NSOperation - Forcing an operation to wait others dynamically

Question

I am trying to implement an operation queue and I have the following scenario:

NSOperation A
NSOperation B
NSOperation C
NSOperation D
NSOperationQueue queue         


        

Answer 1

Once an NSOperation is in its main method you have to go through with it. There is no paused state, only finished or cancelled.

I would implement a NSCopying on operation A which would copy the entire state into a new instance. You would have a delegate method or block which is able to communicate that this operation cannot go through because it is missing info from operation B.

So the process would go such:

• Create Operation A, set delegate
• you cannot proceed, delegate method fires
• the delegate creates a new operation B, creates a copy of operation A, sets the dependency such that A will wait for B's completion
• then the delegate cancels the original op A

Inside the delegate you have to make sure to suspend the queue to avoid a race condition. After the above steps you resume the queue. In operation A you would have multiple places where you check for isCancelled to actually not do any more work in main when it has been cancelled.