OS X inter thread communication question

Question

I am developing a multi-threaded application in Cocoa. The main thread takes values from the user, and when a button is clicked I invoke a secondary thread in which a long calculation takes place. Now from this thread I have to return the output of every step of the calculation to the main thread. I want to periodically send data from one thread to the other. I can't find any simple example that does this. Any ideas?

Answer 1

There are a number of ways to do this, in rough order of complexity (easiest first):

• use NSObject's performSelectorOnMainThread:withObject:waitUntilDone: which is pretty self explanatory.
• use performSelector:onThread:withObject:waitUntilDone:, which will let you go the other way
• use an NSNotification (and NSDistributedNotificationCenter), though you can easily run into a race condition if you're not careful
• Use NSPorts to send data back and forth

Check out the doc that Abizer mentioned for details on all of these.

Answer 2

performSelectorOnMainThread:withObject:waitUntilDone: is often the easiest way to update the UI with a background thread's progress. You could also create your own storage area that's safe to access between threads using NSLock or a similar mechanism, or even use distributed objects (which also works between processes or over a network).

Then there's NSOperationQueue and NSOperation which does help a lot to simplify multi-threaded programming, although a lot of programmers have been avoiding it since it can cause a crash in certain circumstances under Leopard.