src/Checkout/CheckoutComponentConfig.php
public function getFormFields()
    {
        $fields = FieldList::create();
        foreach ($this->getComponents() as $component) {
            if ($cfields = $component->getFormFields($this->order)) {
                $fields->merge($cfields);
            } else {
                user_error('getFields on  ' . get_class($component) . ' must return a FieldList');
            }
        }
        return $fields;
    }
public function getFormFields()
    {
        $fields = FieldList::create();
        foreach ($this->getComponents() as $component) {
            if ($cfields = $component->getFormFields($this->order)) {
                $fields->merge($cfields);
            } else {
                user_error('getFields on  ' . get_class($component) . ' must return a FieldList');
            }
        }
        return $fields;
    }
Get combined form fields

@return FieldList namespaced fields