Question

At some point in a code one may add something like

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

How do I test if this notification is already active on the queue or has been removed, to prevent adding a duplicate?

thanks.

Answer 1

If you mean “testing for whether you’ve already registered as an observer for the notification”, I don’t think there’s an easy way apart from posting the notification and seeing you get a callback (with possibly disastrous effects).

If there’s a danger of a double-add, I usually use [[NSNotificationCenter defaultCenter] removeObserver:self name:foo object:bar] before the add.

Registering for notifications does not happen on a queue.