code: private OnValueChangeListener createNumberPickerListener() {
          return new OnValueChangeListener() {
  
              @Override
              public void onValueChange(final android.widget.NumberPicker numberPicker,
                                        final int oldValue, final int newValue) {
                  numberPicker.setValue(newValue);
                  currentIndex = getMinNumber() + newValue * getStepSize();
              }
  
          };
      }
docs: Creates and returns a listener, which allows to observe the {@link NumberPicker}, which is
  used by the preference.
  
  @return The listener, which has been created, as an {@link OnValueChangeListener}