On Aug 14, 2015, at 8:43 PM, Carl Hoefs <newsli...@autonomy.caltech.edu> wrote:
> On Aug 14, 2015, at 6:24 PM, Ken Thomases <k...@codeweavers.com> wrote:
>>
>> -performSelector:withObject:afterDelay: depends on the run loop. The
>> threads that service any non-main NSOperationQueue don't run their run loop.
>> In fact, you can't rely on them surviving past the end of your operation's
>> code returning.
>>
>> You will need to shunt this to a thread which reliably runs its run loop.
>> The obvious candidate is the main thread.
>>
>> You could do some combination of
>> -performSelectorOnMainThread:withObject:waitUntilDone: with
>> -performSelector:withObject:afterDelay:, but it's really much simpler to use
>> GCD.
>>
>> One approach is to use dispatch_after(). You could schedule a task to
>> invoke -doStatusChecks on the main thread:
>>
>> dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 60ULL * NSEC_PER_SEC),
>> dispatch_get_main_queue(), ^{
>> [self doStatusChecks];
>> });
>>
>> Depending on why you're using a custom operation queue, you might skip the
>> middle man and do:
>>
>> dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 60ULL * NSEC_PER_SEC),
>> dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
>> [self checkStatus];
>> });
>>
>> Finally, since this is a recurring task, you can use a dispatch timer source
>> to do it all:
>>
>> dispatch_source_t timer =
>> dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0,
>> dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0));
>> dispatch_source_set_timer(timer, DISPATCH_TIME_NOW, 60ULL * NSEC_PER_SEC,
>> 1ULL * NSEC_PER_SEC);
>> dispatch_source_set_event_handler(timer, ^{
>> [self checkStatus];
>> });
>> dispatch_resume(timer);
>>
>
> Thanks, Ken. The reason I'm using a custom operation queue instead of GCD is
> that I need to permit only exclusive access to the device (e.g., to obtain
> its status requires writing to it), but allow other operations enqueued to
> use the device, not just the status checks. So I have an NSOperationQueue
> that has maxConcurrentOperationCount set to 1, but I need a status-check
> operation to be automatically queued up 1 minute after the last one has run.
>
> So, if the device is accessed otherwise the status checks won't interfere
> with that because only 1 operation can be active in the queue, thus
> everything stays well ordered and exclusive access is permitted to all, one
> at a time. I just can't figure a way to have the status check operation
> trigger another one to be magically enqueued 60 seconds later.
You can use a serial GCD queue for the same purpose with my above suggestions.
But even if you want to keep using an NSOperationQueue, you can still use my
first dispatch_after() suggestion. It works just like your code except for
replacing -performSelector:withObject:afterDelay: with dispatch_after(). This
changes doesn't affect anything about how -doStatusChecks works. It will still
use your NSOperationQueue.
You don't need to use the main GCD queue for my dispatch_after() suggestion,
you can use a global concurrent queue. Either works.
Regards,
Ken
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