How to stop a DispatchWorkItem in GCD?

GCD does not perform preemptive cancelations. So, to stop a work item that has already started, you have to test for cancelations yourself. In Swift, cancel the DispatchWorkItem. In Objective-C, call dispatch_block_cancel on the block you created with dispatch_block_create. You can then test to see if was canceled or not with isCancelled in Swift (known as dispatch_block_testcancel in Objective-C).

func testDispatchItems() {    let queue = DispatchQueue.global()    var item: DispatchWorkItem?    // create work item    item = DispatchWorkItem { [weak self] in        for i in 0 ... 10_000_000 {            if item?.isCancelled ?? true { break }            print(i)            self?.heavyWork()        }        item = nil    // resolve strong reference cycle of the `DispatchWorkItem`    }    // start it    queue.async(execute: item!)    // after five seconds, stop it if it hasn't already    queue.asyncAfter(deadline: .now() + 5) {        item?.cancel()        item = nil    }}

Or, in Objective-C:

- (void)testDispatchItem {    dispatch_queue_t queue = dispatch_get_global_queue(QOS_CLASS_DEFAULT, 0);    static dispatch_block_t block = nil;  // either static or property    __weak typeof(self) weakSelf = self;    block = dispatch_block_create(0, ^{        for (long i = 0; i < 10000000; i++) {            if (dispatch_block_testcancel(block)) { break; }            NSLog(@"%ld", i);            [weakSelf heavyWork];        }        block = nil;    });    // start it    dispatch_async(queue, block);    // after five seconds, stop it if it hasn't already    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{        if (block) { dispatch_block_cancel(block); }    });}

There is no asynchronous API where calling a "Cancel" method will cancel a running operation. In every single case, a "Cancel" method will do something so the operation can find out whether it is cancelled, and the operation must check this from time to time and then stop doing more work by itself.

I don't know the API in question, but typically it would be something like

        for i in 0...10000000 {            if (self?.cancelled)                break;            print(i)            self?.heavyWork()        }