Main Thread UI and Detached Thread

FROM : Mike
DATE : Sun Mar 30 20:55:29 2008

I have all my UI running on my app's main thread. I have a worker thread
that I detach with detachNewThreadSelector:toTarget:withObject: (my
worker thread).

In my worker thread I do a tight processing loop and one of the things I
do in the loop is call two methods in the main thread to update the
display (a text message and progress bar) - via
performSelectorOnMainThread:withObject:waitUntilDone:modes.

However, when the loop runs in the spawned thread, the display doesn't
get updated. If I insert a sleep(1) call into the loop, then the display
  updates.

Why doesn't the main thread process the changes to the UI unless I call
sleep? I thought the whole idea of using a separate thread was so that
the main thread could continue to run on its own?

Thanks,

Mike