取消包裹在 NSOperation 中的 Alamofire 请求会导致多个 KVO？
Cancelling an Alamofire Request Wrapped In NSOperation Causes Multiple KVO?
我的Xcode版本：6.3.2
Alamofire 版本：1.2.2（通过 Cocoapods 安装）
为了在NSOperationQueue中设置maxConcurrentOperationCount限制并发数，我把我的Alamofire download request in a NSOperation just like Rob suggested.
包裹起来
NSOperation的基本子类是这样的：
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 KVO 观察器观察 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，如果我将 maxConcurrentOperationCount 更改为上述非默认值，并且队列收到 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）改变默认的maxConcurrentOperationCount（2）不要request?.cancel() for 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。
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，将导致 operations 上的单个 KVN。事实上，可以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 cancel 消息，并且每个操作负责响应在自己的时间；我不希望 operations 在操作最终真正完成之前进行更新）。
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。
如果您的目的是了解所有这些操作何时完成，我就不会依赖 operations KVN。相反，我可能会创建一个完成操作，这取决于所有其他请求：
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)
我的Xcode版本：6.3.2
Alamofire 版本：1.2.2（通过 Cocoapods 安装）
为了在NSOperationQueue中设置maxConcurrentOperationCount限制并发数，我把我的Alamofire download request in a NSOperation just like Rob suggested.
NSOperation的基本子类是这样的：
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 KVO 观察器观察 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，如果我将 maxConcurrentOperationCount 更改为上述非默认值，并且队列收到 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）改变默认的maxConcurrentOperationCount（2）不要request?.cancel() for 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。
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，将导致 operations 上的单个 KVN。事实上，可以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 cancel 消息，并且每个操作负责响应在自己的时间；我不希望 operations 在操作最终真正完成之前进行更新）。
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。
如果您的目的是了解所有这些操作何时完成，我就不会依赖 operations KVN。相反，我可能会创建一个完成操作，这取决于所有其他请求：
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)