iOS并发：如何使用OperationQueue代替barrier？

Question

学习OperationQueue时，是否可以使用OperationQueue代替gcd barrier？

情况如下：

upload 3 images , then upload other 3 images

与 barrier、gcd 完美搭配

    func workFlow(){
        let queue = DispatchQueue(label: "test.concurrent.queue", qos: .background, attributes: .concurrent, autoreleaseFrequency: .workItem)
        queue.async {
            self.uploadImg(idx: "A_0")
        }
        queue.async {
            Thread.sleep(forTimeInterval: 2)
            self.uploadImg(idx: "A_1")
        }
        queue.async {
            self.uploadImg(idx: "A_2")
        }
        queue.async(qos: queue.qos, flags: .barrier) {
            print("group A done")
        }
        print("A: should not be hanged")
        queue.async {
            self.uploadImg(idx: "B_0")
        }
        queue.async {
            self.uploadImg(idx: "B_1")
        }
        queue.async {
            self.uploadImg(idx: "B_2")
        }
        
        queue.async(qos: queue.qos, flags: .barrier) {
            print("group B done")
        }
        print("B: should not be hanged")
    }
    
    
    func uploadImg(idx info: String){
        Thread.sleep(forTimeInterval: 1)
        print("img \(info) uploaded")
    }

与OperationQueue相比，这里有一点小瑕疵

主队列被挂起，检查打印

"A/B: should not be hanged"

  lazy var uploadQueue: OperationQueue = {
       var queue = OperationQueue()
       queue.name = "upload queue"
       queue.maxConcurrentOperationCount = 5
       return queue
     }()


   func workFlow(){
        let one = BlockOperation {
            self.uploadImg(idx: "A_0")
        }
        let two = BlockOperation {
            Thread.sleep(forTimeInterval: 3)
            self.uploadImg(idx: "A_1")
        }
        let three = BlockOperation {
            self.uploadImg(idx: "A_2")
        }
        uploadQueue.addOperations([one, two, three], waitUntilFinished: true)
        print("A: should not be hanged")

        uploadQueue.addOperation {
            print("group A done")
        }
        let four = BlockOperation {
            self.uploadImg(idx: "B_0")
        }
        let five = BlockOperation {
            self.uploadImg(idx: "B_1")
        }
        let six = BlockOperation {
            self.uploadImg(idx: "B_2")
        }
        uploadQueue.addOperations([four, five, six], waitUntilFinished: true)
        print("B: should not be hanged")
        uploadQueue.addOperation {
            print("group B done")
        }
    }

如何使用 OperationQueue 做得更好？

Answer 1

如果不希望加入队列的操作阻塞当前线程，waitUntilFinished必须是false。但是如果你设置为true，它会阻塞当前线程，直到添加的操作完成。

显然，如果你不等待，它不会阻塞主线程，但你也会失去屏障行为。但是 iOS 13 和 macOS 10.15 引入了 addBarrierBlock。如果你真的需要障碍并且必须支持更早的 OS 版本，那么你将不得不使用依赖项。但是，如果您之前使用 GCD 屏障只是为了限制并发程度，那么 maxConcurrentOperationCount 可能会使屏障变得毫无意义。这完全取决于您为什么要对这些 uploads/downloads 使用障碍。（看到 upload/download 队列的障碍有点不寻常，因为它会降低效率。）

How to do it better with OperationQueue?

我假设 uploadImg 同步下载图像。我会将其重构为自己的 Operation 子类，它执行必要的 Operation KVO，例如。这包含了运行中的下载任务，但您也可以对上传或数据任务执行相同的操作（尽管数据任务对内存的影响要大得多）。

但始终建议避免同步网络请求，以 (a) 确保不占用工作线程； (b) 使请求可取消。

iOS并发：如何使用OperationQueue代替barrier？

iOS concurrent: how to use OperationQueue instead of barrier?

concurrency

nsoperationqueue

swift