Re: Wrapping my head around pthreads, nsthreads, and nsnotificationcenter

FROM : Chris Kane
DATE : Sat Nov 03 16:40:58 2007

NSThread is just an object layer on top of pthreads.  There's a 
pthread underneath every NSThread.

If you're having trouble receiving notifications, the problem is 
unlikely to be thread-related.  It's more likely to be a problem with 
addObserver:... or removeObserver:... usage.  For example, upon 
receiving the first notification if the something is calling 
removeObserver:..., it may be doing so specifying parameters that are 
too broad and unintentionally wiping out other observers in the process.

People have a lot of confusion around notification and threads, 
because they expect somehow there is a relationship between 
notification observation and threads.  Actually, it is usually that 
they are hoping for that, so they can use notification posting as an 
inter-thread communication mechanism.  So it is not that 
notifications "can only be received" on the thread from which they 
are sent, it's that they simply are, because the posting algorithm 
looks like this:
    for each observer matching given name+object,
        [observer performSelector:observersSelector 
withObject:notification];


Chris Kane
Cocoa Frameworks, Apple


On Nov 2, 2007, at 12:28 PM, Ian Archer wrote:

> I have an app that needs to remain portable across windows and unix,
> although I'm trying to put Cocoa in whenever I can on the mac side.
>
> pthreads are prevalent, although mac-specific stuff is all done in 
> nsthread.
>
> Much of the mac code relies on NSNotificationCenter.  I'm finding that
> one of my objects is not receiving messages from the center (or, in
> some cases, receives the first and receives no more).
>
> I guessed this was due to either 1) NSNotificationCenter's thread
> separation (e.g. messages can only be received on the thread from
> which they are sent), or 2) some weird incompatibility between pthread
> and nsthread.
>
> In the object "A" which should be receiving messages but isn't, I
> added a thread startup in its init method which basically does nothing
> except spin and sleep.  Now, this object is receiving messages from
> the notificaton center!  Does anyone know what might be causing this
> strange behavior?
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