Link to dedicated URL as display option for referenced YAML form

I needed this as well, but eventually ended up going a different route.

Here is the initial patch if someone wants to continue working with this.

It makes complete sense that in certain situations a link (instead of the form) to a YAML form from a node would make a lot of sense. The link should also include the source entity type and id (aka the YAML Form node) so that the submission can properly track this information.

For example, the link from the YAML Form node should be /form/contact?source_entity_type=node&source_entity_id=12345

The ability to populate a YAML Form submission source_entity should initially be limited to only valid YAML Form node, this would prevent end-users from injecting an unwanted source entity.

Once #2792681: Allow a YAML Form's source entity to be (optionally) populated using query string parameters. is done we should start working on the link formatter.

@tuutti The patch is good starting point, thanks

Just looking at this for the first time, so thinking aloud, any pointers appreciated:

If you select 'Reference -> other' and then make an 'entity_reference' field to the YAML Form config entity, you have the default standard ID, Label, Rendered Entity FieldFormatters. Which includes the basic label link to the entity.

I assume the reason YAML form then implements it's own 'yamlform' FieldType is to add the schema for 'default_data' and 'status'. I'm guessing there are use cases where this should be related to the embedded Rendered Entity: embedding the same form multiple times as a rendered entity with differing defaults. And that there was no way of squeezing this data somewhere in the standard 'entity_reference' settings.

Core seem to generally implement FieldFormatters for 'entity_reference' but then restrict them to particular entity types with FormatterInterface::isApplicable().

So this could be both a FieldFormatter for 'entity_reference' and 'yamlform' extending EntityReferenceLabelFormatter adding in 'key'='[token:value]' (or something) query strings.
For 'entity_reference' it actually sounds like I generic module? My alternative thought, on the project only basis, here was it would be as quick to us 'entity_reference' with the Label FieldFormatter and just alter the output in some hook, or similar, on the way out.

So i just spent some time thinking about some other approaches and concluded that @tuutti's patch from #2 is the best solution. It allows a YAML form to be linked to from node teasers.

I was also able to get the 'default_data' to load correctly when a YAML Form is linked to with a valid source entity passed in the query string.

Let's see if the attach patch passes all the existing tests and then I have write some new tests.

@tuutti I also applied some of your code improvements to the YamlFormEntityReferenceEntityFormatter. Thanks again for the patch.

Same patch + some tests.

Tests are passing and there is no risk for data loss, so I committed the patch to 8.x-1.x.

Please checkout 8.x-1.x or download the dev release.

Sorry to nitpick, but you forgot to change YamlFormEntityReferenceEntityFormatter::__construct() comments to match the actual class.

Absolutely not a nitpicky patch. Thanks.