On Feb 16, 2012, at 12:57 PM, Matt Neuburg <[email protected]> wrote:
> On Feb 16, 2012, at 12:45 PM, Greg Parker wrote:
>> On Feb 16, 2012, at 12:42 PM, Matt Neuburg <[email protected]> wrote:
>>> On Feb 16, 2012, at 12:05 PM, Greg Parker wrote:
>>>>
>>>> The question is, who retains the observer? The __block __weak variable
>>>> does not, because it's weak. NSNotificationCenter does not, as I
>>>> understand it.
>>>
>>> It does, actually; thanks for pressing me on this point.
>>
>> The NSNotificationCenter documentation says:
>> "Important: The notification center does not retain its observers…"
>
> We may be talking at cross purposes.
>
> If you register by saying addObserver:selector:name:object:, the notification
> center does not retain the observer object named in the first param.
>
> But if you register by saying addObserverForName:object:queue:usingBlock:, it
> returns an observer object, and the notification center *does* retain that
> observer.
Aha. That's the part I didn't see, and it contradicts the "Important" note at
the top of the NSNotificationCenter documentation. I'll file a documentation
bug report.
With that clarified, the example you posted should be fine.
There's an alternative if you are using ARC and you know the notification will
fire exactly once:
// ARC required.
__block id observer = [[NSNotificationCenter defaultCenter]
addObserverForName:@"MyMandelbrotOperationFinished"
object:op queue:[NSOperationQueue mainQueue]
usingBlock:^(NSNotification *note) {
MyMandelbrotOperation* op2 = note.object;
// do stuff
[[NSNotificationCenter defaultCenter] removeObserver:observer
name:@"MyMandelbrotOperationFinished" object:op2];
observer = nil;
}];
This version does not use __weak. Instead, the block object creates a retain
cycle by retaining the observer via the __block variable. When the block object
executes, it breaks the cycle by setting the __block variable to nil, releasing
the observer.
If you have a specific time that you can break the cycle, you can use this
pattern. The pattern would fail if you didn't know when you could break the
cycle (for example, if the block might run zero times).
--
Greg Parker [email protected] Runtime Wrangler
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