Magento2 Dynamic System->Configuration Comments

Question

In Magento 1.x you could have dynamic comments on system->configuration fields by declaring your field in system.xml like this:

<field_code_here translate="label">
    <label>Label Here</label>
    <frontend_type>text</frontend_type>
    <sort_order>10</sort_order>
    <show_in_default>1</show_in_default>
    <show_in_website>1</show_in_website>
    <show_in_store>1</show_in_store>
    <comment>
        <model>[module]/field_comment</model><!-- this made it dynamic -->
    </comment>
</field_code_here>

Then you just need to create the [Namespace]/[Module]/Model/Field/Comment.php file with this content:

class [Namespace]_[Module]_Model_Field_Comment 
{
    public function getCommentText()  //the method has to be named getCommentText
    {
        //do some calculations here
        return 'Some string based on the calculations';
    }
}

This way the field would have as comment what the getCommentText method from above returned.
Is there an alternative for Magento 2? It seams that I cannot attach a model to a comment tag. The system.xml is not validated by Magento/Backend/etc/system_file.xsd if I do.

Answer 1

[EDIT]
In the latest magento version this is possible in a similar way as in M2. I fixed it :)

[Original Answer]

It seams that this feature is partially implemented in Magento 2. It doesn't work yet.
In the code that generates the configuration form there is the method Magento\Backend\Model\Config\Structure\Element\Field::getComment:

public function getComment($currentValue = '')
{
    $comment = '';
    if (isset($this->_data['comment'])) {
        if (is_array($this->_data['comment'])) {
            if (isset($this->_data['comment']['model'])) {
                $model = $this->_commentFactory->create($this->_data['comment']['model']);
                $comment = $model->getCommentText($currentValue);
            }
        } else {
            $comment = parent::getComment();
        }
    }
    return $comment;
}

This method should handle the case when the comment is generated by a model, but the validation schema does not allow a model tag inside the comment tag because the comment is defined like this:

<xs:element name="comment" type="xs:string" />

I already opened a ticked on github