Fist version of the module.

README.md

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+## Block field extra

block_field_extra.info.yml

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+name: 'Block field extra'
+type: module
+description: 'Extends the block field functionality'
+core: 8.x
+package: 'block_field'
+dependencies:
+  - block_field

block_field_extra.module

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+<?php
+
+/**
+ * @file
+ * Contains block_field_extra.module.
+ */
+
+use Drupal\Core\Routing\RouteMatchInterface;
+
+/**
+ * Implements hook_help().
+ */
+function block_field_extra_help($route_name, RouteMatchInterface $route_match) {
+  switch ($route_name) {
+    // Main module help for the block_field_extra module.
+    case 'help.page.block_field_extra':
+      $output = '';
+      $output .= '<h3>' . t('About') . '</h3>';
+      $output .= '<p>' . t('Extends the block field functionality') . '</p>';
+      return $output;
+
+    default:
+  }
+}
+
+
+/**
+ * Implements hook_preprocess().
+ */
+function block_field_extra_preprocess(&$variables, $hook, $info) {
+  if ($hook = 'views_view') {
+    if (isset($variables['view']->block_field_attachments)) {
+      $variables['block_field_attachments'] = $variables['view']->block_field_attachments;
+    }
+  }
+}
+
+/**
+ * Implements hook_views_data_alter().
+ */
+function block_field_extra_views_data_alter(array &$data) {
+  $data['views']['block_field_attachments'] = [
+    'title' => t('Block field attachments'),
+    'help' => t('Block field attachments'),
+    'area' => [
+      'id' => 'block_field_attachments',
+    ],
+  ];
+}

composer.json

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+{
+  "name": "WondrousLLC/block_field_extra",
+  "type": "drupal-module",
+  "description": "Extends the block field functionality",
+  "keywords": ["Drupal"],
+  "license": "GPL-2.0+",
+  "homepage": "https://www.drupal.org/project/block_field_extra",
+  "minimum-stability": "dev",
+  "support": {
+    "issues": "https://www.drupal.org/project/issues/block_field_extra",
+    "source": "http://cgit.drupalcode.org/block_field_extra"
+  },
+  "require": {
+    "drupal/block_field": "^1.0-alpha5"
+  }
+}

src/Plugin/Field/FieldFormatter/BlockFieldAttachmentsFormatter.php

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+<?php
+
+namespace Drupal\block_field_extra\Plugin\Field\FieldFormatter;
+
+use Drupal\block_field\Plugin\Field\FieldFormatter\BlockFieldFormatter;
+use Drupal\Component\Plugin\Exception\ContextException;
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\Core\Field\FormatterBase;
+use Drupal\Core\Plugin\ContextAwarePluginInterface;
+use Drupal\Core\Form\FormStateInterface;
+
+/**
+ * Plugin implementation of the 'block_field_attachments' formatter.
+ *
+ * @FieldFormatter(
+ *   id = "block_field_attachments",
+ *   label = @Translation("Block field with attachments"),
+ *   field_types = {
+ *     "block_field"
+ *   }
+ * )
+ */
+class BlockFieldAttachmentsFormatter extends BlockFieldFormatter {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function viewElements(FieldItemListInterface $items, $langcode) {
+    $settings = $this->getSettings();
+    $entity = $items->getEntity();
+    foreach ($settings['field'] as $field_name) {
+      if ($entity->hasField($field_name) && ($field_value = $this->getValue($entity, $field_name))) {
+        $build = $this->getFieldBuild($entity, $field_name);
+        $block_field_attachments[$field_name] = $build ?: $field_value;
+      }
+    }
+
+    if (!empty($block_field_attachments)) {
+      $block_field_attachments['#context'] = [
+        'field_definition' => $items->getFieldDefinition(),
+        'langcode' => $langcode,
+      ];
+      // Inject the attachment as the field item settings.
+      // This can be useful if you need this data on the field type plugin.
+      foreach ($items as $item) {
+        $settings = $item->settings;
+        $settings['block_field_attachments'] = $block_field_attachments;
+        $item->set('settings', $settings);
+      }
+    }
+
+    // Let the parent to build the elements.
+    // Calling the parent::viewElements will add by default the $attachment settings under
+    // $element #configuration.
+    // see $element[DELTA]['#configuration']['block_field_attachments'][FIELD_NAME]
+    $elements = parent::viewElements($items, $langcode);
+
+    if (isset($block_field_attachments)) {
+      foreach ($elements as &$element) {
+        $element['block_field_attachments'] = $block_field_attachments;
+        if (isset($element['content']['#view'])) {
+          $view = $element['content']['#view'];
+          $view->block_field_attachments = $block_field_attachments;
+        }
+      }
+    }
+
+    return $elements;
+  }
+
+
+  public function getFieldBuild($entity, $field_name) {
+    $build = $entity->{$field_name}->view($this->viewMode);
+
+    return $build;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function defaultSettings() {
+    return parent::defaultSettings() + [
+        'field' => 'field_attachment',
+      ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function settingsForm(array $form, FormStateInterface $form_state) {
+    $element = parent::settingsForm($form, $form_state);
+
+    // Make sure if there's a view options to select.
+    $options = $this->getFieldsList();
+    $element['field'] = [
+      '#title' => $this->t('Attachment field'),
+      '#description' => $this->t('The selected fields will be injected into the block instance'),
+      '#type' => 'select',
+      '#multiple' => TRUE,
+      '#options' => $this->getEntityProperties() + $options,
+      '#default_value' => $this->getSetting('field'),
+    ];
+
+    return $element;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function settingsSummary() {
+    $summary = parent::settingsSummary();
+    return $summary;
+  }
+
+  /**
+   * Get the the entity property or field value.
+   *
+   * @param $entity
+   *   The entity object.
+   * @param $name
+   *   The property or field name.
+   *
+   * @return mixed
+   *   The value or FALSE.
+   */
+  public function getValue($entity, $name) {
+    // Check if it's a field.
+    if ($entity->hasField($name)) {
+      $field_value = $entity->get($name)->getValue();
+      // TODO Uggly solution, find a better way to get the value property key.
+      $value_keys = ['value', 'target_id'];
+      foreach ($value_keys as $key) {
+        if (isset($field_value[0][$key])) {
+          return $field_value[0][$key];
+        }
+      }
+    }
+    else {
+      // Check for property.
+      if (isset($entity->{$name})) {
+        return $entity->{$name}->value;
+      }
+    }
+
+    return FALSE;
+  }
+
+  /**
+   * Gets the list of the host entity properties.
+   *
+   * @return array
+   *   The list of available properties.
+   */
+  function getEntityProperties() {
+    // TODO Get the properties dynamically.
+    return ['id' => $this->t('Entity ID')];
+  }
+
+  /**
+   * Gets the list fields from the host entity.
+   *
+   * @return array
+   *   List of fields.
+   */
+  public function getFieldsList() {
+    $fields = [];
+    $entity_field_definitions = $this->getFieldDefinitions();
+    $current_field = $this->fieldDefinition->getName();
+
+    // Build the arguments array;
+    foreach ($entity_field_definitions as $entity_field_definition) {
+      if ($entity_field_definition instanceof \Drupal\field\Entity\FieldConfig && $entity_field_definition->getName() != $current_field ) {
+        $fields[$entity_field_definition->getName()] = $entity_field_definition->label();
+      }
+    }
+
+    return $fields;
+  }
+
+  /**
+   * Gets the field definitions of the host entity.
+   *
+   * @return array
+   *   The field definitions.
+   */
+  public function getFieldDefinitions() {
+    $field_definition = $this->fieldDefinition;
+    $entity_field_manager = \Drupal::service('entity_field.manager');
+    $entity_type = $field_definition->get('entity_type');
+    $bundle = $field_definition->get('bundle');
+
+    // Get the list fields from the host entity.
+    $entity_field_definitions = $entity_field_manager->getFieldDefinitions($entity_type, $bundle);
+
+    return $entity_field_definitions;
+  }
+
+
+}

src/Plugin/views/area/BlockFieldAttachments.php

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+<?php
+
+namespace Drupal\block_field_extra\Plugin\views\area;
+
+use Drupal\views\Plugin\views\area\AreaPluginBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Link;
+
+/**
+ * Views area block_field_attachments handler.
+ *
+ * @ingroup views_area_handlers
+ *
+ * @ViewsArea("block_field_attachments")
+ */
+class BlockFieldAttachments extends AreaPluginBase {
+
+  /**
+   * The entity object
+   *
+   * @var object
+   */
+  public $entity = NULL;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function defineOptions() {
+    $options = parent::defineOptions();
+    $options['title'] = ['default' => ''];
+    $options['label_from_argument'] = ['default' => []];
+    $options['link_label'] = ['default' => 'See all'];
+    $options['link_label_html_tag'] = ['default' => 'h2'];
+    return $options;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildOptionsForm(&$form, FormStateInterface $form_state) {
+    parent::buildOptionsForm($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function render($empty = FALSE) {
+    return isset($this->view->block_field_attachments) ? $this->view->block_field_attachments : [];
+  }
+
+}