Which tasks are more suitable to NSOperation than GCD? [duplicate]

Question

This question already has an answer here:

• NSOperation vs Grand Central Dispatch 8 answers

Which tasks would be better suited to using NSOperation as opposed to using GCD when programming for the iPhone?

To me they seem to do the same thing. I can't see the strengths and weaknesses one has over the other.

Answer 1

NSOperation is built on top of GCD, so the question is more about whether you use NSOperation or pass a block directly to GCD.

An NSOperation is bulky and needs more boiler-plate codes to set it up, but it has a lot more functionality. You can create the same NSOperation subclass in various parts of your code and put them into the queue and run it.

Passing a block to GCD by e.g. dispatch_async is quick and disposable. You typically don't reuse a block anywhere else; you just set up a block which is executed only at that point of the code, passes it to the GCD or other APIs, and quickly go on.

So each has its merits.

Answer 2

Apparently, NSOperationQueue is built on GCD as of iOS 4; the docs just haven't been updated. Check this posting by an Apple employee here: https://devforums.apple.com/message/352770 (You may need to create an account) So, you should follow Mike Abdullah's advice and use the simplest API for the task at hand. dispatch_async is lower level, usually C-type stuff (but not limited to), and is good for one-shot and sequential type deals (fire this block on this queue, FTW). NSOperationQueues are higher level, Objective-C stuff, and are good if you are adding a lot of operations at various points in your code, and/or need to manage concurrency, priorities and dependencies. At least that's how I use them.

Answer 3

As always with such questions, use the simplest API available. Measure if it's a performance problem and then reevaluate if needed.

Answer 4

One thing that I don't believe has been mentioned here is that NSOperations can be cancelled during execution, whereas a block is guaranteed to complete once execution has begun. Having said that, a GCD queue can be suspended (dispatch_suspend()), so that any blocks following the currently executing blocks will not be executed.