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); }
});
}
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