On a related topic from my last post, looking at the equivalent situation but using GCD instead of runloops, the problem appears to be impossible. That is:
* Code running on dispatch queue A starts some async task that will run on another queue B, and which will signal completion by using dispatch_async to call a completion block on queue A, passing it some result. * After the task starts, the logic on queue A decides it needs to block until the task finishes and returns its result. (This might be part of an implementation of the 'future' design pattern, for example.) * Now, how does queue A wait for the callback? It seems to be impossible: the only way for code running on queue A to receive the callback is to return back to the queue dispatcher, so if it tries to wait it simply blocks the queue, causing a deadlock. This is exactly why runloops have the ability to run a "nested" loop, to block but still allow incoming messages. Of course that comes with its own set of complications, like the necessity for multiple "modes", and issues like the one I just brought up. But given that dispatch queues can't do this, what's the solution to this design problem, i.e. turning an async call into a sync one? —Jens _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
