How do I call an object in a prepressor only once In drupal8 is there a way to call a function only once in the theme layer. Can I call this function only once and not on other preprocessor hooks or 
am I using the wrong hook?

function template_preprocess(&amp;$vars){
    //-- can I call this function only once and not on other preprocessor hooks
    _updateFieldOnNode('field_metatag_title');
}
If you want to call the preprocess hook on nodes only you can use template_preprocess_node().