Cancel GCD block working in thread

问题

I have the following block which performs a request in the background.
How may I cancel this request before it has completed?

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_async(queue, ^{
        NSData *thumbnailData = [NSURLConnection sendSynchronousRequest:request];
        ...
    });

回答1:

You can't. You have to use the asynchronous interface of NSURLConnection to be able to cancel requests.

回答2:

You can't cancel once you've dispatched...

You can use a workaround, like:

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
__block BOOL isCanceled = NO;
dispatch_async(queue, ^{

    if (isCanceled)
        return;

    NSData *thumbnailData = [NSURLConnection sendSynchronousRequest:request];
    ...
});

回答3:

You can use the higher level "NSOperationQueue", after adding the operation to queue, then you can cancelAllOperations.

来源：https://stackoverflow.com/questions/6090406/cancel-gcd-block-working-in-thread