objective c - How to know when a job that require multiple threading has finished? -

one thing is:

-(void)grabbingprocess:(void (^)())block; {     self.othergrabbingindicator +=1;     block();     dispatch_after(dispatch_time(dispatch_time_now, 2 * nsec_per_sec), dispatch_get_current_queue(), ^{         self.othergrabbingindicator -=1;     }); } 

everytime want run may take long time in background,

[grabclass grab] grabbingprocess: ^{ //do grab data etc. }];

there many such functions , many such data. example, @ first grab businesses id. grab details on businesses on separate thread.

i want know time when threads have finished , post suitable notification.

the problem solution after while, quite value of self.othergrabbingindicator hover around 2 or 3 never going down though of thread have finished.

somehow of self.othergrabbingindicator +=1; "leaked" , not matched self.othergrabbingindicator -=1. wonder how can leak ever happen?

if want run blocks asynchronously , find out when they're done, appropriate tool dispatch_group. can dispatch blocks onto specific group , queue using dispatch_group_async(), , group track when block finished. can either wait synchronously on group finish dispatch_group_wait() or register block invoked when group done dispatch_group_notify().

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the reason why solution above doesn't work because aren't accessing/mutating counter in threadsafe way. here's trivial series of events cause problem:

start: self.othergrabbingindicator 1
thread a: reads self.othergrabbingindicator
thread b: reads self.othergrabbingindicator
thread a: increments read value , writes self.othergrabbingindicator
thread b: increments read value , writes self.othergrabbingindicator
end: self.othergrabbingindicator 2

even though 2 threads tried increment it, 1 "succeeded" , end losing other increment. can happen during decrement. if use dispatch groups problem goes away.

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as side note, should never use dispatch_get_current_queue(). debugging function never supposed used in actual code. primary reason because aren't running on 1 queue, you're running on whole hierarchy of queues simultaneously, function can tell 1 queue. in addition, may running on someone's private queue , should not dispatching onto yourself.