I've RTFM'd (specifically performed a full-text search in the core
library for @synchronized) and STFA'd, and I'm still confused about
using @synchronized in a thread-safe accessor. I can't use @synthesize
because the set method must do more than just set a variable.
With NS(Recursive)Lock, this isn't difficult:
- (id)myVariable
{
id returnValue;
[ivarLock lock];
returnValue = [[ivarMyVariable retain] autorelease];
[ivarLock unlock];
return returnValue;
}
- (void)setMyVariable:(id)newMyVariable
{
@try
{
[ivarLock lock];
if (newMyVariable != ivarMyVariable)
{
[ivarMyVariable release];
ivarMyVariable = [newMyVariable retain];
// do whatever else we need to do here
}
}
@finally
{
[ivarLock unlock];
}
}
OK, now what is the best way to do this using @synchronized instead of
an NSLock? The getter method is obvious, but the setter isn't. My
first thought was something like this:
- (void)setMyVariable:(id)newMyVariable
{
@synchronized(ivarMyVariable)
{
if (newMyVariable != ivarMyVariable)
{
[ivarMyVariable autorelease];
ivarMyVariable = [newMyVariable retain];
// then go do whatever needs to be done
}
}
}
But would this create problems if:
(1) ivarMyVariable is nil (the documentation doesn't say what happens
when @synchronized(nil) or @synchronized(<some nil variable>) is
called, so I'll assume this behavior is undefined either way), or
(2) some other thread is waiting for the set method to finish in its
own @synchronized() block, and after the setter is finished, the old
value of ivarMyVariable gets deallocated by the autorelease pool/
garbage collector before the thread-safe block is done?
If the answer to either is yes, then should I try using some alternate
mutex for an object, and if so, then what? Advice welcomed...
Nick Zitzmann
<http://www.chronosnet.com/>
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