How to get triggering_element I want to get triggering_element element on form_alter after form rebuild, Also I know that this could be used on submit handler on Ajax controller to findout which element cause form submit or Ajax triggered.

So how can I get triggering_element after Ajax call or form submissions?
Eyal is correct. FormState has a method getTriggeringElement. See the original change record for details.

$form_state-&gt;getTriggeringElement();


and if you want to get id try 

$triggerdElement = $form_state-&gt;getTriggeringElement();
$htmlIdofTriggeredElement = $triggerdElement['#id'];