iOS 8 Dismiss keyboard for swift

i'm a beginner in swift and i have been using this dismissKeyboard() method but it is not working for the keyboard extension.

@IBAction func donePressed (sender: UIButton) {

      dismissKeyboard()


    }

can anyone tell me why this doesn't work?

thanks.

EDIT: full code

import UIKit

class KeyboardViewController: UIInputViewController {

var keyboardView: UIView!

@IBOutlet var nextKeyboardButton: UIButton!

override func updateViewConstraints() {
    super.updateViewConstraints()

    // Add custom view sizing constraints here
}

override func viewDidLoad() {
    super.viewDidLoad()

  self.loadInterface()





}


func loadInterface() {

    var keyboardNib = UINib(nibName: "KeyboardView", bundle: nil)

    self.keyboardView = keyboardNib.instantiateWithOwner(self, options: nil)[0] as UIView

    view.addSubview(self.keyboardView)

    view.backgroundColor = self.keyboardView.backgroundColor

    self.nextKeyboardButton.addTarget(self, action: "advanceToNextInputMode", forControlEvents: .TouchUpInside)

    self.nextKeyboardButton.addTarget(self, action: "advanceToNextInputMode", forControlEvents: .TouchUpInside)
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated
}

override func textWillChange(textInput: UITextInput) {
    // The app is about to change the document's contents. Perform any preparation here.
}

override func textDidChange(textInput: UITextInput) {
    // The app has just changed the document's contents, the document context has been updated.

}

@IBAction func buttonPressed (sender: UIButton) {

    let title = sender.titleForState(.Normal)

    var proxy = textDocumentProxy as UITextDocumentProxy

    proxy.insertText(title!)

}

@IBAction func spacePressed (sender: UIButton) {

    var proxy = textDocumentProxy as UITextDocumentProxy

    proxy.insertText(" ")

}

@IBAction func deletePressed (sender: UIButton) {

    var proxy = textDocumentProxy as UITextDocumentProxy

    proxy.deleteBackward()

}

@IBAction func donePressed (sender: UIButton) {

resignFirstResponder()


}

}