nsoperationqueue - Do NSOperations and their completionBlocks run concurrently?

Question

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.

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

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?

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.

Answer 1

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.

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

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

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.

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: