How to safely dispatch code after delay on the main thread, using GCD?

Question

Is it safe to dispatch a block of code with delay on the main thread, if you are already on the main thread?

dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, seconds * NSEC_PER_SEC);
dispatch_after(popTime, dispatch_get_main_queue(), theBlock);

Or is there a safer way? Do I have to perform any checks if I am already on main queue (main thread) when executing this?

It's not `greatest-common-divisor`, it's `grand-central-dispatch`, I believe. — Bhavin, Jun 28 '13 at 11:20
Yes. No. You may want to use weakSelf to avoid retaining an out of date view controller etc. — David H, Jun 28 '13 at 11:29

score 5 · Accepted Answer · answered Jun 28 '13 at 11:42

5

You generally don't have to check whether you're already on the main thread if the block is enqueued asynchronously, which dispatch_after does:

This function waits until the specified time and then asynchronously adds block to the specified queue.

You would have to check however, if you were using a synchronous function like dispatch_sync. That would otherwise result in a deadlock.

answered Jun 28 '13 at 11:42

omz

I'm calling the above code simply from the main thread (after user presses a button). Can you give an example with dispatch_sync that causes a deadlock? – openfrog Jun 28 '13 at 11:44
dispatch_get_current_queue is deprecated. – openfrog Jun 28 '13 at 13:24
You can use `[NSThread isMainThread]` instead of `dispatch_get_current_queue` (obviously only for the main thread queue). – omz Jun 28 '13 at 17:33

score 0 · Answer 2 · answered Jun 28 '13 at 11:31

Yes, it is safe. There are other ways to perform actions on main thread, but I don't guess they are safer. You can use, for example:

[[NSOperationQueue mainQueue] addOperationWithBlock: YOUR_BLOCK_HERE ];

I really prefer NSOperationQueue when the extra features of GCD are not needed. It is easier.

2 Answers2