Lets say, i have a complex calculation running in NSOperation block. I have paused it. Closed the app. Then restarted the app. Can i recover the last state and continue from there? Is there existing solution for such a problem or it can be only custom built for certain purposes?
The question is a bit vague, so it's hard to say without knowing all of the code in play. With that said, I may approach the problem by:
Option 1. In your subclass of NSOperation, add your own atomic KVO property "isPaused". Within the operation itself, observe that property and handle accordingly if it ever changes.
Option 2. Are you ever suspending the Operation Queue itself? If so, consider observing that property from within your operations, and each one independently can take action if that value changes.
Option 3. Cancel all operations in the queue, and if the view appears again, just restart with new operations.
Overall, though, there is no magic bullet for pausing operations already in progress. You'll have to bake your own solution. The damage shouldn't be too bad though.
Suspending and Resuming Queues If you want to issue a temporary halt to the execution of operations, you can suspend the corresponding operation queue using the setSuspended: method.
Suspending a queue does not cause already executing operations to pause in the middle of their tasks. It simply prevents new operations from being scheduled for execution. You might suspend a queue in response to a user request to pause any ongoing work, because the expectation is that the user might eventually want to resume that work.
For more detail Refer this link apple docs: http://developer.apple.com/library/mac/#documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationObjects/OperationObjects.html