You are not allowed to change the content if it is not new message.
@param ModelEvent $event
485806
public function onContentChanged($event)
    {
        $sender = $event->sender;
        // If it is permitted to change content, then it will return directly.
        if ($sender->permitChangeContent) {
            return;
        }
        // The messgage will be reversed if it changed (current message isn't
        // same as the old).
        $cAttribute = $sender->contentAttribute;
        $oldContent = $sender->getOldAttribute($cAttribute);
        if ($oldContent != $sender->$cAttribute) {
            $sender->$cAttribute = $oldContent;
        }
    }
351528
public function onPreContentView(PreContentViewEvent $event): void
    {
        $currentRequest = $this->requestStack->getCurrentRequest();
        if (!$currentRequest instanceof Request) {
            return;
        }

        $viewType = $currentRequest->attributes->get('viewType');

        $event->getContentView()->addParameters(
            [
                'view_type' => !empty($viewType) ? $viewType : '',
            ]
        );
    }
731526
public function removeEvent($event, $table = null): void
    {
        unset($this->events[$table ?? static::TABLE_ANY][$event]);
    }
780049
public static function onPreUpdate(Event $event)
    {
        $page = $event->getTarget();
        $eventArgs = $event->getEventArgs();
        $updateParents = false;
        $new = $old = null;

        $pageCountModifier = -1;
        if ($eventArgs instanceof PreUpdateEventArgs) {
            if ($page instanceof Page && $eventArgs->hasChangedField('_section')) {
                $old = $eventArgs->getOldValue('_section');
                $new = $eventArgs->getNewValue('_section');

                if ($new->getUid() === $page->getUid()) {
                    return;
                }
                $updateParents = true;
            }

            if ($page instanceof Page && $eventArgs->hasChangedField('_state')) {
                if ($page->getParent() !== null) {
                    if ($eventArgs->getNewValue('_state') >= 4) {
                        $old = $page->getParent()->getSection();
                        if (!$page->isLeaf()) {
                            $pageCountModifier = 0;
                        }
                    } else {
                        $new = $page->getParent()->getSection();
                    }
                    $updateParents = true;
                }
            }

            if ($page instanceof Section && $eventArgs->hasChangedField('_parent')) {
                $old = $eventArgs->getOldValue('_parent');
                $new = $eventArgs->getNewValue('_parent');
                $updateParents = true;
            }

            if ($updateParents) {
                $em = $event->getApplication()->getEntityManager();

                self::setSectionHasChildren($em, $old, $pageCountModifier);
                self::setSectionHasChildren($em, $new, +1);
            }
        }
    }
641891
public function beforeValidationOnUpdate(Event $event, Model $model)
    {
        if($model instanceof ITimestampable) {
            $now = $this->getNow();
            $model->setUpdatedAt($now);
        }
    }
758770
public function addMessage(AddMessageEvent $event)
    {
        /** @var Message $messageAdapter */
        $messageAdapter = $this->framework->getAdapter(Message::class);

        $messageAdapter->add($event->getContent(), 'TL_' . strtoupper($event->getType()));
    }
1004697
public function triggerModels($name, ModelEvent $event = null)
    {
        if ($event === null) {
            $event = new ModelEvent();
        }
        foreach ($this->models as $model) {
            $model->trigger($name, $event);
        }

        return $event->isValid;
    }
793454
public static function clear(string $event = '') {
		if (empty($event)) {
			self::$events = [];
		} else {
			self::$events[$event] = [];
		}
	}
832258
public static function onFlushContent(Event $event)
    {
        $content = $event->getTarget();
        if (!($content instanceof AbstractClassContent)) {
            return;
        }

        $page = $content->getMainNode();
        if (null === $page) {
            return;
        }

        $newEvent = new Event($page, $content);
        $newEvent->setDispatcher($event->getDispatcher());
        self::onFlushPage($newEvent);
    }
31588
public function onAllowedThemeList(AllowListEvent $event)
    {
        if ($group = $this->findMostRelevantGroup()) {
            if ($themes = $group->getAttribute('allowed_themes')) {
                $event->removeNotIn($themes);
            }
        }
    }
85211
public function setEvent($event)
  {
    if ($event === '?' || $event === '') {
      $this->event = null;
    } else {
      $event = Util::foreignLettersToEnglishLetters($event);
      $event = Util::titleCaseIfCurrentlyAllCaps($event);
      $this->event = $event === '?' ? null : $event;
    }
  }
826789
public function onPreContentView(PreContentViewEvent $event): void
    {
        $currentRequest = $this->requestStack->getCurrentRequest();
        if (!$currentRequest instanceof Request) {
            return;
        }

        $view = $currentRequest->attributes->get('view');
        if (!$view instanceof ContentValueView) {
            return;
        }

        if ($currentRequest->attributes->get('viewType') !== 'full') {
            return;
        }

        $content = $view->getSiteContent();
        if (!$content->hasField('css_class') || $content->getField('css_class')->isEmpty()) {
            return;
        }

        $fieldValue = $content->getField('css_class')->value;
        if (!$fieldValue instanceof TextLineValue) {
            return;
        }

        $event->getContentView()->addParameters(
            [
                'page_css_class' => trim($fieldValue->text),
            ]
        );
    }
883339
public function after($event = []) {
		$modelDynSchema = ClassRegistry::init('CakeLdap.DynSchema');
		$modelDynSchema->updateSchema($event, $this->connection);
	}
929516
function(event, model, collection, options) {
      if ((event === 'add' || event === 'remove') && collection !== this) return;
      if (event === 'destroy') this.remove(model, options);
      if (event === 'change') {
        var prevId = this.modelId(model.previousAttributes());
        var id = this.modelId(model.attributes);
        if (prevId !== id) {
          if (prevId != null) delete this._byId[prevId];
          if (id != null) this._byId[id] = model;
        }
      }
      this.trigger.apply(this, arguments);
    }
262943
public function handleUpdateAttribute(PostPersistModelEvent $event)
    {
        $model = $event->getModel();

        if (('file' !== $model->getProperty('type'))
            || (!$model->getProperty('file_multiple'))
            || ('tl_metamodel_attribute' !== $event->getEnvironment()->getDataDefinition()->getName())
        ) {
            return;
        }

        $metaModelsName = $this->getFactory()->translateIdToMetaModelName($model->getProperty('pid'));
        $metaModel      = $this->getFactory()->getMetaModel($metaModelsName);
        $attributeName  = $model->getProperty('colname') . '__sort';
        $tableColumns   = $this->connection->getSchemaManager()->listTableColumns($metaModel->getTableName());

        if (\array_key_exists($attributeName, $tableColumns)) {
            return;
        }

        $this->tableManipulator->createColumn($metaModel->getTableName(), $attributeName, 'blob NULL');
    }
922430
public function beforeRender(Event $event)
    {
        parent::beforeRender($event);

        $this->set('detect', $this->detect);
    }
553489
function(event, model, collection, options) {
      if ((event == 'add' || event == 'remove') && collection != this) return;
      if (event == 'destroy') {
        this.remove(model, options);
      }
      if (model && event === 'change:' + model.idAttribute) {
        delete this._byId[model.previous(model.idAttribute)];
        this._byId[model.id] = model;
      }
      this.trigger.apply(this, arguments);
    }