NSOperations及其完成块是否同时运行？ [英] Do NSOperations and their completionBlocks run concurrently?

问题描述

我在 NSOperationQueue 中添加了一堆 NSOperations 。操作队列将 maxConcurrentOperationCount 设置为1，以便 NSOperations 一个接一个地运行。

I've got a bunch of NSOperations added to a NSOperationQueue. The operation queue has the maxConcurrentOperationCount set to 1, so that the NSOperations run one after the other.

现在，在 NSOperation 的 completionBlock 中，我想取消所有通过在 NSOperationQueue 上调用 cancelAllOperations 来挂起 NSOperations 。

Now, in the completionBlock of a NSOperation I want to cancel all pending NSOperations by calling cancelAllOperations on the NSOperationQueue.

这样做安全吗？我可以确定只有在上一个操作的 completionBlock 具有后，才能调用下一个操作的 start 方法被完全执行？还是上一个操作的 completionBlock 和当前操作的任务同时运行？

Is it safe to do this? Can I be sure that the start-method of the next operation is called only after the completionBlock of the previous operation has been fully executed? Or do the completionBlock of the previous operation and the task of the current operation run concurrently?

我为什么问：我使用AFNetworking执行一批 AFHTTPRequestOperations ，并且仅在该批次的所有先前请求均成功时才要执行一个请求。

The reason why I'm asking: I use AFNetworking to execute a batch of AFHTTPRequestOperations and want to perform one request only if all previous requests of the batch were successful.

推荐答案

我在下面的发现似乎不再是事实。我已在iOS 8和iOS 9上重新运行测试，并且操作的完成部分始终与下一个操作同时运行。目前，我看不到要等待上一个完成块完成的操作的方法。

My findings below no longer seem to be true. I've re-run the tests on iOS 8 and iOS 9 and the completion block of an operation always runs concurrently with the next operation. Currently, I don't see a way to make an operation wait for the previous completion block to finish.

我只是在一个示例项目中尝试了这种情况。结果如下：

I just tried this scenario in a sample project. Here is the result:

如果 NSOperationQueue 的 maxConcurrentOperationCount 设置为1，一个 NSOperation 的 completionBlock 和下一个 NSOperation 在队列中同时运行。

If the NSOperationQueue's maxConcurrentOperationCount is set to 1, an NSOperation's completionBlock and the next NSOperation in the queue run simultaneously.

但是，如果每个 NSOperation 通过调用 addDependency：，操作的执行要等到前一个操作的 completionBlock 完成。

But, if every NSOperation is linked to its previous operation by calling addDependency:, the execution of an operation waits until the previous operation's completionBlock has finished.

，如果要在当前操作的 completionBlock 中取消下一个操作，并确保在开始操作之前取消了该操作，则必须在 NSOperations 通过调用 addDependency：

So, if you want to cancel the next operation in the completionBlock of the current operation and be sure that it is cancelled before it is started, you have to set dependencies between the NSOperations by calling addDependency:

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