Settings Definition (How to Use On_Change?)
on_change is executed when setting value is changed. You can either set it 

 'on_change' =&gt; [
   ['CLASS_NAME', 'functionName'],
 ],


Else 

'on_change' =&gt; [
  'CLASS_NAME::functionName',
],


A good example would be https://github.com/civicrm/civicrm-core/blob/master/settings/Localization.setting.php#L145

HTH

Pradeep