Do Cocoa bindings retain their own values?

FROM : Frank Reiff
DATE : Mon Nov 19 22:42:24 2007

Hi everybody,

During my latest hunt for irreproducible crashes, I've run into lots 
of thread-safety issues with a dialog that displays the progress of a 
worker thread.

Cocoa bindings are not thread-safe, so you have to be extra careful 
about memory management, retain/release cycles and autorelease pools.

I think I've pretty much fixed the problem, but now I'm in full 
paranoia mode and every single line of code looks like a potential 
crashing bug..

From the worker thread I call the method updateStageText directly:


- (void) updateStageText: (NSString*) text;{

   [self performSelectorOnMainThread: 
@selector(internalUpdateStageText:) withObject: text waitUntilDone: 
YES];
   
}

This calls a private method but on the main thread:

- (void) internalUpdateStageText: (NSString*) text;{
   
   id old = paramStageText;
   [self willChangeValueForKey: @"stageText"];
   paramStageText = [text copy];
   [self didChangeValueForKey: @"stageText"];
   [old release];
}

All this is to make sure that KVC notifications go into the main 
thread and that the allocation itself is safe.

As you notice, however, I call performSelectorOnMainThread with 
waitUntilDone: YES, which is extra safe but also extra slow.

Now for the question: can I call it with waitUntilDone: NO !?

The code as it stands wouldn't be thread safe because the NSString 
parameter might already have been de-allocated by the worker thread by 
the time [text copy] is executed on the main thread.

This could be solved by retaining the text parameter like this:

- (void) updateStageText: (NSString*) text;{

   [text retain];
   [self performSelectorOnMainThread: 
@selector(internalUpdateStageText:) withObject: text waitUntilDone: 
YES];
   [text release];
}

Is this safe? I don't know whether the retain/release mechanism itself 
is thread-safe, but I presume it is.

More worrying is the original

[old release];

line. This is perfectly fine if the Cocoa bindings NSTextField retains 
its value, but does it?

I hope I’m only paranoid, but I would appreciate the insights from the 
Cocoa threading gurus :-)

Best regards,

Frank_______________________________________________

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