On Wed, 15 Feb 2012 16:20:50 -0800, Greg Parker <gpar...@apple.com> said:
>On Feb 15, 2012, at 12:58 PM, Matt Neuburg <m...@tidbits.com> wrote:
>Are you using GC or ARC? I think this is safe with GC but not safe with ARC,
>but I'm not an expert in the NSNotificationCenter machinery.
ARC, but I don't see how it might be unsafe. I sure *hope* it isn't, because
that's what the __weak is for. You need that because if your observer doesn't
vanish in good order, neither will +self+, because the observer retains +self+,
not just while registered with the notification center but for as long as it
lives (by way of the block, I presume). *That* is something I *do* understand,
having spent many hours watching the retain counts at work.
>Without __block, the block object snapshots the value of the observer variable
>when the block object is created. But in your code the block object is created
>before the call to -addObserverForName:..., and at that point the observer
>variable is not yet initialized. Boom.
>
>With __block, the block object always uses the current value of the observer
>variable. The variable is initialized before the block object actually
>executes, so everything works.
Thank you! (So, the fact that we are async is crucial to this working. I was
missing *that* piece of the mental puzzle too, though of course I would not
have used sync in any case.)
I feel this wasn't quite made clear by the WWDC videos and the docs. Perhaps
it's in the latter and I missed it. This is not a criticism (I would never
criticize the GCD/runtime people about how they explain GCD; au contraire, I
think you guys are the best explainers associated with Apple), just a
suggestion. A simple sentence would suffice: You can use the result of a call
to a method that takes a block inside the block, but only if it is typed as
__block (and only if the block is executed later).
The same issue came up again later the same day:
__block UIBackgroundTaskIdentifier bti = [[UIApplication sharedApplication]
beginBackgroundTaskWithExpirationHandler: ^{
[[UIApplication sharedApplication] endBackgroundTask:bti];
}];
Without __block, bti is invalid, and you won't find that out easily because
it's unlikely that you'll actually expire. :) m.
--
matt neuburg, phd = m...@tidbits.com, <http://www.apeth.net/matt/>
A fool + a tool + an autorelease pool = cool!
Programming iOS 5! http://shop.oreilly.com/product/0636920023562.do
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