How programmatically create custom field type with some base fields?

Question

I want to emphasise that I don't want to use field_collection.

I want to create custom field type that can be reused in different entities and bundles. This should be kept in one custom module. So when I enable it and go to 'manage fields' page on any entity type then I should be able to pick up my custom field type (e.g.: foo). This field should be combined with base image field and simple one line text field. I don't want to use field_collection module because as far as I see I can not recreate my field, even features give me ability to export it in field_base section. After flushing my drupal to clean instance and enabling this feature again I can not see it in 'Add existing field' in [structure > content types > CONTENT_TYPE > manage fields] page. Maybe I'm doing something wrong. (?)

So how can I programmatically create custom field type with existing base fields? e.g. Custom foo field type with image field and text field?

OR (if I done something wrong with features..)

How can I export custom field collection with sub fields, only as a field definition (not attached to any content type/entity) using features?

Answer 1

What you want to do is create a field type with multiple properties. This is indeed more efficient than a field collection, because a property only needs one more column in the field table, while a field collection is a complete new entity in the database.

This is the documentation how to build a custom field type from scratch in a custom module:

https://www.drupal.org/docs/8/creating-custom-modules/create-a-custom-field-type

As you want to add a property to an existing field type use this tutorial

https://www.lullabot.com/articles/extending-a-field-type-in-drupal-8

and extend the class ImageItem for the new field type. You also need to extend ImageFormatter to display and ImageWidget to provide a form field for the new property.