Custom Yii2 serializer that allows masking and setting default fields and expands

CustomSerializer.php

<?php

namespace app\components;

use yii\base\InvalidConfigException;

/**
 * Class CustomSerializer
 *
 * Serializer
 *
 * @license   https://opensource.org/licenses/GPL-3.0  GPL-3.0
 * @copyright 2018-2021 Linus Kohl <linus@munichresearch.com>
 */
class CustomSerializer extends \yii\rest\Serializer
{

    /** @var string[] $defaultFields */
    public $defaultFields = [];
    /** @var string[] $maskedFields */
    public $maskedFields  = [];
    /** @var string[] $defaultExpand */
    public $defaultExpand = [];
    /** @var string[] $maskedExpands */
    public $maskedExpands = [];

    /** {@inheritdoc} */
    public function init()
    {
        parent::init();
        // validate config
        if (!is_array($this->defaultFields) ||
            !is_array($this->maskedFields)  ||
            !is_array($this->defaultExpand) ||
            !is_array($this->maskedExpands)) {
            throw new InvalidConfigException();
        }
    }

    /** {@inheritdoc} */
    protected function getRequestedFields()
    {
        $fields = [];
        $expand = [];

        // values passed by request
        $fields_string = $this->request->get($this->fieldsParam);
        $expand_string = $this->request->get($this->expandParam);

        if (!is_null($fields_string)) {
            // slit to array of strings
            $fields = preg_split('/\s*,\s*/', $fields_string, -1, PREG_SPLIT_NO_EMPTY);
        } else {
            // set to default
            $fields = $this->defaultFields;
        }
        // filter masked fields
        $fields = array_diff($fields, $this->maskedFields);

        if (!is_null($expand_string)) {
            // slit to array of strings
            $expand = preg_split('/\s*,\s*/', $expand_string, -1, PREG_SPLIT_NO_EMPTY);
        } else {
            // set to default
            $fields = $this->defaultExpand;
        }
        // filter expands
        $expand = array_diff($expand, $this->maskedExpands);

        return [$fields, $expand];
    }
}

Example usage:
public $serializer = [
'class' => CustomSerializer::class,
'defaultFields' => ['id', 'created_at', 'updated_at'],
'defaultExpand' => ['n1'],
'maskedExpands' => ['n2','n3'],
];