CODEWITHSUNDEEP
swiftnsnotificationcenter
Jimmery
Jimmery

Reputation: 10139

NSNotificationCenter: Removing an Observer in Swift

I have a view controller with a button. When the button is pressed it adds an observer, like so:

func buttonPress(sender:UIButton){
    NSNotificationCenter.defaultCenter().addObserverForName("buttonPressEvent", object:nil, queue:nil, usingBlock:{(notif) -> Void in
        // code
    })
}

When I dismiss this view controller, and then return to it and press the button the //code is executed twice. If I go away and come back again the //code is executed three times, and so on.

What I want to do is to remove the Observer before I add it again, so this code doesn't execute twice. Ive gone through the documentation here and Ive added this line of code just above where I add the Observer:

    NSNotificationCenter.defaultCenter().removeObserver(self, name:"buttonPressEvent", object:nil)

But this isnt working.

Can anyone tell me where I'm going wrong?

Upvotes: 9

Views: 8725

Answers (2)

Shahid
Shahid

Reputation: 61

Implemented it like this, seems to be working fine. 

override func viewDidLoad() 
{
        super.viewDidLoad()    
       AddScreenShotNotification() 
}

 func AddScreenShotNotification() {

NSNotificationCenter.defaultCenter().addObserver(
           self,
            selector: #selector(MyViewController.ScreenShotTaken),
            name: UIApplicationUserDidTakeScreenshotNotification,
            object: nil)
}

func ScreenShotTaken()
    {        
 // do something     
    }

 override func viewWillDisappear(animated: Bool) {
        NSNotificationCenter.defaultCenter().removeObserver(self)    
    }

Upvotes: -1

Mike Pollard
Mike Pollard

Reputation: 10195

When you use the 'blocks' based approach to observing notifications then self isn't in fact the observer. The function returns an object which acts as the observer:

func addObserverForName(_ name: String?,
                 object obj: AnyObject?,
                  queue queue: NSOperationQueue?,
             usingBlock block: (NSNotification!) -> Void) -> NSObjectProtocol

You need to keep a reference to this returned object and pass it in as the observer when you call removeObserver

It's explained well in the Apple Doc here

Upvotes: 30

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