framework/base/ArrayableTrait.php
protected function resolveFields(array $fields, array $expand)
    {
        $fields = $this->extractRootFields($fields);
        $expand = $this->extractRootFields($expand);
        $result = [];

        foreach ($this->fields() as $field => $definition) {
            if (is_int($field)) {
                $field = $definition;
            }
            if (empty($fields) || in_array($field, $fields, true)) {
                $result[$field] = $definition;
            }
        }

        if (empty($expand)) {
            return $result;
        }

        foreach ($this->extraFields() as $field => $definition) {
            if (is_int($field)) {
                $field = $definition;
            }
            if (in_array($field, $expand, true)) {
                $result[$field] = $definition;
            }
        }

        return $result;
    }
protected function resolveFields(array $fields, array $expand)
    {
        $fields = $this->extractRootFields($fields);
        $expand = $this->extractRootFields($expand);
        $result = [];

        foreach ($this->fields() as $field => $definition) {
            if (is_int($field)) {
                $field = $definition;
            }
            if (empty($fields) || in_array($field, $fields, true)) {
                $result[$field] = $definition;
            }
        }

        if (empty($expand)) {
            return $result;
        }

        foreach ($this->extraFields() as $field => $definition) {
            if (is_int($field)) {
                $field = $definition;
            }
            if (in_array($field, $expand, true)) {
                $result[$field] = $definition;
            }
        }

        return $result;
    }
Determines which fields can be returned by [[toArray()]].
This method will first extract the root fields from the given fields.
Then it will check the requested root fields against those declared in [[fields()]] and [[extraFields()]]
to determine which fields can be returned.
@param array $fields the fields being requested for exporting
@param array $expand the additional fields being requested for exporting
@return array the list of fields to be exported. The array keys are the field names, and the array values
are the corresponding object property names or PHP callables returning the field values.