Canceling OperationQueue Swift

Question

I'm learning OperationQueue. What I want to do is to prevent network calls if there's an ongoing call. The problem is that when I tap on the button, a new operation is being added to the queue.

class ViewController: UIViewController {

    private var queue = OperationQueue()

    func networkCall(completion: (()->Void)?) {
        DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(3)) {
            completion?()
        }
    }

    lazy var button: UIButton = {
        let b = UIButton(type: .custom)
        b.backgroundColor = .black
        b.setTitleColor(.white, for: .normal)
        b.setTitle("CLICK!~", for: .normal)
        b.addTarget(self, action: #selector(self.boom), for: .touchUpInside)
        return b
    }()

    @objc func boom() {
        print("BOOM!")

        self.queue.maxConcurrentOperationCount = 1

        let block = BlockOperation()
        block.addExecutionBlock {
            self.networkCall {
                print("DONE NETWORK CALL!")
            }
        }

        self.queue.addOperation(block)

        DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(1)) {
            self.queue.cancelAllOperations()
        }
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        self.view.backgroundColor = .gray
        self.view.addSubview(self.button)
        self.button.translatesAutoresizingMaskIntoConstraints = false
        self.button.centerYAnchor.constraint(equalTo: self.view.centerYAnchor).isActive = true
        self.button.centerXAnchor.constraint(equalTo: self.view.centerXAnchor).isActive = true
        self.button.widthAnchor.constraint(equalToConstant: 100.0).isActive = true
        self.button.heightAnchor.constraint(equalToConstant: 50.0).isActive = true
    }
}

Am I missing something?

Answer 1

Unfortunately, you can’t just add an asynchronous method to a BlockOperation. The operation, itself, is going to complete as soon as the asynchronous network request is submitted (even though you’ve deferred the completion?() an arbitrary 3 seconds later).

Instead, you’re going to want to wrap the network request in a custom, asynchronous, Operation subclass method that does all the KVO notifications for proper Operation behaviors. See the Operation documentation for details.

See https://stackoverflow.com/a/32322851/1271826 for example with downloadTask implementation. Or see https://stackoverflow.com/a/48104095/1271826 for general discussion.

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By the way, even though you won’t be using BlockOperation, in those cases where you do, be mindful of the addExecutionBlock of BlockOperation. You can, for example, have a serial queue, create a BlockOperation, and then add multiple addExecutionBlock for a given operation, and they will not be serial. Individual operations will behave serially, but not individual blocks you add to a given BlockOperation. I’d generally advise people to avoid addExecutionBlock with BlockOperation until you really grok what’s going on there. Better to just use the main closure when you initialize BlockOperation.