Question

This could be a much more generic question abut how to best cancel blocking jobs on other threads, but I’m interested in a solution in the context of Grand Central Dispatch. I have the need to call a function which basically blocks until it gets data from the network; it could potentially be blocked forever. I have it set up now so that this blocked call happens on a private dispatch queue, and when i do get data, i put a block back on the main queue. Th e problem is that once I dispatch my private-queue-block and blocking call, I can never really cancel that. Imagine this ability was tied to a user setting toggle. If they toggled off, I would want this blocking job and execution block to essentially just end. Is there a good solution to this type of problem?

Thanks

- (void)_beginListeningForNetworkJunk
{
    dispatch_async(my_private_queue, ^{
        // blocks until it gets data
        id data = [NetworkListener waitForData];
        dispatch_async(dispatch_get_main_queue(), ^{
            [self _handleNetworkData:data];
        });
    });
}

- (void)_endListeningForNetworkJunk
{
   // How do I kill that job that is blocked on my private queue?
}

Answer 1

You can’t. The problem is in NetworkListener in its blocking-and-uninterruptible interface.

Normally, you’d code the block to service the network connection asynchronously and also monitor some other signalling mechanism, such as a custom run loop source (or NSPort or pipe file descriptor or …). When the network connection had activity, that would be serviced. When the signalling mechanism fired, you would shut down the network connection and exit the block.

In that way, the block could be cancellable with its cooperation.

Since your block is stuck in -waitForData, it can’t cooperate. There’s no mechanism for canceling blocks without their cooperation. The same is true of NSOperation and NSThread. The reason is that it’s basically infeasible to terminate another thread’s activity without its cooperation.

You need a different design for your networking code.