framework/behaviors/AttributeBehavior.php
protected function getValue($event)
    {
        if ($this->value instanceof Closure || (is_array($this->value) && is_callable($this->value))) {
            return call_user_func($this->value, $event);
        }

        return $this->value;
    }
protected function getValue($event)
    {
        if ($this->value instanceof Closure || (is_array($this->value) && is_callable($this->value))) {
            return call_user_func($this->value, $event);
        }

        return $this->value;
    }
Returns the value for the current attributes.
This method is called by [[evaluateAttributes()]]. Its return value will be assigned
to the attributes corresponding to the triggering event.
@param Event $event the event that triggers the current attribute updating.
@return mixed the attribute value