[WordPress] Conditional Meta Box / Custom fields depending on page template in WP Admin

admin.js

jQuery( document ).ready( function ( $ ) {
    var $pageTemplate = $( "#page_template" ),
    	$metaBoxID = $( "#meta_box_id" );

    $pageTemplate.on( "change", function () {
        if ( $( this ).val() == "templates/page-custom.php" ) {
                $metaBoxID.show();
            } else {
                $metaBoxID.hide();
            }
    }).change();
});

wp-meta-box.php

<?php
/**
 * Run code in WP Admin depending on page template
 */
if ( isset( $_GET['post'] ) ) {
	$post_id = intval( $_GET['post'] );
} elseif (isset( $_POST['post_ID'] ) ) {
	$post_id = intval( $_POST['post_ID'] );
} else {
	$post_id = 0;
}

if ( get_post_meta( $post_id, '_wp_page_template', true ) == 'templates/page-custom.php' ) {
	
	// This code will run only on pages with page-custom.php template
	
}