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I'm using manual reference counting and Xcode 4.5.

I declared an ivar:

UIAccelerometer *accelerometer;

accelerometer was NOT a property. And I had this code in one method:

accelerometer = [UIAccelerometer sharedAccelerometer];
accelerometer.delegate = self;

In a second method I had:

accelerometer.delegate = nil;

and in a another method that runs later I had:

accelerometer.delegate = self;

With this setup I occasionally (rarely and seemingly randomly) got the following abort:

'NSInvalidArgumentException', reason: '-[NSPathStore2 setDelegate:]: unrecognized selector sent to instance xxx

After lots of frustration looking for the reason for this abort I tried putting a retain on accelerometer when it was first used and after that there have been no aborts. But I'm worried that the retain didn't fix the real problem. The reason for thinking of this "solution" is that the abort message was complaining about a call to setDelegate and in the section of code that seemed to be relevant the only delegate I was setting was on accelerometer. But I don't understand why this should fix it and I'm worried that the retain is just shifting code around in a way that happens to hide the problem.

The way I understand it is that 

accelerometer = [UIAccelerometer sharedAccelerometer];

creates a singleton instance that will have the same address for the duration of the instance it is in, and that address is being assigned to accelerometer, which also exists for the duration of the instance. If that is the case why does retain change anything?

Can the sharedAccelerometer singleton get deallocated before the instance it is in gets deallocated?

nielsbot
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RobertL
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  • Sorry but backtics don't seem to be working right today. I've edited this 3 times to change the backtics but can't get it to look right. All my backtics are on separate lines but sometimes they appear here without the newline. – RobertL Oct 12 '12 at 16:07
  • Have you considered switching to ARC? Manual reference counting is tedious and error prone. – nielsbot Oct 12 '12 at 16:11
  • Yes I have but this isn't the time. This game already has 10,000 lines of code, much of it written more than once, plus 170 png files, and I'm under pressure to get it out ASAP. It seems solid now so I'm reluctant to make unnecessary changes. If it gets many users I'll convert to ARC for updates. – RobertL Oct 12 '12 at 17:28
  • well for such a large project, converting to ARC could be painful... I just say this because I feel like I'm seeing so many questions on SO where people start new projects w/o ARC. – nielsbot Oct 12 '12 at 18:07
  • fixed your formatting BTW. Backtick is for quoting code inline.. for code blocks, begin each line with 4 spaces. – nielsbot Oct 12 '12 at 18:09

1 Answers1

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If accelerometer is an ivar, it is correct to retain whatever object you put there... So this code

accelerometer = [UIAccelerometer sharedAccelerometer];
accelerometer.delegate = self;

Should be

accelerometer = [ [ UIAccelerometer sharedAccelerometer ] retain ] ;
accelerometer.delegate = self;

And in -dealloc, add

[ accelerometer release ] ;
accelerometer = nil ;
nielsbot
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