<?php
function listen_to_mepr_events($event) {
  $obj = $event->get_data();
  //$obj might be a MeprTransaction object or a MeprSubscription object

  if(!($obj instanceof MeprTransaction) && !($obj instanceof MeprSubscription)) {
    return; // nothing here to do if we're not dealing with a txn or sub
  }

  $member = $obj->user();

  if($member->is_active_on_membership($obj)) {
    //member is active on membership
  }
  else {
    //member is no longer active on this membership
  }
}
add_action('mepr-event-create', 'listen_to_mepr_events');