NSProgress何时删除观察员

Question

NSProgress是一个用于跟踪队列进度的有用类。它依赖于观察"fractionCompleted"属性。但是，你应该何时/如何移除观察者？我尝试了dispatch_group和dispatch_barrier，但在所有工作完成之前，观察者仍然被删除。

这是我目前掌握的代码。

override func observeValueForKeyPath(keyPath: String!,
    ofObject object: AnyObject!,
    change: [NSObject : AnyObject]!,
    context: UnsafeMutablePointer<Void>)
{
    if keyPath == "fractionCompleted"
    {
        progressBar.doubleValue = (object as NSProgress).fractionCompleted
        println((object as NSProgress).localizedDescription)
    }
    else
    {
        super.observeValueForKeyPath(keyPath, ofObject: object, change: change, context: context)
    }
}

var overallProgress : NSProgress?
@IBAction func start(sender: AnyObject)
{
    overallProgress = NSProgress(totalUnitCount: 100)
    overallProgress?.cancellable = true
    overallProgress?.cancellationHandler = {() -> () in
        println("cancelled")
    }
    overallProgress?.addObserver(self,
        forKeyPath: "fractionCompleted",
        options: (.Initial | .New),
        context: nil)
    var dispatchGroup = dispatch_group_create()
    dispatch_group_async(dispatchGroup, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {

    //dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) 
  { [unowned self] in
        if !(self.overallProgress?.cancelled ?? false)
        {
            self.overallProgress?.becomeCurrentWithPendingUnitCount(50)
            self.doWork(2)
            self.overallProgress?.resignCurrent()
        }
        if !(self.overallProgress?.cancelled ?? false)
        {
            self.overallProgress?.becomeCurrentWithPendingUnitCount(50)
            self.doWork(1)
            self.overallProgress?.resignCurrent()
        }
    }
    dispatch_group_notify(dispatchGroup, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)){
        dispatch_async(dispatch_get_main_queue()) {
            println("remove")
            self.overallProgress?.removeObserver(self, forKeyPath: "fractionCompleted")
        }
    }
  //    dispatch_barrier_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {
 //dispatch_async(dispatch_get_main_queue()) {
 //println("remove")
 //self.overallProgress?.removeObserver(self, forKeyPath: "fractionCompleted")
 //         }
 //     }
}

@IBAction func cancel(sender: AnyObject)
{
    overallProgress?.cancel()
}

func doWork(sleeptime : UInt32)
{
    let privateProgess = NSProgress(totalUnitCount: 5)
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) { [unowned self] in
        for index : Int64 in 0...5
        {
            sleep(sleeptime)
            privateProgess.completedUnitCount = index
        }
    }
}

Answer 1

doWork中的工作与您在start中调度的工作不在同一个dispatch_group中。如果您希望dispatch_group_notify在所有工作完成后发生，那么doWork也需要使用dispatch_group_async来调度内部工作。否则，对doWork的两个调用将立即返回，然后组中唯一的块将完成，从而导致您的dispatch_group_notify块立即执行。最简单的方法可能是向doWork添加一个参数以传入dispatch_group_t，这样内部工作也可以作为该组的一部分来完成。