CODEWITHSUNDEEP
objective-cnsoperationnsoperationqueue
tolgamorf
tolgamorf

Reputation: 809

Adding an NSOperationQueue to an NSOperation

Is it safe to add an NSOperationQueue to an NSOperation, and then add this operation to another NSOperationQueue?

Here is some code to visualize what I am trying to do.

NSOperationQueue *mainQueue = [NSOperationQueue alloc] init];

// Here I declare some NSBlockOperation's, i.e. parseOperation1-2-3
// and also another operation called zipOperation, which includes
// an NSOperationQueue itself. This queue takes the processed (parsed) files
// and write them to a single zip file. Each operation's job is to write the data
// stream and add it to the zip file. After all operations are done,
// it closes the zip.

[zipOperation addDependency:parseOperation1];
[zipOperation addDependency:parseOperation2];
[zipOperation addDependency:parseOperation3];

[mainQueue addOperation:parseOperation1];
[mainQueue addOperation:parseOperation2];
[mainQueue addOperation:parseOperation3];
[mainQueue addOperation:zipOperation];

Upvotes: 3

Views: 1314

Answers (1)

Rory O'Bryan
Rory O'Bryan

Reputation: 1904

I have used this approach and have it running in live code deployed on the App Store. I haven't experienced any issues during development or in the last 2 months since the code has been live.

In my case I had a high level series of operations, some of which contained a set of sub operations. Rather than expose the detail of each sub operation into the high level code, I created NSOperations which themselves contained NSOperationQueues and enqueued their own sub operations. The code I ended up with was much cleaner and easier to maintain.

I read extensively into NSOperation and have not seen any commentary that warns against this approach. I reviewed a lot of information online, the Apple documentation, and WWDC videos.

The only possible "drawback" might be the added complexity of understanding and implementing a Concurrent operation. Embedding an NSOperationQueue in an NSOperation means that operation becomes Concurrent.

So that's a 'YES' from me.

Additional details about concurrent operations:

An NSOperationQueue calls the start method on a normal (non-concurrent) NSOperation and expects the operation to be finished by the time the start call returns. For instance some piece of code you supplied to NSBlockOperation is complete at the end of the block.

If the work will not be finished by the time the start call returns then you configure the NSOperation as a Concurrent operation, so the NSOperationQueue knows that it has to wait until you tell it that the operation is finished at some later point in time.

For example, concurrent operations are often used to run asynchronous network calls; the start method only starts the network call, which then runs in the background, and calls back to the operation when its finished. You then change the isFinished property of the NSOperation to flag that the work is now complete.

So.... Normally when you add operations to an NSOperationQueue that queue runs those operations in the background. So if you put an NSOperationQueue inside an NSOperation then that operations work will be done in the background. Therefore the operation is concurrent and you need to flag when the internal NSOperationQueue has finished processing all it's operations.

Alternatively there are some methods on NSOperationQueue such as waitUntilAllOperationsAreFinished which could be used to ensure all the work was done before the start call returns, however these involve blocking threads and I avoided them, you may feel more comfortable with that approach, and making sure you don't have any side effects from blocking threads.

In my case I was already familiar with Concurrent operations so it was straightforward just to set it up as a Concurrent operation.

Some documentation about concurrent operations:

Concurrency Programming Guide: Configuring Operations for Concurrent Execution

In this example they are detaching a thread to perform work in the background, in our case we would be starting the NSOperationQueue here.

Upvotes: 5

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