ios - Does dispatch_group_wait allow cutting in line? -

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this question grand central dispatch, , dispatch_group_wait() in particular.

assume dispatch_group called group 10 tasks in waiting performed.

elsewhere have task needs wait tasks in group finish before executes. make happen, use dispatch_group_wait(group, dispatch_time_forever).

to distinguish tasks in group i'll call lonelytask.

if task gets added group while lonelytask waiting, gets executed first, lonelytask or task added group? in other words, tasks added group while task waiting execute "cut in line" ahead of waiting task, or order in called maintained?

i have searched documentation, haven't been able find answer question...

group +/- counter (semaphore) fires each time reaches zero. unaware of tasks, because dispatch_group_async() wrapper enter()s group before task submission , enqueues new block call task's block , leave() group. that's all. groups may used without queues asynchronous retain-counters or sml that.

so in general "you can't".

but if able [re]target (dispatch_set_target_queue()) related queues single concurrent queue, dispatch_barrier_async() on queue may want, if called in proper isolation. note group has nothing @ all.

also, original lonelytask not task group's point of view — unrelated thread of execution waits until group balanced. synonym "cutting in line" "barrier".

another approach create private group each taskset , wait specific group. not insert barrier though — following tasks not wait completion of lonelytask , uncontrolled async()s may "break rules", leading funny debugging sessions.


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