OperationQueue的'progress'属性在iOS 13中不起作用

Question

iOS 13在progress类中引入了OperationQueue属性。同时，Apple将operations和operationCount属性标记为已弃用，这表明它们不应再用于报告队列中的进度。

我的问题是，我无法使progress属性正常工作（基本上是开箱即用的）。另外，我找不到有关此新属性的任何文档（现在不存在）。

我试图让它在新的SingleView项目中工作，该项目在主UIProgressView上有一个UIViewController。该示例受到https://nshipster.com/ios-13/的极大启发。

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var progressView: UIProgressView!

    private let operationQueue: OperationQueue = {

        let queue = OperationQueue()
        queue.maxConcurrentOperationCount = 1
        queue.underlyingQueue = .global(qos: .background)

        return queue

    }()

    override func viewDidLoad() {

        super.viewDidLoad()

        self.progressView.observedProgress = operationQueue.progress

        self.operationQueue.cancelAllOperations()
        self.operationQueue.isSuspended = true

        for i in 0...9 {

            let operation = BlockOperation {
                sleep(1)
                NSLog("Operation \(i) executed.")
            }

            self.operationQueue.addOperation(operation)

        }

    }

    override func viewDidAppear(_ animated: Bool) {

        self.operationQueue.isSuspended = false

    }

}

控制台显示队列正在按预期方式运行（作为串行队列，但是进度条上没有移动。

progress属性上的KVO也直接不起作用，所以我怀疑OperationQueue的progress属性是问题的原因，而不是UIProgressView。

知道我在这里缺少什么吗？还是iOS 13中的错误？模拟器和运行iOS 13.3.1的iPhone 6s Plus均存在此问题。谢谢！

Answer 1

我刚刚从Apple收到有关此问题的反馈。该文档当前很好地隐藏在头文件NSOperation.h中。这是遇到相同问题的任何人的摘要：

/// @property progress
/// @discussion     The `progress` property represents a total progress of the operations executed in the queue. By default NSOperationQueue
/// does not report progress until the `totalUnitCount` of the progress is set. When the `totalUnitCount` property of the progress is set the
/// queue then opts into participating in progress reporting. When enabled, each operation will contribute 1 unit of completion to the
/// overall progress of the queue for operations that are finished by the end of main (operations that override start and do not invoke super
/// will not contribute to progress). Special attention to race conditions should be made when updating the `totalUnitCount` of the progress
/// as well as care should be taken to avoid 'backwards progress'. For example; when a NSOperationQueue's progress is 5/10, representing 50%
/// completed, and there are 90 more operations about to be added and the `totalUnitCount` that would then make the progress report as 5/100
/// which represents 5%. In this example it would mean that any progress bar would jump from displaying 50% back to 5%, which might not be
/// desirable. In the cases where the `totalUnitCount` needs to be adjusted it is suggested to do this for thread-safety in a barrier by
/// using the `addBarrierBlock:` API. This ensures that no un-expected execution state occurs adjusting into a potentially backwards moving
/// progress scenario.
///
/// @example
/// NSOperationQueue *queue = [[NSOperationQueue alloc] init];
/// queue.progress.totalUnitCount = 10;