Question

Sorry, I’m not sure of the right language here, but when methods are called because they are either delegate methods, or methods called as a result of being listed as the target of an observer, are they executed on the main thread?

I’m wondering if I can just make UI changes in these methods, or do I have to wrap them in

    dispatch_async(dispatch_get_main_queue(), ^{ UI stuff });

TIA: John

Answer 1

For delegates this can vary. If the documentation does not specify, then usually they are sent on the main thread. Traditionally UIKit must be used on the main thread so those delegates will almost always be called from the main thread.

For notifications I think you want this little snip.

A notification center delivers notifications to observers synchronously. In other words, the postNotification: methods do not return until all observers have received and processed the notification. To send notifications asynchronously use NSNotificationQueue. In a multithreaded application, notifications are always delivered in the thread in which the notification was posted, which may not be the same thread in which an observer registered itself.

From http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Classes/nsnotificationcenter_Class/Reference/Reference.html

And finally for KVO, the notifications can come in from other threads. Here is what an Apple Engineer had to say about handling them.

http://lists.apple.com/archives/cocoa-dev/2007/May/msg00022.html