StackOverflower StackOverflower - 25 days ago 13
Swift Question

Why my NSOperation is not cancelling?

I have this code to add a

NSOperation
instance to a queue

let operation = NSBlockOperation()
operation.addExecutionBlock({
self.asyncMethod() { (result, error) in
if operation.cancelled {
return
}

// etc
}
})
operationQueue.addOperation(operation)


When user leaves the view that triggered this above code I cancel operation doing

operationQueue.cancelAllOperations()


When testing cancelation, I'm 100% sure cancel is executing before async method returns so I expect
operation.cancelled
to be true. Unfortunately this is not happening and I'm not able to realize why

I'm executing cancellation on
viewWillDisappear


EDIT

asyncMethod
contains a network operation that runs in a different thread. That's why the callback is there: to handle network operation returns. The network operation is performed deep into the class hierarchy but I want to handle NSOperations at root level.

Answer

Calling the cancel method of this object sets the value of this property to YES. Once canceled, an operation must move to the finished state.

Canceling an operation does not actively stop the receiver’s code from executing. An operation object is responsible for calling this method periodically and stopping itself if the method returns YES.

You should always check the value of this property before doing any work towards accomplishing the operation’s task, which typically means checking it at the beginning of your custom main method. It is possible for an operation to be cancelled before it begins executing or at any time while it is executing. Therefore, checking the value at the beginning of your main method (and periodically throughout that method) lets you exit as quickly as possible when an operation is cancelled.

import Foundation

let operation1 = NSBlockOperation()
let operation2 = NSBlockOperation()
let queue = NSOperationQueue()
operation1.addExecutionBlock { () -> Void in
    repeat {
        usleep(10000)
        print(".", terminator: "")
    } while !operation1.cancelled
}
operation2.addExecutionBlock { () -> Void in
    repeat {
        usleep(15000)
        print("-", terminator: "")
    } while !operation2.cancelled
}
queue.addOperation(operation1)
queue.addOperation(operation2)
sleep(1)
queue.cancelAllOperations()

try this simple example in playground.

if it is really important to run another asynchronous code, try this

operation.addExecutionBlock({
if operation.cancelled {
            return
        }    
self.asyncMethod() { (result, error) in


        // etc
    }
})
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