On Feb 16, 2012, at 4:16 PM, Greg Parker wrote:
> On Feb 16, 2012, at 12:57 PM, Matt Neuburg <m...@tidbits.com> wrote:
>> On Feb 16, 2012, at 12:45 PM, Greg Parker wrote:
>>> On Feb 16, 2012, at 12:42 PM, Matt Neuburg <m...@tidbits.com> 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).
Right, good point (several good points). Thanks again. m.
--
matt neuburg, phd = m...@tidbits.com, http://www.apeth.net/matt/
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