Question

What is the best/simplest way to detect if a queue I have created is currently in use? My intent here is to defer a particular action until all queued tasks have completed.

Thanks,
Doug

UPDATE

Here is a bit more context. I have an async serial queue (private dispatch queue) that is a property of my object. When this queue completes its work it calls dispatch_async(dispatch_get_main_queue(), {...}) to up date the UI. Currently the app runs only in landscape interface orientation. I am now adding support for both portrait & landscape interface orientation. For now, I prefer not to allow the interface to rotate to/from portrait/landscape while my GCD async task is running. So in my controller’s shouldAutorotateToInterfaceOrientation: method I want to return NO during those periods.

Answer 1

There’s no direct support for that in GCD. You can probably get the effect you’re looking for in other ways, though.

• Do you just want to run a block after other blocks have completed? Put the blocks in a dispatch_group, and submit your “after everything’s done action” using dispatch_group_notify. This works with concurrent or sequential queues.

• Similarly, put your blocks in a group, and call dispatch_group_wait(group, DISPATCH_TIME_NOW). If it returns non-zero, then there are outstanding blocks in the group.

• Is your queue sequential, and you want to wait until the other blocks have completed, blocking the current thread until then? Just issue your handler using dispatch_sync on the same queue.

There are other possibilities, depending on your exact situation.

Edit: Based on your update, there’s an even easier way:

• add a BOOL property to your view controller, called allowsOrientationChange
• override -shouldAutorotateToInterfaceOrientation: to return self.allowsOrientationChange
• at the start of your work block, do a dispatch_async to set the property to NO
• at the end of your work block, do a dispatch_async to set the property to YES

(There’s a little bit of a race condition between when the property is set to NO on the main thread, and when the rest of your work happens in the queue. You could make it a dispatch_sync if that really matters. Depends on what your reasons are for preventing rotation, I suppose.)