[generate:entity:content] Add 'view_builder' to config entity annotation template. Fix #3129.

[jall]

This is a patch for #3129.

This allows content entities with bundles to be exposed through the core REST API without throwing errors of the form:

Route entity.{{entityBundleType}}.canonical does not exist.

Steps to reproduce & fix manually have been well described by @tahirm in the linked issue, so I won't re-explain them here.

This patch adds a default 'view_builder' to all config entity definitions. It uses the Core Drupal\Core\Entity\EntityViewBuilder & doesn't bother defining a custom extension class in the same way as, for example, the 'list_builder': Drupal\{{ module }}\{{ entity_class }}ListBuilder as I haven't come across a use case for this level of flexibility yet.

I checked Core to see where it extends EntityViewBuilder rather than using it directly and found of the >30 Drupal Core config entities, only 2 have 'view_builder' defined:

• \Drupal\tour\Entity\Tour => Drupal\tour\TourViewBuilder
• \Drupal\block\Entity\Block => Drupal\block\BlockViewBuilder

These both appear to be bundle config entities, which fits with the use case being fixed here.

I was concerned that adding 'view_builder' unconditionally to entity.php.twig might be too far reaching, as it doesn't look like a config entity needs it defined unless they specify a canonical route. However, it appears that the only usage of this template is for adding content entity's bundle classes in src/Generate/EntityConfigGenerator, so I believe this patch will only affect what it should.

[jmolivas]

@jall Thanks for your contribution, your PR was merged. This will be included on the next release.

[davereid]

I think this should be reverted. This line is not necessary on the majority of config entities.

[jall]

@davereid I can see why a view_builder would be dead code for many config entities which are not exposed/rendered, but does it introduce any performance impact or have other negative impacts other than cleanliness?

I raised this PR because I was generating a lot of bundle-able entities and exposing them as REST resources, & it was quite irritating having them fail by default before this patch.

I know very little about the bigger picture entity system but a lot of the other generated entity code from Drupal console seems be fairly verbose boilerplate & possibly unnecessary for many use cases, so I didn't think this would be any different.

[jmolivas]

We can probably pass some flags to decide which code to generate.