class DeferJs

class DeferJs {
    
    /**
     * The defer type to use from advagg_mod configuration.
     *
     * @var int
     */
    protected $deferType;
    
    /**
     * The global count to use from advagg configuration.
     *
     * @var int
     */
    protected $counter;
    
    /**
     * The Drupal module handler.
     *
     * @var \Drupal\Core\Extension\ModuleHandlerInterface
     */
    protected $moduleHandler;
    
    /**
     * A list of file cids to skip.
     *
     * @var array
     */
    protected $skipList;
    
    /**
     * DeferCss constructor.
     *
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   The config factory.
     * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
     *   The Drupal module handler.
     */
    public function __construct(ConfigFactoryInterface $config_factory, ModuleHandlerInterface $module_handler) {
        $this->deferType = $config_factory->get('advagg_mod.settings')
            ->get('css_defer_js_code');
        $this->counter = $config_factory->get('advagg.settings')
            ->get('global_counter');
        $this->moduleHandler = $module_handler;
        $this->skipList = [];
        // Admin Toolbar 8x fails when deferred.
        if ($this->moduleHandler
            ->moduleExists('admin_toolbar')) {
            $this->skipList[] = Crypt::hashBase64(drupal_get_path('module', 'admin_toolbar') . '/js/admin_toolbar.js' . $this->counter);
        }
    }
    
    /**
     * Add defer attribute to script tags.
     *
     * @param string $content
     *   The response content.
     *
     * @return string
     *   Updated content.
     */
    public function defer($content) {
        // Admin Toolbar 8x fails when deferred.
        foreach ($this->skipList as $cid) {
            if (strstr($content, $cid)) {
                return $content;
            }
        }
        // Only defer local scripts.
        if ($this->deferType === 2) {
            $pattern = '/<script src="\\/[a-zA-Z0-0].*"/';
        }
        else {
            $pattern = '/<script src=".*"/';
        }
        return preg_replace_callback($pattern, [
            $this,
            'callback',
        ], $content);
    }
    
    /**
     * Callback to replace individual stylesheet links.
     *
     * @param array $matches
     *   Array from matches from preg_replace_callback.
     *
     * @return string
     *   Updated html string.
     */
    protected function callback(array $matches) {
        return "{$matches[0]} defer";
    }

}