Batch update a post meta field value of each post in Post Type

Take a look at the AJAX documentation, there are plenty of examples on this site and across the Googles to trigger a server event without refreshing the page.

In this case, WP is listening to an AJAX call that looks like http://example.com/wp-admin/admin-ajax.php?action=wpse20160318_batch_update. When triggered, it’ll run through your batch process an output a JSON message that can be consumed by JavaScript after the call is completed.

The major modifications are just adding an AJAX listener to trigger the batch_update_meta, removing any echos, and returning JSON when complete.

function batch_update_meta() {

    $post_type        = "listings";
    $post_type_object = get_post_type_object( $post_type );
    $label            = $post_type_object->label;
    $processed = array ();

    $posts = get_posts( array (
        'post_type'        => $post_type,
        'post_status'      => 'publish',
        'suppress_filters' => false,
        'posts_per_page'   => - 1,
    ) );

    foreach ( $posts as $post ) {

        $key1 = 'my_meta_key_1';
        $key2 = 'my_meta_key_2';

        $meta_value1 = get_post_meta( $post->ID, $key1, true );

        if ( ! empty( $meta_value1 ) ) {

            // Returns a string after it finishes process.
            $meta_value2 = media_process( $meta_value1, $post->ID );

            // Update the meta
            update_post_meta( $post->ID, $key2, $meta_value2, $meta_value1 );

            // Add to our success list
            $processed[] = $post->post_title;
        }
        else {
            //...
        }
    }

    // give a response for AJAX
    wp_send_json_success( array (
        'label'     => $label,
        'processed' => $processed,
        'message'   => 'Batch Completed.',
    ) );
}

// callback for AJAX function
function wpse20160318_batch_update_meta() {

    // run your batch update
    batch_update_meta();
}

// must be logged in to trigger - http://example.com/wp-admin/admin-ajax.php?action=wpse20160318_batch_update
add_action( 'wp_ajax_wpse20160318_batch_update', 'wpse20160318_batch_update_meta' );