How to avoid adding multiple NSNotification observer?

Question

Right now the API doesn't seem to provide a way to detect if an observer has already been added for a particular NSNotification. What's the best way to avoid adding multiple NSNotification observers other than maintaining a flag on your end to keep track? Has anyone already created a category to facilitate this?

Answer 1

One way to prevent duplicate observers from being added is to explicitly call removeObserver for the target / selector before adding it again. I imagine you can add this as a category method:

@interface NSNotificationCenter (UniqueNotif)  - (void)addUniqueObserver:(id)observer selector:(SEL)selector name:(NSString *)name object:(id)object {          [[NSNotificationCenter defaultCenter] removeObserver:observer name:name object:object];         [[NSNotificationCenter defaultCenter] addObserver:observer selector:selector name:name object:object];  }  @end 

This assumes that the you will only add one unique observer to each target for any notification name, as it will remove any existing observers for that notification name.

Answer 2

Swift 5:

import Foundation  extension NotificationCenter {   func setObserver(_ observer: Any, selector: Selector, name: Notification.Name, object: Any?) {     removeObserver(observer, name: name, object: object)     addObserver(observer, selector: selector, name: name, object: object)   } } 

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Swift 3-4:

import Foundation  extension NotificationCenter {   func setObserver(_ observer: AnyObject, selector: Selector, name: NSNotification.Name, object: AnyObject?) {     removeObserver(observer, name: name, object: object)     addObserver(observer, selector: selector, name: name, object: object)   } } 

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Swift 2:

import Foundation  extension NSNotificationCenter {   func setObserver(observer: AnyObject, selector: Selector, name: String?, object: AnyObject?) {     removeObserver(observer, name: name, object: object)     addObserver(observer, selector: selector, name: name, object: object)   } }