最初删除非活动队列

问题描述

我有一些需要下载的照片，但是下载功能一次只能下载一张照片。我希望在下载其他照片之前确保下载功能已完成。我创建.initialyInactive DispatchQueue，并在下载功能的完成块中激活了队列。这很不错，照片已下载，但是我的问题是如何取消下载过程？我可以以某种方式删除未激活的队列吗？

我的代码逻辑如下所示。

I have array of photos that needs to be downloaded, but download function can download only one photo at a time. I neeed to make sure that download function is completed before I call it for other photo. I create .initialyInactive DispatchQueue and queue is activated in completion block of download function. This is woking, and photos are downloaded, but my question is how to cancel download process? Can I somehow to remove not activated queues?
My code logic looks something like this..

 func downloadPhoto(photo: Photo, completion: (_ success: Bool) -> Void) {
    ... completion(true) ..
}
for photo in photos {
      let queue = DispatchQueue(label: "Download queue\(counter)", qos: .userInitiated, attributes: .initiallyInactive)
       self.inactiveQueues[counter] = queue

       queue.async {

           self.downloadPhoto(photo: photo) { (success) in
                  nextQueue?.activate()
                   if success {
                          ...
                   } else {
                          ...
                   }
           }
}

解决我的问题的其他方法也很棒。谢谢。

Ant other solution to my problem is great too. Thanks.

推荐答案

有几种选择：

1. 从历史上看，我们将使用自定义的异步 Operation 子类。请参见（或请参见（用于替代 AsynchronousOperation 的实现）。这使我们可以取消操作，并且我们可以轻松地控制并发程度。

1. Historically we’d use a custom, asynchronous, Operation subclass. See https://stackoverflow.com/a/38469350/1271826 (or see https://stackoverflow.com/a/48104095/1271826 for alternate AsynchronousOperation implementation). That gives us a cancelable operation and we can easily control the degree of concurrency.

如果要一次下载一个，可以设置队列的 maxConcurrentOperationCount 到1。然后，您可以将下载操作添加到单个 OperationQueue 中，如果要取消下载操作，请致电。

If you want to download them one at a time, you could set the queue’s maxConcurrentOperationCount to 1. Then you can just add your download operations to a single OperationQueue, and if you want to cancel them, you’d call queue.cancelAllOperations().

如果您的目标是iOS 13及更高版本，请使此操作更加容易。请参阅，以了解如何使用Combine下载一系列图像的示例（两者

If you are targeting iOS 13 and later, Combine makes this even easier. See https://stackoverflow.com/a/61195234/1271826 for example of downloading a series of images using Combine (both sequentially as well as with a controlled degree of concurrency).

我已经说了所有这些，我鼓励您需要重新考虑顺序下载图像的选择。在我的中，将允许的最大并发率提高到6，导致下载速度总体上是串行下载速度的两倍行为。

All of this having been said, I’d encourage you to reconsider the choice to download the images sequentially. In my random test, increasing the max allowed concurrency to 6 resulted in downloads that were, overall, twice as fast as the serial behavior.

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