iphone - NSOperationQueue out of range exception -

i have strange error pop recently. nsoperationqueue says has 1 object in cannot access nsoperation object inside it.

    if ([[queue operations] count] > 0)     op = [queue.operations objectatindex:0]; 

but reason ends in following exception: index 0 beyond bounds empty array'

i understand error message surprised since checking queue count before asking object itself.

any ideas please?

remember operations able run on separate threads , are. nsoperationqueue has own method getting count called operationcount , provides word of caution:

the value returned method reflects instantaneous number of objects in queue , changes operations completed. result, time use returned value, actual number of operations may different. should therefore use value approximate guidance , should not rely on object enumerations or other precise calculations.

what running concurrency issue. 1 thing consider copy operations array.

nsarray *ops = [queue.operations copy]; if ([ops count] > 0) {     op = [ops objectatindex:0];     //you can check if has finished using [op isfinished];     //and need here } [ops release]; 

update:

here example of why may see happening often

//set , start nsoperation ...  //here call operations put sort of lock //around operations retrieve them operation //finished , waiting lock complete remove //the operation. operations call returns copy. if([[que operations] count] > 0) {     //now operation queue can access operations , remove     //the item lock released (it can access     //as before call , count)      //uh oh there 0 operations     op = [queue.operations objectatindex:0];  }