addobserver / removeobserver query

Is it OK to use -removeObserver: first and then call -addObserver: with the same name? Or is it a rule to have -addObserver: first before -removeObserver:?

I tried it using OS 4.0 and it seems OK (no crash, warnings... etc.).

-(void) setObserver
{
[[NSNotificationCenter defaultCenter] removeObserver:self 
                                                name:OBSERVER_NAME object:nil]; 

[[NSNotificationCenter def开发者_开发问答aultCenter] addObserver:self
                                         selector: @selector(selectorName)
                                             name:OBSERVER_NAME 
                                           object:nil]; 
}

The reason is to prevent two observers with the same selectorName method being called twice assuming that -setObserver method was called again if its inside -viewDidLoad and a memory warning was issued.

Also, do I need to invoke -removeObserver: during -dealloc?

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If you're getting -selectorName invoked twice, there's probably some other issue with your code. And even if Cocoa will be graceful about removing observers you haven't added yet, I wouldn't actually do that.

As user tonklon says in a comment, you should probably remove the observer in -viewDidUnload. As to your last question, it's also reasonable to remove the observer in -dealloc, though it's good practice to structure your code such that things like removing observers happen at a deterministic time rather than as a side-effect of memory management.