How to stop a DispatchWorkItem in GCD?

Question

I am currently playing around with Grand Central Dispatch and discovered a class called DispatchWorkItem. The documentation seems a little incomplete so I am not sure about using it the right way. I created the following snippet and expected something different. I expected that the item will be cancelled after calling cancel on it. But the iteration continues for some reason. Any ideas what I am doing wrong? The code seems fine for me.

@IBAction func testDispatchItems() {     let queue = DispatchQueue.global(attributes:.qosUserInitiated)     let item = DispatchWorkItem { [weak self] in         for i in 0...10000000 {             print(i)             self?.heavyWork()         }     }      queue.async(execute: item)     queue.after(walltime: .now() + 2) {         item.cancel()     } } 

Answer 1

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); }     }); }