Blocks do retain objects that are passed in, so that's probably what's
happening. Also, keep in mind that autoreleased objects aren't freed
immediately; it happens when the run loop completes. You should always act as
if it does happen immediately, but it isn't always true.
Jeff Kelley
On Jun 18, 2010, at 11:00 AM, Jonny Taylor wrote:
> I've just been looking back at some code that has been working fine for me
> for a while, and as far as I can see it shouldn't actually work! I'd be
> interested for peoples' comments. The code is as follows:
>
> dispatch_async(queue1,
> ^{
> NSImage *theImage = [frame GetNSImage];
> NSData *tiffRep = [theImage TIFFRepresentation];
> dispatch_async(queue2,
> ^{
> [tiffRep writeToFile:[NSString stringWithFormat:@"%...@%d.tif",
> [[frame Camera] ExportFilePrefix],
> [frame FrameNumber]]
> atomically:YES];
> });
> [theImage release];
> });
>
> Work running on serial queue "queue1" calculates a TIFF representation for an
> image, and then schedules work on serial queue "queue2" to write that data to
> disk. What I can't work out is why tiffrep isn't autoreleased as soon as the
> outer block completes. Is the compiler/runtime being clever enough to retain
> it because it is going to be needed in the inner block (if so: very clever!)?
> If not, am I just getting lucky here with exactly when/how grand central does
> its autorelease cleanup? Or maybe the TIFF representation and/or my frame
> data is still being retained elsewhere for a while (possible, depending on
> how the thread timings work out...).
>
> I'm pretty new to Cocoa so I'm keen to understand this properly - I'd be
> interested to hear peoples thoughts on this: is what I am doing ok, or do I
> need to add some explicit retain/releasing of tiffRep?
>
> Cheers
> Jonny
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