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If you don't accept a queue, devs will usually just dispatch to that queue in the callback anyway. Same problems, right?

No, actually. Worried about composition wrt target queues, APIs which mutate queues, and resilience against future concurrent APIs.

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dispatch_queue_set_target; dispatch_suspend. It’s dangerous to pass around a private queue.

I can understand deadlock if the API runs the callback synchronously. Is there still a problem if it's async?

No, but then implementing a cancellation API becomes very dangerous…

Only if you have to guarantee that no callbacks will received after cancellation, right?

That’s pretty much always necessary, unless the thing generating these callbacks lives as long as your program does.

Not sure I understand. Isn't it enough to live as long as the callback wants to use it? Can block capture solve this?

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Block capture handles memory management, but that’s not necessarily the same as semantic invalidation.

4:56 PM · Jul 8, 2013 from Cupertino, CATweetbot for Mac