
推荐答案
让我概述一系列替代方案.第一个只是解决您问题中的眼前战术问题，后两个是进一步完善，其复杂性不断提高.
Let me outline a series of alternatives. The first is just addressing the immediate tactical problem in your question, and the latter two being further refinements, of increasing complexity.
-
一旦操作开始，您将无法调用
addExecutionBlock.因此，只需创建一个新的Operation.
You cannot call
addExecutionBlockonce an operation has started. So just create a newOperation.
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()
}
}
值得注意的是，这会不必要地变慢，连续加载图像.您可以同时加载它们，例如:
It's worth noting that this is going to be unnecessarily slow, loading the images consecutively. You could load them concurrently with something like:
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()
}
}
即使有问题.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.
Even that has issues. The other problem is that when you "cancel", it may well continue trying to download the resource currently being downloaded because you're not using a cancelable network request.
一种更好的方法是将下载文件(通过URLSession执行)包装在其自己的异步Operation子类中，并使其可取消，例如:
An even better approach would be to wrap the download (which is to be performed via URLSession) in its own asynchronous Operation subclass, and make it cancelable, e.g.:
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()
}
}
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 {
return stateLock.withCriticalScope { _executing }
}
set {
willChangeValue(forKey: "isExecuting")
stateLock.withCriticalScope { _executing = newValue }
willChangeValue(forKey: "isExecuting")
}
}
private var _finished: Bool = false
override private(set) public var isFinished: Bool {
get {
return 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()
}
}
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