protocol HashTagPickerDelegate {

   func hastagPicked(hashtag:String)
}

class HashtagPicker : UIViewController
{

  var delegate: HashTagPickerDelegate?

}

delegate.hashtagPicked(pickedHashtag)

class ParentView:UIViewController, HashTagPickerDelegate
{

}

func prepare(for segue: UIStoryboardSegue, sender: AnyObject?)
{
  if segue.identifier == "HastagPickerSegue" // use your segue id here
  {
    let hashtagPicker = segue.destinationViewController as! HashtagPickerView
    hashtagPicker.delegate = self
   }
}

NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(ParentViewController.someActionToBePerformed), name: "myNotification", object: nil)

func someActionToBePerformed () {
    // this will be called when hashTag is changed
    // do something when hashTag is changed
}

NSNotificationCenter.defaultCenter().postNotificationName("myNotification", object: nil)

// note the ": class" part
protocol HashTagPickerDelegate: class {
  func picked(hashtag: String)
}

class HashtagPicker: UIViewController {
  // if HashTagPickerDelegate wouldn't be limited to class,
  // we couldn't have made a weak reference here!
  weak var delegate: HashTagPickerDelegate?

  // at some point, you call the delegate, it can be anywhere, this is just an example
  @IBAction func tappedHashtag(_ sender: Any) {
    delegate?.picked(hashtag: "bla")
  }
}

class ParentView: UIViewController, HashTagPickerDelegate {
  func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    // we are presenting the nested controller
    if segue.identifier == "SegueHastagPickerContainer",
       let destinationController = segue.destination as? HashtagPicker {
      destinationController.delegate = self
    }
  }
}