On Mar 16, 2009, at 2:32 PM, Michael Tsai wrote:
On Mar 16, 2009, at 2:08 PM, Bill Cheeseman wrote:
I am looking for a strategy to avoid implementing a -finalize
method in a garbage-collected Objective-C class that declares a
CFTypeRef-style Core Foundation instance variable. I believe this
is a fairly common problem, because any Cocoa wrapper class for a
Core Foundation API would have to do it, but I haven't seen a
usable solution. Apple's GC documentation stops just short of
suggesting an answer.
I believe you're supposed to declare your CFTypeRef ivar as __strong
and call CFMakeCollectable so that the garbage collector will be
responsible for releasing it. You can still use CFRelease in -
dealloc for when you aren't running under garbage collection.
After thinking on this overnight, I have concluded that in a mixed-
mode framework the best practice is indeed to call CFMakeCollectable
as soon as the CFTypeRef ivar is created so that garbage-collected
clients can ignore memory management issues.
As a corollary, when the problem domain served by the framework is
such that there is no convenient way to know when to call an -
invalidate method, simply put all the cleanup code in -finalize and be
done with it.
But perhaps even in this situation, it would be good practice to
provide -invalidate and -isValid methods, and write -finalize so that
it checks -isValid before calling -invalidate itself. This way,
clients who do know when to call -invalidate can do so.
This leaves unresolved the need in some cases to do cleanup work
before -finalize because of order-of-execution issues with -finalize.
My tentative conclusion is that this is a huge can of worms for
garbage collection when CFTypeRef ivars are involved.
--
Bill Cheeseman
b...@cheeseman.name
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