Question

This isn t so much a question as a pondering thought - why does NSNotificationCenter throw an exception when it s released? I m still new to iPhone development, and thus don t know the innards of Cocoa yet, so it d be good to understand why.

I m assigning the defaultCenter to a variable, calling addObserver:selector:name:object and then releasing the previous variable, but the call to [notify release] crashes the app. I m not doing anything strange in the code, so it d be interesting to find out exactly why it s doing this.

Anyone ran into this problem?

Answer 1

From what I know of NSNotifcation you shouldn t be assigning the defaultcenter to a variable but rather doing something like:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(showLogin) name:@"IncorrectLogin" object:nil];

In that snippet your calling the default centre and registering the current object for a certain message.

And then to post a message to the notification centre you can use:

[[NSNotificationCenter defaultCenter] postNotificationName:@"IncorrectLogin" object:nil];

I use the above in all my code and don t have any problems with it.

Answer 2

There should only be one default notification center for your app, so none of your classes should be retaining or releasing it. You wouldn t want your notification center to disappear on you, right?

Answer 3

I don t think you own the object, and therefore should not release it.

Remember the NARC: New Alloc, Retain, Copy. If you do one of these, you have to release it.

Answer 4

It s not yours to release.

Remember the NARC rule--you ONLY release things that you brought into existence using:

New Allocate Retain, or Copy.

NARC. See?

What you re doing with NSNotificationCenter is you re getting a copy of the singleton that represents the default notification center. It d be worth reading up on singletons.

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