Cancel actions in dispatch_async when they're no longer needed

Question

I have a UIViewController that does the following in viewDidLoad

dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
    items = [[DataFetcher sharedInstance] getItems handler:^(NSArray *currentItems){
        if (currentItems.count % 30 == 0) { //don't wait for all the items to be ready show by chunks of 30
            items = currentItems;
            [tableView reloadData];
        }
        items = currentItems;
    }];//Pretty complex call that takes some time to finish there is WebService calls, data parsing, storing some results ...
    dispatch_async(dispatch_get_main_queue(), ^{
        [tableView reloadData];
    });
});

What I need to do is to stop getItems when I pop this viewController. It's pointless and it takes CPU Time and energy (This call may take up to a minute on some cases).

I am assuming I should be doing this in viewWillDisappear but how exactly?

score 1 · Accepted Answer · answered Sep 17 '14 at 15:49

You can use NSBlockOperation. Periodically check if it's been cancelled, and stop doing work if it has been:

- (void)getItemsWithHandler:(void (^)(NSArray *currentItems))handler {
    self.operation = [NSBlockOperation blockOperationWithBlock:^{
        if (self.operation.isCancelled) {
            return;
        }

        // Do something expensive

        if (self.operation.isCancelled) {
            return;
        }

        // Do something else expensive

        for (int i = 0; i < 10000; i++) {
            if (self.operation.isCancelled) {
                return;
            }

            // Do expensive things in a loop
        }
    }];
}

- (void) cancelGetItemsRequest {
    [self.operation cancel];
    self.operation = nil;
}

Alternatively, you can put a bunch of NSBlockOperations in an NSOperationQueue. You can set dependencies for the work, and cancel the entire queue at once if you want.

Note that strongly capturing `self` in an operation also owned by `self` creates a retain cycle. This example clears the dependency in the cancellation case, but not in the successful completion case. — ipmcc, Sep 17 '14 at 16:11
Yeah, I should've mentioned that. Thanks for pointing it out. See [this question for further discussion](http://stackoverflow.com/q/8113268/1445366). — Aaron Brager, Sep 17 '14 at 16:18

score 0 · Answer 2 · answered Sep 17 '14 at 15:37

0

Cancelling asynchronous operations is nicely supported in NSOperation and NSOperationQueue, but it's quite a bit more complicated. In any case, your asynchronous code has to check from time to time whether it is cancelled or should still continue.

Best thing is to have a property "cancelled" somewhere, that you set when you don't want any more work to be done, and whenever you try to do more work, you check that property.

answered Sep 17 '14 at 15:37

gnasher729

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I think it would actually be *less* complicated with [`NSBlockOperation`](https://developer.apple.com/library/mac/documentation/cocoa/reference/NSBlockOperation_class/Reference/Reference.html). – Aaron Brager Sep 17 '14 at 15:40
(It doesn't even require an operation queue, and already has a cancelled property.) – Aaron Brager Sep 17 '14 at 15:40

Cancel actions in dispatch_async when they're no longer needed

2 Answers2