public function addValueChangedListener(ValueChangedListenerInterface $listener)
    {
        if (!$this->listeners->hasItem($listener)) {
            $this->listeners->push($listener);
        }
    }
public function addValueChangedListener(ValueChangedListenerInterface $listener)
    {
        if (!$this->listeners->hasItem($listener)) {
            $this->listeners->push($listener);
        }
    }
public function addValueChangedListener ( ValueChangedListenerInterface $ listener ) { if ( ! $ this -> listeners -> hasItem ( $ listener ) ) { $ this -> listeners -> push ( $ listener ) ; } }
Registers a given listener as a recipient of value changed events.

@param ValueChangedListenerInterface $listener
Registers a given listener as a recipient of value changed events .