start_previewing_theme example

        add_action( 'after_setup_theme', array( $this, 'establish_loaded_changeset' ), 5 );

        /* * Import theme starter content for fresh installations when landing in the customizer. * Import starter content at after_setup_theme:100 so that any * add_theme_support( 'starter-content' ) calls will have been made. */
        if ( get_option( 'fresh_site' ) && 'customize.php' === $pagenow ) {
            add_action( 'after_setup_theme', array( $this, 'import_theme_starter_content' ), 100 );
        }

        $this->start_previewing_theme();
    }

    /** * Establishes the loaded changeset. * * This method runs right at after_setup_theme and applies the 'customize_changeset_branching' filter to determine * whether concurrent changesets are allowed. Then if the Customizer is not initialized with a `changeset_uuid` param, * this method will determine which UUID should be used. If changeset branching is disabled, then the most saved * changeset will be loaded by default. Otherwise, if there are no existing saved changesets or if changeset branching is * enabled, then a new UUID will be generated. * * @since 4.9.0 * * @global string $pagenow The filename of the current screen. */