How to define a custom field as a BaseFieldDefinition

I'm not 100% sure as I havn't tried it myself but would assume you would use the setSettings() method to define the columns/field_settings as it would be in configs? https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Field%21B...

I would probably create a regular field first and look at the field configs to see if you can make it work from that as an example. If you figure it out, feel free to create a documentation page. Also, be aware that I'm working on a 4.x version that is going to completely refactor the form settings into more of the "drupal" way of handling so the config structure is inevitable going to change.

I believe I have it working as follows, for a custom field with two elements:
* String field
* Serialized - Text (plain)

    $fields['target_types'] = BaseFieldDefinition::create('custom')
      ->setLabel(new TranslatableMarkup('Vehicle Types'))
      ->setDescription(new TranslatableMarkup('Vehicle Types'))
      ->setSetting('field_settings', [
        'brand' => [
          'type' => 'text',
          'weight' => 0,
          'check_empty' => FALSE,
          'widget_settings' => [
            'label' => 'Brand',
            'translatable' => FALSE,
            'settings' => [
              'description' => '',
              'description_display' => 'after',
              'size' => 60,
              'placeholder' => '',
              'maxlength' => 255,
              'prefix' => '',
              'suffix' => '',
              'required' => FALSE,
            ],
          ],
        ],
        'models' => [
          'type' => 'map_text',
          'weight' => 0,
          'check_empty' => FALSE,
          'widget_settings' => [
            'label' => 'Model(s)',
            'translatable' => FALSE,
            'settings' => [
              'description' => '',
              'description_display' => 'after',
              'table_empty' => '',
              'required' => FALSE,
            ],
          ],
        ],
      ])
      ->setSetting('columns', [
        'brand' => [
          'name' => 'brand',
          'type' => 'string',
          'length' => 255,
        ],
        'models' => [
          'name' => 'models',
          'type' => 'map_string',
        ],
      ])
      ->setCardinality(FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED)
      ->setDisplayConfigurable('view', TRUE)
      ->setDisplayConfigurable('form', TRUE)
      ->setDisplayOptions('form', [
        'type' => 'custom_flexbox',
        'region' => 'content',
        'settings' => [
          'label' => TRUE,
          'wrapper' => 'div',
          'open' => TRUE,
        ],
      ])
      ->setDisplayOptions('view', [
        'type' => 'custom_formatter',
        'label' => 'above',
        'settings' => [
          'fields' => [],
        ],
        'third_party_settings' => [],
        'region' => 'content',
      ]);

Oh good! I figured that was all that was needed. It will get simpler in 4.x as the array of settings will be flattened out and easier to work with. Just be aware of that when that version rolls out.

It would be great if you could create a documentation page with a full example.

Done: https://www.drupal.org/docs/extending-drupal/contributed-modules/contrib... →

Perfect. Thank you!