NSNotificationCenter: Removing an Observer in Swift - swift

Question

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?

Answer 1

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

Answer 2

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)
}