ios - 操作完成了 isFinished=YES 没有被它所在的队列启动

Question

概览

• 有异步操作子类
• 将此操作添加到队列中。
• 我在开始之前取消了这个操作。

运行时错误/警告:

SomeOperation isFinished=YES 没有被它所在的队列启动

问题:

1. 这是可以忽略的事情还是严重的事情？
2. 如何解决这个问题？
3. 最后提供的解决方法/解决方案是否有效？

代码:

public class SomeOperation : AsyncOperation {

    //MARK: Start

    public override func start() {

        isExecuting = true

        guard !isCancelled else {
            markAsCompleted() //isExecuting = false, isFinished = true
            return
        }

        doSomethingAsynchronously { [weak self] in

            self?.markAsCompleted() //isExecuting = false, isFinished = true
        }
    }

    //MARK: Cancel

    public override func cancel() {

        super.cancel()
        markAsCompleted() //isExecuting = false, isFinished = true
    }
}

添加到队列和取消:

//someOperation is a property in a class
if let someOperation = someOperation {
    queue.addOperation(someOperation)
}

//Based on some condition cancelling it
someOperation?.cancel()

这是有效的解决方案吗？

public override func cancel() {

    isExecuting = true //Just in case the operation was cancelled before starting

    super.cancel()
    markAsCompleted()
}

注意事项:

• markAsCompleted 设置 isExecuting = false 和 isFinished = true
• isExecuting, isFinished 是同步的属性 KVO

Answer 1

关键问题是您的markAsCompleted 在操作未isExecuting 时触发isFinished。我建议您将 markAsCompleted 修改为仅在 isExecuting 为真时执行此操作。这减轻了子类执行任何复杂状态测试以确定它们是否需要转换到 isFinished 的负担。

话虽如此，我在编写可取消异步操作时看到了三种基本模式:

1. 如果我正在处理某种模式，其中取消任务将阻止它把正在执行的操作转换为 isFinished 状态。

   在那种情况下，我必须让 cancel 实现手动完成执行操作。例如:

   class FiveSecondOperation: AsynchronousOperation {
       var block: DispatchWorkItem?
   
       override func main() {
           block = DispatchWorkItem { [weak self] in
               self?.finish()
               self?.block = nil
           }
   
           DispatchQueue.main.asyncAfter(deadline: .now() + 5, execute: block!)
       }
   
       override func cancel() {
           super.cancel()
   
           if isExecuting {
               block?.cancel()
               finish()
           }
       }
   }
   

   关注 cancel 实现，因为如果我取消 DispatchWorkItem 它不会完成操作，因此我需要确保 cancel 将明确地完成操作本身。

2. 有时，当您取消一些异步任务时，它会自动为您调用其完成处理程序，在这种情况下，cancel 除了取消该任务外不需要做任何事情，并且叫 super 。例如:

   class GetOperation: AsynchronousOperation {
       var url: URL
       weak var task: URLSessionTask?
   
       init(url: URL) {
           self.url = url
           super.init()
       }
   
       override func main() {
           let task = URLSession.shared.dataTask(with: url) { data, _, error in
               defer { self.finish() }  // make sure to finish the operation
   
               // process `data` & `error` here
           }
           task.resume()
           self.task = task
       }
   
       override func cancel() {
           super.cancel()
           task?.cancel()
       }
   }
   

   再次关注cancel，在这种情况下我们不触及“finished”状态，而只是取消dataTask(它将调用它的完成处理程序，即使您取消请求)并调用 super 实现。

3. 第三种情况是您有一些定期检查 isCancelled 状态的操作。在这种情况下，您根本不必实现 cancel，因为默认行为就足够了。例如:

   class DisplayLinkOperation: AsynchronousOperation {
       private weak var displayLink: CADisplayLink?
       private var startTime: CFTimeInterval!
       private let duration: CFTimeInterval = 2
   
       override func main() {
           startTime = CACurrentMediaTime()
           let displayLink = CADisplayLink(target: self, selector: #selector(handleDisplayLink(_:)))
           displayLink.add(to: .main, forMode: .commonModes)
           self.displayLink = displayLink
       }
   
       @objc func handleDisplayLink(_ displayLink: CADisplayLink) {
           let percentComplete = (CACurrentMediaTime() - startTime) / duration
   
           if percentComplete >= 1.0 || isCancelled {
               displayLink.invalidate()
               finish()
           }
   
           // now do some UI update based upon `elapsed`
       }
   }
   

   在这种情况下，我在操作中包装了一个显示链接，以便我可以管理依赖项和/或将显示链接封装在一个方便的对象中，我不必实现 cancel完全没有，因为默认实现会为我更新 isCancelled，我可以检查一下。

这是我通常看到的三种基本 cancel 模式。话虽如此，更新 markAsCompleted 以仅在 isExecuting 时触发 isFinished 是一个很好的安全检查，以确保您永远不会遇到您描述的问题.

---

顺便说一句，我在上面的例子中使用的AsynchronousOperation如下，改编自Trying to Understand Asynchronous Operation Subclass .顺便说一句，你所谓的 markAsCompleted 被称为 finish，听起来你正在触发 isFinished 和 isExecuting KVO 通过不同的机制，但思想基本相同。在触发 isFinished KVO 之前检查当前状态:

open class AsynchronousOperation: Operation {

    /// State for this operation.

    @objc private enum OperationState: Int {
        case ready
        case executing
        case finished
    }

    /// Concurrent queue for synchronizing access to `state`.

    private let stateQueue = DispatchQueue(label: Bundle.main.bundleIdentifier! + ".rw.state", attributes: .concurrent)

    /// Private backing stored property for `state`.

    private var rawState: OperationState = .ready

    /// The state of the operation

    @objc private dynamic var state: OperationState {
        get { return stateQueue.sync { rawState } }
        set { stateQueue.sync(flags: .barrier) { rawState = newValue } }
    }

    // MARK: - Various `Operation` properties

    open         override var isReady:        Bool { return state == .ready && super.isReady }
    public final override var isExecuting:    Bool { return state == .executing }
    public final override var isFinished:     Bool { return state == .finished }
    public final override var isAsynchronous: Bool { return true }

    // MARK: - KVN for dependent properties

    open override class func keyPathsForValuesAffectingValue(forKey key: String) -> Set<String> {
        if ["isReady", "isFinished", "isExecuting"].contains(key) {
            return [#keyPath(state)]
        }

        return super.keyPathsForValuesAffectingValue(forKey: key)
    }

    // MARK: - Foundation.Operation

    public final override func start() {
        if isCancelled {
            state = .finished
            return
        }

        state = .executing

        main()
    }

    /// Subclasses must implement this to perform their work and they must not call `super`. The default implementation of this function throws an exception.

    open override func main() {
        fatalError("Subclasses must implement `main`.")
    }

    /// Call this function to finish an operation that is currently executing

    public final func finish() {
        if isExecuting { state = .finished }
    }
}