NSOperation和调度组不创建串行队列

Question

我已经将NSOperation子类化为尝试在串行队列中运行一些异步任务，但是，尽管我努力使用信号量和调度组，但NSOperationQueue仍然允许多个操作同时运行。我设置了maxConcurrentOperationCount = 1。

在下面的代码中，我设置了我的调度组，它们确实使代码顺序运行。

如果我运行1个操作控制台打印：

[0, 0] : Enter : 1
[0, 0] : Enter : 2
[0, 0] : Enter : 3
[0, 0] : Leave : 1
[0, 0] : Leave : 2
[0, 0] : Leave : 3
QUEUE FINISHED

如果我运行2个操作控制台打印：

[0, 1] : Enter : 1
[0, 0] : Enter : 2
[0, 1] : Enter : 2
[0, 0] : Enter : 3
[0, 0] : Leave : 1
[0, 0] : Leave : 2
[0, 0] : Leave : 3
QUEUE FINISHED
[0, 1] : Enter : 3
[0, 1] : Leave : 1
[0, 1] : Leave : 2
[0, 1] : Leave : 3
QUEUE FINISHED

无法理解......

override func main() {

    GlobalDownloadQueue.async() {

        if(self.isCancelled) {
            return
        }

        let downloadGroup = DispatchGroup()

        let downloadedAudio = PFObject(className: "downloadedAudio")
        let selectedObjectId = GlobalOperationStore.sharedInstance.globalMultiPartArray[self.collectionView.tag][self.indexPath.item].id
        let selectedPartName = GlobalOperationStore.sharedInstance.globalMultiPartArray[self.collectionView.tag][self.indexPath.item].name

       // let semaphore = DispatchSemaphore(value: 0)

        downloadGroup.enter() // add task 1
        print("\(self.indexPath) : Enter : 1")

        let query = PFQuery(className: "Part")
        query.whereKey("objectId", equalTo: selectedObjectId)
        query.getFirstObjectInBackground { (object, error) in
            if error != nil || object == nil {
                print("No object for the index selected.")
                downloadGroup.leave() // remove task 1 if error with query
            } else {
                //print("there is an object, getting the file.")
                downloadedAudio.add(object?.object(forKey: "partAudio") as! PFFile, forKey: selectedPartName)
                let downloadedFile = object?.object(forKey: "partAudio") as! PFFile

                downloadGroup.enter() // add task 2
                print("\(self.indexPath) : Enter : 2")

                // get the data first so we can track progress
                downloadedFile.getDataInBackground({ (success, error) in
                    if (success != nil) {

                        downloadGroup.enter() // add task 3
                        print("\(self.indexPath) : Enter : 3")

                        // pin the audio if there is data
                        downloadedAudio.pinInBackground(block: { (success, error) in
                            if success {
                                // only reload if vc visible
                                if (DownloadCollectionController().isViewLoaded && (DownloadCollectionController().view.window != nil)) {
                                    self.mainQueue.addOperation {
                                        // reload the cell
                                        self.collectionView.reloadItems(at: [self.indexPath])
                                        self.collectionCell.isUserInteractionEnabled = true
                                    }
                                }
                                //semaphore.signal()
                                downloadGroup.leave() // remove task 1 when complete
                                print("\(self.indexPath) : Leave : 1")
                                downloadGroup.leave() // remove task 2 when complete
                                print("\(self.indexPath) : Leave : 2")
                                downloadGroup.leave() // remove task 3 when complete
                                print("\(self.indexPath) : Leave : 3")
                            }
                        })
                    }
                    //downloadGroup.enter() // add task 4
                    //print("\(self.indexPath) : Enter : 4")
                }, progressBlock: { (percent) in
                    self.mainQueue.addOperation {
                        //print("\(downloadedFile.name)::::\(percent)")
                        //self.activityIndicatorView.stopAnimating()
                        if let downloadingCell = self.collectionView.cellForItem(at: self.indexPath) as? InnerCollectionCell {
                            downloadingCell.progressBar.isHidden = false
                            downloadingCell.contentView.bringSubview(toFront: downloadingCell.progressBar)
                            downloadingCell.progressBar.setProgress(Float(percent) / Float(100), animated: true)
                            downloadingCell.setNeedsDisplay()
                            downloadingCell.isUserInteractionEnabled = false
                        }
                    }
                })
            }
        }

        //semaphore.wait()

        downloadGroup.notify(queue: GlobalDownloadQueue, execute: {
            print("QUEUE FINISHED")

        })


    } // async

} // main