framework/behaviors/SluggableBehavior.php
protected function getValue($event)
    {
        if (!$this->isNewSlugNeeded()) {
            return $this->owner->{$this->slugAttribute};
        }

        if ($this->attribute !== null) {
            $slugParts = [];
            foreach ((array) $this->attribute as $attribute) {
                $part = ArrayHelper::getValue($this->owner, $attribute);
                if ($this->skipOnEmpty && $this->isEmpty($part)) {
                    return $this->owner->{$this->slugAttribute};
                }
                $slugParts[] = $part;
            }
            $slug = $this->generateSlug($slugParts);
        } else {
            $slug = parent::getValue($event);
        }

        return $this->ensureUnique ? $this->makeUnique($slug) : $slug;
    }
protected function getValue($event)
    {
        if (!$this->isNewSlugNeeded()) {
            return $this->owner->{$this->slugAttribute};
        }

        if ($this->attribute !== null) {
            $slugParts = [];
            foreach ((array) $this->attribute as $attribute) {
                $part = ArrayHelper::getValue($this->owner, $attribute);
                if ($this->skipOnEmpty && $this->isEmpty($part)) {
                    return $this->owner->{$this->slugAttribute};
                }
                $slugParts[] = $part;
            }
            $slug = $this->generateSlug($slugParts);
        } else {
            $slug = parent::getValue($event);
        }

        return $this->ensureUnique ? $this->makeUnique($slug) : $slug;
    }
{@inheritdoc}