问题
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()
}
}
回答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 { break }
print(i)
self?.heavyWork()
}
item = nil // resolve strong reference cycle
}
// start it
queue.async(execute: item)
// after five seconds, stop it if it hasn't already
queue.asyncAfter(deadline: .now() + 5) { [weak item] in
item?.cancel()
}
}
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); }
});
}
回答2:
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()
}
来源:https://stackoverflow.com/questions/38370035/how-to-stop-a-dispatchworkitem-in-gcd