swift - 取消包裹在 NSOperation 中的 Alamofire 请求会导致多个 KVO？

Question

我的 Xcode 版本:6.3.2
Alamofire版本:1.2.2(通过Cocoapods安装)

为了设置 maxConcurrentOperationCount 以限制 NSOperationQueue 中的并发操作数，我将 Alamofire 包装起来NSOperation 中的下载请求只是 like Rob suggested .

NSOperation 的基本子类如下:

class ConcurrentOperation : NSOperation {

    override var concurrent: Bool {
        return true
    }

    override var asynchronous: Bool {
        return true
    }

    private var _executing: Bool = false
    override var executing: Bool {
        get {
            return _executing
        }
        set {
            if (_executing != newValue) {
                self.willChangeValueForKey("isExecuting")
                _executing = newValue
                self.didChangeValueForKey("isExecuting")
            }
        }
    }

    private var _finished: Bool = false;
    override var finished: Bool {
        get {
            return _finished
        }
        set {
            if (_finished != newValue) {
                self.willChangeValueForKey("isFinished")
                _finished = newValue
                self.didChangeValueForKey("isFinished")
            }
        }
    }

    /// Complete the operation
    ///
    /// This will result in the appropriate KVN of isFinished and isExecuting

    func completeOperation() {
        executing = false
        finished  = true
    }

    override func start() {
        if (cancelled) {
            finished = true
            return
        }

        executing = true

        main()
    }
}

我的子类包装了这样一个 Alamofire 下载请求:

class DownloadImageOperation : ConcurrentOperation {
    let URLString: String
    let downloadImageCompletionHandler: (responseObject: AnyObject?, error: NSError?) -> ()
    weak var request: Alamofire.Request?

    init(URLString: String, downloadImageCompletionHandler: (responseObject: AnyObject?, error: NSError?) -> ()) {
        self.URLString = URLString
        self.downloadImageCompletionHandler = downloadImageCompletionHandler
        super.init()
    }

    override func main() {
        let destination = Alamofire.Request.suggestedDownloadDestination(directory: .DocumentDirectory, domain: .UserDomainMask)
        request = Alamofire.download(.GET, URLString, destination).response { (request, response, responseObject, error) in
            if self.cancelled {
                println("Alamofire.download cancelled while downlading. Not proceed.")
            } else {
                self.downloadImageCompletionHandler(responseObject: responseObject, error: error)
            }
            self.completeOperation()
        }
    }

    override func cancel() {
        request?.cancel()
        super.cancel()
    }
}

它会覆盖 cancel() 并在 NSOperation 被取消时尝试取消 Alamofire 请求。

我使用了一个 KVO 观察器来观察 NSOperationQueue 的完成。

private var testAlamofireContext = 0

class TestAlamofireObserver: NSObject {
    var queue = NSOperationQueue()

    init(delegate: ImageDownloadDelegate) {
        super.init()
        queue.addObserver(self, forKeyPath: "operations", options: .New, context: &testAlamofireContext)
    }

    deinit {
        queue.removeObserver(self, forKeyPath: "operations", context: &testAlamofireContext)
    }

    override func observeValueForKeyPath(keyPath: String, ofObject object: AnyObject, change: [NSObject: AnyObject], context: UnsafeMutablePointer<Void>) {
        if context == &testAlamofireContext {
            if self.queue.operations.count == 0 {
                println("Image Download Complete queue. keyPath: \(keyPath); object: \(object); context: \(context)")
            }
        } else {
            super.observeValueForKeyPath(keyPath, ofObject: object, change: change, context: context)
        }
    }
}

我开始了这样的下载列表:

func downloadImages() {
    let imgLinks = [
        "https://farm4.staticflickr.com/3925/18769503068_1fc09427ec_k.jpg",
        "https://farm1.staticflickr.com/338/18933828356_4f57420df7_k.jpg",
        "https://farm4.staticflickr.com/3776/18945113685_ccec89d67a_o.jpg",
        "https://farm1.staticflickr.com/366/18333992053_725f21166e_k.jpg",
        "https://farm4.staticflickr.com/3777/18962702032_086453ee7a_k.jpg",
        "https://farm1.staticflickr.com/373/18930501406_4753ac021a_k.jpg",
        "https://farm1.staticflickr.com/283/18772907409_56ffbe573b_k.jpg",
        "https://farm1.staticflickr.com/314/18940901785_b0564b1c9b_o.jpg",
        "https://farm1.staticflickr.com/502/18949263495_88d75d2d2f_k.jpg",
        "https://farm4.staticflickr.com/3912/18938184302_6e0ca9ad31_k.jpg",
        "https://farm1.staticflickr.com/356/18957923475_3dc9df7634_k.jpg",
        "https://farm1.staticflickr.com/378/18925014986_e87feca9c7_o.jpg",
        "https://farm1.staticflickr.com/461/18949863812_ddf700bd03_o.jpg",
        "https://farm1.staticflickr.com/303/18920711216_4684ff4295_k.jpg",
        "https://farm1.staticflickr.com/558/18935058546_fc10d10855_k.jpg",
        "https://farm1.staticflickr.com/384/18955290345_fb93d17828_o.jpg",
        "https://farm1.staticflickr.com/366/18333992053_725f21166e_k.jpg",
        "https://farm4.staticflickr.com/3777/18962702032_086453ee7a_k.jpg",
        "https://farm1.staticflickr.com/373/18930501406_4753ac021a_k.jpg",
        "https://farm1.staticflickr.com/283/18772907409_56ffbe573b_k.jpg",
        "https://farm1.staticflickr.com/314/18940901785_b0564b1c9b_o.jpg",
        "https://farm1.staticflickr.com/502/18949263495_88d75d2d2f_k.jpg",
        "https://farm4.staticflickr.com/3912/18938184302_6e0ca9ad31_k.jpg",
        "https://farm1.staticflickr.com/356/18957923475_3dc9df7634_k.jpg",
        "https://farm1.staticflickr.com/378/18925014986_e87feca9c7_o.jpg",
        "https://farm1.staticflickr.com/461/18949863812_ddf700bd03_o.jpg",
        "https://farm1.staticflickr.com/303/18920711216_4684ff4295_k.jpg",
        "https://farm1.staticflickr.com/558/18935058546_fc10d10855_k.jpg",
        "https://farm1.staticflickr.com/366/18333992053_725f21166e_k.jpg",
        "https://farm4.staticflickr.com/3777/18962702032_086453ee7a_k.jpg",
        "https://farm1.staticflickr.com/373/18930501406_4753ac021a_k.jpg",
        "https://farm1.staticflickr.com/283/18772907409_56ffbe573b_k.jpg",
        "https://farm1.staticflickr.com/314/18940901785_b0564b1c9b_o.jpg",
        "https://farm1.staticflickr.com/502/18949263495_88d75d2d2f_k.jpg",
        "https://farm4.staticflickr.com/3912/18938184302_6e0ca9ad31_k.jpg",
        "https://farm1.staticflickr.com/356/18957923475_3dc9df7634_k.jpg",
        "https://farm1.staticflickr.com/378/18925014986_e87feca9c7_o.jpg",
        "https://farm1.staticflickr.com/461/18949863812_ddf700bd03_o.jpg",
        "https://farm1.staticflickr.com/303/18920711216_4684ff4295_k.jpg",
        "https://farm1.staticflickr.com/558/18935058546_fc10d10855_k.jpg",
        "https://farm4.staticflickr.com/3777/18962702032_086453ee7a_k.jpg",
        "https://farm1.staticflickr.com/373/18930501406_4753ac021a_k.jpg",
        "https://farm1.staticflickr.com/283/18772907409_56ffbe573b_k.jpg",
        "https://farm1.staticflickr.com/314/18940901785_b0564b1c9b_o.jpg",
        "https://farm1.staticflickr.com/502/18949263495_88d75d2d2f_k.jpg",
        "https://farm4.staticflickr.com/3912/18938184302_6e0ca9ad31_k.jpg",
        "https://farm1.staticflickr.com/356/18957923475_3dc9df7634_k.jpg",
        "https://farm1.staticflickr.com/378/18925014986_e87feca9c7_o.jpg",
        "https://farm1.staticflickr.com/461/18949863812_ddf700bd03_o.jpg",
        "https://farm1.staticflickr.com/303/18920711216_4684ff4295_k.jpg",
        "https://farm1.staticflickr.com/558/18935058546_fc10d10855_k.jpg",
        "https://farm1.staticflickr.com/266/18956724112_6e61a743a5_k.jpg"
    ]

    var testAlamofireObserver = TestAlamofireObserver()
    testAlamofireObserver!.queue.maxConcurrentOperationCount = 5

    for imgLink in imgLinks {
        let operation = DownloadImageOperation(URLString: imgLink) {
            (responseObject, error) in

            if responseObject == nil {
                // handle error here
                println("failed: \(error)")
            } else {
                println("\(responseObject?.absoluteString) downloaded.")
            }
        }
        testAlamofireObserver!.queue.addOperation(operation)
    }
}

如果队列完成而没有收到任何取消，日志输出应该是:

2015-06-22 17:11:04.206 RSS Wallpaper Switchr[46250:714702] Optional(Optional("https://farm1.staticflickr.com/461/18949863812_ddf700bd03_o.jpg")) downloaded.
...
...
...
2015-06-22 17:11:56.979 RSS Wallpaper Switchr[46250:714702] Optional(Optional("https://farm1.staticflickr.com/461/18949863812_ddf700bd03_o.jpg")) downloaded.
2015-06-22 17:11:56.979 RSS Wallpaper Switchr[46250:714702] Image Download Complete queue. keyPath: operations; object: <NSOperationQueue: 0x6180002354a0>{name = 'NSOperationQueue 0x6180002354a0'}; context: 0x000000010007eb70

如果队列接收到cancelAllOperations()，日志输出应该是:

2015-06-22 17:16:29.691 RSS Wallpaper Switchr[46467:720630] Optional(Optional("https://farm1.staticflickr.com/366/18333992053_725f21166e_k.jpg")) downloaded.
2015-06-22 17:16:32.632 RSS Wallpaper Switchr[46467:720630] Alamofire.download cancelled while downlading. Not proceed.
...
...
2015-06-22 17:16:32.642 RSS Wallpaper Switchr[46467:720630] Alamofire.download cancelled while downlading. Not proceed.
2015-06-22 17:16:32.643 RSS Wallpaper Switchr[46467:720630] Image Download Complete queue. keyPath: operations; object: <NSOperationQueue: 0x600000024c20>{name = 'NSOperationQueue 0x600000024c20'}; context: 0x000000010007eb70

但是，如果我如上所述将 maxConcurrentOperationCount 更改为非默认值，并且队列接收到 cancelAllOperations()，则日志变为:

2015-06-22 17:17:56.427 RSS Wallpaper Switchr[46606:722523] Optional(Optional("https://farm4.staticflickr.com/3777/18962702032_086453ee7a_k.jpg")) downloaded.
2015-06-22 17:17:58.675 RSS Wallpaper Switchr[46606:722523] Alamofire.download cancelled while downlading. Not proceed.
...
...
2015-06-22 17:17:58.677 RSS Wallpaper Switchr[46606:722523] Alamofire.download cancelled while downlading. Not proceed.
2015-06-22 17:17:58.678 RSS Wallpaper Switchr[46606:722720] Image Download Complete queue. keyPath: operations; object: <NSOperationQueue: 0x608000424ee0>{name = 'NSOperationQueue 0x608000424ee0'}; context: 0x000000010007eb70
2015-06-22 17:17:58.678 RSS Wallpaper Switchr[46606:722560] Image Download Complete queue. keyPath: operations; object: <NSOperationQueue: 0x608000424ee0>{name = 'NSOperationQueue 0x608000424ee0'}; context: 0x000000010007eb70
2015-06-22 17:17:58.678 RSS Wallpaper Switchr[46606:722574] Image Download Complete queue. keyPath: operations; object: <NSOperationQueue: 0x608000424ee0>{name = 'NSOperationQueue 0x608000424ee0'}; context: 0x000000010007eb70
2015-06-22 17:17:58.678 RSS Wallpaper Switchr[46606:722719] Image Download Complete queue. keyPath: operations; object: <NSOperationQueue: 0x608000424ee0>{name = 'NSOperationQueue 0x608000424ee0'}; context: 0x000000010007eb70
2015-06-22 17:17:58.678 RSS Wallpaper Switchr[46606:722721] Image Download Complete queue. keyPath: operations; object: <NSOperationQueue: 0x608000424ee0>{name = 'NSOperationQueue 0x608000424ee0'}; context: 0x000000010007eb70
2015-06-22 17:17:58.678 RSS Wallpaper Switchr[46606:722572] Image Download Complete queue. keyPath: operations; object: <NSOperationQueue: 0x608000424ee0>{name = 'NSOperationQueue 0x608000424ee0'}; context: 0x000000010007eb70

KVO observeValueForKeyPath 是从多个不同的线程执行的。线程数可以是可变的。这将导致 KVO 的完成函数被执行多次。如果我不更改 maxConcurrentOperationCount 的默认值或不为 Alamofire.Request request?.cancel()，则不会发生这种情况。

为什么我关心 KVO 完成函数的不止一次执行？我的目的是启动一个下载队列，当足够的下载完成后，取消剩余的操作，即使是未启动或正在下载，然后为下载做一些事情。完成函数应该只执行一次，有两个因素 (1) 更改 maxConcurrentOperationCount 的默认值 (2) 不要为 request?.cancel() Alamofire.Request 可能与它有关。我想知道为什么以及如何纠正这个问题。

Answer 1

我发现您描述的多个 KVN 行为并不令人惊讶。文档中没有任何内容表明当它取消所有操作时，operations 将产生单个 KVN。事实上，人们可能会安全地推断出您描述的行为应该是预期的(因为它不会抢先杀死所有这些工作线程，而是向每个工作线程发送一条 cancel 消息，并且每个操作都负责在自己的时间对此做出回应；并且我不希望 operations 在操作最终实际完成之前进行更新)。

就我个人而言，我建议完全放弃这种观察者模式。您的代码不应取决于 NSOperationQueue 是否一次删除所有操作。相反，我建议您改为依赖现有的 downloadImageCompletionHandler 闭包，无论请求是否完成都调用它。只需让闭包查看 error 对象，以确定它是否被取消或是否由于其他原因而失败。

---

如果您的意图是了解所有这些操作何时完成，我不会依赖operations KVN。相反，我可能会根据所有其他请求创建一个完成操作:

let completionOperation = NSBlockOperation() {                    // create completion operation
    // do whatever you want here
}

for imgLink in imgLinks {
    let operation = DownloadImageOperation(URLString: imgLink) { responseObject, error in
        if error != nil {
            if error!.code == NSURLErrorCancelled && error!.domain == NSURLErrorDomain {
                println("everything OK, just canceled")
            } else {
                println("error=\(error)")
            }
        }
        if responseObject != nil {
            println("\(responseObject?.absoluteString) downloaded.")
        }
    }

    completionOperation.addDependency(operation)                 // add dependency

    testAlamofireObserver!.queue.addOperation(operation)
}

NSOperationQueue.mainQueue().addOperation(completionOperation)   // schedule completion operation on some other queue (so that when I cancel everything on that other queue, I don't cancel this, too)