WP_Customize_Manager::customize_preview_init PHP Method

    public function customize_preview_init()
    {
        /*
         * Now that Customizer previews are loaded into iframes via GET requests
         * and natural URLs with transaction UUIDs added, we need to ensure that
         * the responses are never cached by proxies. In practice, this will not
         * be needed if the user is logged-in anyway. But if anonymous access is
         * allowed then the auth cookies would not be sent and WordPress would
         * not send no-cache headers by default.
         */
        if (!headers_sent()) {
            nocache_headers();
            header('X-Robots: noindex, nofollow, noarchive');
        }
        add_action('wp_head', 'wp_no_robots');
        add_filter('wp_headers', array($this, 'filter_iframe_security_headers'));
        /*
         * If preview is being served inside the customizer preview iframe, and
         * if the user doesn't have customize capability, then it is assumed
         * that the user's session has expired and they need to re-authenticate.
         */
        if ($this->messenger_channel && !current_user_can('customize')) {
            $this->wp_die(-1, __('Unauthorized. You may remove the customize_messenger_channel param to preview as frontend.'));
            return;
        }
        $this->prepare_controls();
        add_filter('wp_redirect', array($this, 'add_state_query_params'));
        wp_enqueue_script('customize-preview');
        add_action('wp_head', array($this, 'customize_preview_loading_style'));
        add_action('wp_head', array($this, 'remove_frameless_preview_messenger_channel'));
        add_action('wp_footer', array($this, 'customize_preview_settings'), 20);
        add_filter('get_edit_post_link', '__return_empty_string');
        /**
         * Fires once the Customizer preview has initialized and JavaScript
         * settings have been printed.
         *
         * @since 3.4.0
         *
         * @param WP_Customize_Manager $this WP_Customize_Manager instance.
         */
        do_action('customize_preview_init', $this);
    }

 /**
  * Do Customizer boot actions.
  */
 function do_customize_boot_actions()
 {
     $_SERVER['REQUEST_METHOD'] = 'POST';
     do_action('setup_theme');
     do_action('after_setup_theme');
     do_action('init');
     do_action('customize_register', $this->wp_customize);
     $this->wp_customize->customize_preview_init();
     do_action('wp', $GLOBALS['wp']);
 }