yii\base\Event::trigger PHP Method

    public static function trigger($class, $name, $event = null)
    {
        if (empty(self::$_events[$name])) {
            return;
        }
        if ($event === null) {
            $event = new static();
        }
        $event->handled = false;
        $event->name = $name;
        if (is_object($class)) {
            if ($event->sender === null) {
                $event->sender = $class;
            }
            $class = get_class($class);
        } else {
            $class = ltrim($class, '\\');
        }
        $classes = array_merge([$class], class_parents($class, true), class_implements($class, true));
        foreach ($classes as $class) {
            if (!empty(self::$_events[$name][$class])) {
                foreach (self::$_events[$name][$class] as $handler) {
                    $event->data = $handler[1];
                    call_user_func($handler[0], $event);
                    if ($event->handled) {
                        return;
                    }
                }
            }
        }
    }

 /**
  * Parse a Less file to CSS
  */
 public function parse($src, $dst, $options)
 {
     $this->auto = isset($options['auto']) ? $options['auto'] : $this->auto;
     $variables = $this->variables;
     $assetManager = Yii::$app->assetManager;
     // Final url of the file being compiled
     $assetUrl = substr($dst, strpos($assetManager->basePath, $assetManager->baseUrl));
     $variables['published-url'] = '"' . $assetUrl . '"';
     // Root for the published folder
     $variables['published-base-url'] = '"/' . implode('/', array_slice(explode('/', ltrim($assetUrl, '/')), 0, 2)) . '"';
     $less = new \lessc();
     $less->setVariables($variables);
     // Compressed setting
     if ($this->compressed) {
         $less->setFormatter('compressed');
     }
     \Less_Parser::$default_options['compress'] = $this->compressed;
     \Less_Parser::$default_options['relativeUrls'] = $this->relativeUrls;
     // Send out pre-compile event
     $event = new \yii\base\Event();
     $this->runtime = ['sourceFile' => $src, 'destinationFile' => $dst, 'compiler' => $less];
     $event->sender = $this;
     Event::trigger($this, self::EVENT_BEFORE_COMPILE, $event);
     try {
         if ($this->auto) {
             /* @var FileCache $cacheMgr */
             $cacheMgr = Yii::createObject('yii\\caching\\FileCache');
             $cacheMgr->init();
             $cacheId = 'less#' . $dst;
             $cache = $cacheMgr->get($cacheId);
             if ($cache === false || @filemtime($dst) < @filemtime($src)) {
                 $cache = $src;
             }
             $newCache = $less->cachedCompile($cache);
             if (!is_array($cache) || $newCache["updated"] > $cache["updated"]) {
                 $cacheMgr->set($cacheId, $newCache);
                 file_put_contents($dst, $newCache['compiled']);
             }
         } else {
             $less->compileFile($src, $dst);
         }
         // If needed, respect the users configuration
         if ($assetManager->fileMode !== null) {
             @chmod($dst, $assetManager->fileMode);
         }
         unset($this->less);
     } catch (\Exception $e) {
         throw new \Exception(__CLASS__ . ': Failed to compile less file : ' . $e->getMessage() . '.');
     }
 }