How to cancel NSBlockOperation

Question

I have a long running loop I want to run in the background with an NSOperation. I\'d like to use a block:

NSBlockOperation *operation = [NSBlock         


        

Answer 1

With Swift 5, you can create a cancellable BlockOperation with addExecutionBlock(_:). addExecutionBlock(_:) has the following declaration:

func addExecutionBlock(_ block: @escaping () -> Void)

Adds the specified block to the receiver’s list of blocks to perform.

---

The example below shows how to implement addExecutionBlock(_:):

let blockOperation = BlockOperation()

blockOperation.addExecutionBlock({ [unowned blockOperation] in
    for i in 0 ..< 10000 {
        if blockOperation.isCancelled {
            print("Cancelled")
            return // or break
        }
        print(i)
    }
})

Note that, in order to prevent a retain cycle between the BlockOperation instance and its execution block, you have to use a capture list with a weak or unowned reference to blockOperation inside the execution block.

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The following Playground code shows how to cancel a BlockOperation subclass instance and check that there is no retain cycle between it and its execution block:

import Foundation
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

class TestBlockOperation: BlockOperation {
    deinit {
        print("No retain cycle")
    }
}

do {
    let queue = OperationQueue()

    let blockOperation = TestBlockOperation()
    blockOperation.addExecutionBlock({ [unowned blockOperation] in
        for i in 0 ..< 10000 {
            if blockOperation.isCancelled {
                print("Cancelled")
                return // or break
            }
            print(i)
        }
    })

    queue.addOperation(blockOperation)

    Thread.sleep(forTimeInterval: 0.5)
    blockOperation.cancel()
}

This prints:

0
1
2
3
...
Cancelled
No retain cycle