Does replace_tag fire for single tags if there is no $data?

Question

I'm having a problem with a custom field type where I want to return 'some value' on the condition of there being no value in data. This works when I use a tag pair, but not when I use a single tag.

i.e.

{my_field}something here{/my_field}

will call replace_tag even when $data is empty, but...

{my_field}

will not even call replace_tag unless $data is not empty. In this case I want to force a 'default' value if the field is empty but it appears impossible.

Is this a bug or intended behaviour to improve performance? Is there some workaround I can use to trick ExpressionEngine into thinking $data has some value before the template is parsed to force it to call replace_tag on single tags?

Edit:

Trawling through the ExpressionEngine core files gives me the impression that what I'm trying to achieve is impossible though I guess there's a mild chance that an extension might be able to rescue me.

Answer 1

It does not fire if the field data is empty. You could probably use the channel_entries_row hook to modify your field value to show the default if blank. I wouldn't do this, though, as that hook will run on every channel:entries loop and can be quite expensive.

I'd suggest updating your database manually with the default value, and then change your FT's save method to add a default value if blank.

SQL to set a default value on an existing field (change XX to your actual field ID):

UPDATE exp_channel_data SET field_id_XX = 'your default' WHERE field_id_XX = '';

Save function setting a default if blank:

public function save($data)
{
    return $data ?: 'your default';
}