如何在GCD中停止DispatchWorkItem？

Question

我目前正在玩 Grand Central Dispatch
，发现了一个名为的课程DispatchWorkItem。该文档似乎有些不完整，所以我不确定是否正确使用它。我创建了以下代码段，并期望有所不同。我预计该项目将在调用后被取消cancel。但是由于某种原因，迭代仍在继续。有什么想法我做错了吗？该代码对我来说似乎很好。

@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不会执行抢先取消。因此，要停止已经开始的工作项，您必须自己测试取消。在斯威夫特cancel的DispatchWorkItem。在Objective-
C中，调用dispatch_block_cancel用创建的块dispatch_block_create。然后，您可以测试isCancelled在Swift中是否被取消（dispatch_block_testcancel在Objective-
C中称为）。

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
    }
}

或者，在Objective-C中：

- (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); }
    });
}