WP_Customize_Manager example

    $is_customize_save_action = (
        wp_doing_ajax()
        &&
        isset( $_REQUEST['action'] )
        &&
        'customize_save' === wp_unslash( $_REQUEST['action'] )
    );
    $settings_previewed       = ! $is_customize_save_action;

    require_once ABSPATH . WPINC . '/class-wp-customize-manager.php';
    $GLOBALS['wp_customize'] = new WP_Customize_Manager(
        compact(
            'changeset_uuid',
            'theme',
            'messenger_channel',
            'settings_previewed',
            'autosaved',
            'branching'
        )
    );
}