Add a field link formatter

config/schema/field_formatter.schema.yml

@@ -15,3 +15,11 @@ field.formatter.settings.field_formatter_with_inline_settings:
       type: string
     settings:
       type: field.formatter.settings.[%parent.type]
+
+field.formatter.settings.field_link:
+  type: mapping
+  mapping:
+    type:
+      type: string
+    settings:
+      type: field.formatter.settings.[%parent.type]

field_formatter.module

@@ -0,0 +1,12 @@
+<?php
+
+/**
+ * Implements hook_field_formatter_info_alter().
+ */
+function field_formatter_field_formatter_info_alter(array &$info) {
+  /** @var \Drupal\Core\Field\FieldTypePluginManagerInterface $field_type_manager */
+  $field_type_manager = \Drupal::service('plugin.manager.field.field_type');
+  $field_types = array_keys($field_type_manager->getDefinitions());
+
+  $info['field_link']['field_types'] = $field_types;
+}

src/Plugin/Field/FieldFormatter/FieldLink.php

@@ -0,0 +1,41 @@
+<?php
+
+namespace Drupal\field_formatter\Plugin\Field\FieldFormatter;
+
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\Core\Render\Element;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides the field link formatter.
+ *
+ * @FieldFormatter(
+ *   id = "field_link",
+ *   module = "field_linker",
+ *   label = @Translation("Field linker"),
+ *   field_types = {
+ *   },
+ * )
+ *
+ * 
+ */
+class FieldLink extends FieldWrapperBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function viewElements(FieldItemListInterface $items, $langcode) {
+    $field_output = $this->getFieldOutput($items, $langcode);
+
+    $elements = [];
+    foreach (Element::children($field_output) as $key) {
+      $elements[$key] = [
+        '#type' => 'link',
+        '#url' => $items->getEntity()->toUrl(),
+        '#title' => $field_output[$key],
+      ];
+    }
+    return $elements;
+  }
+
+}

src/Plugin/Field/FieldFormatter/FieldWrapperBase.php

@@ -0,0 +1,273 @@
+<?php
+
+namespace Drupal\field_formatter\Plugin\Field\FieldFormatter;
+
+use Drupal\Core\Entity\Entity\EntityViewDisplay;
+use Drupal\Core\Field\BaseFieldDefinition;
+use Drupal\Core\Field\FieldDefinitionInterface;
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\Core\Field\FieldStorageDefinitionInterface;
+use Drupal\Core\Field\FormatterBase;
+use Drupal\Core\Field\FormatterInterface;
+use Drupal\Core\Field\FormatterPluginManager;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Wraps an existing field.
+ */
+abstract class FieldWrapperBase extends FormatterBase implements ContainerFactoryPluginInterface {
+
+  /** @var \Drupal\Core\Entity\Display\EntityViewDisplayInterface */
+  protected $viewDisplay;
+
+  /**
+   * The entity field manager.
+   *
+   * @var \Drupal\Core\Entity\EntityFieldManagerInterface
+   */
+  protected $entityFieldManager;
+
+  /**
+   * The formatter plugin manager.
+   *
+   * @var \Drupal\Core\Field\FormatterPluginManager
+   */
+  protected $formatterPluginManager;
+
+  /**
+   * Constructs a FieldFormatterWithInlineSettings object.
+   *
+   * @param string $plugin_id
+   *   The plugin_id for the formatter.
+   * @param mixed $plugin_definition
+   *   The plugin implementation definition.
+   * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
+   *   The definition of the field to which the formatter is associated.
+   * @param array $settings
+   *   The formatter settings.
+   * @param string $label
+   *   The formatter label display setting.
+   * @param string $view_mode
+   *   The view mode.
+   * @param array $third_party_settings
+   *   Any third party settings.
+   * @param \Drupal\Core\Field\FormatterPluginManager $formatter_plugin_manager
+   *   The formatter plugin manager.
+   */
+  public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, $label, $view_mode, array $third_party_settings, FormatterPluginManager $formatter_plugin_manager) {
+    parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $label, $view_mode, $third_party_settings);
+    $this->formatterPluginManager = $formatter_plugin_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    return new static(
+      $plugin_id,
+      $plugin_definition,
+      $configuration['field_definition'],
+      $configuration['settings'],
+      $configuration['label'],
+      $configuration['view_mode'],
+      $configuration['third_party_settings'],
+      $container->get('plugin.manager.field.formatter')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function defaultSettings() {
+    return [
+      'field_name' => '',
+      'type' => '',
+      'settings' => [],
+    ];
+  }
+
+  /**
+   * Get field definition for given field storage definition
+   *
+   * @param \Drupal\Core\Field\FieldStorageDefinitionInterface $field_storage_definition
+   *   The field storage definition.
+   *
+   * @return \Drupal\Core\Field\BaseFieldDefinition
+   *   The field definition.
+   */
+  protected function getFieldDefinition(FieldStorageDefinitionInterface $field_storage_definition) {
+    return BaseFieldDefinition::createFromFieldStorageDefinition($field_storage_definition);
+  }
+
+  /**
+   * Get all available formatters by loading available ones and filtering out.
+   *
+   * @param \Drupal\Core\Field\FieldStorageDefinitionInterface $field_storage_definition
+   *   The field storage definition.
+   *
+   * @return string[]
+   *   The field formatter labels keys by plugin ID.
+   */
+  protected function getAvailableFormatterOptions(FieldStorageDefinitionInterface $field_storage_definition) {
+    $formatters = $this->formatterPluginManager->getOptions($field_storage_definition->getType());
+    $formatter_instances = array_map(function($formatter_id) {
+      $configuration = [
+        'field_definition' => $this->fieldDefinition,
+        'settings' => [],
+        'label' => '',
+        'view_mode' => '',
+        'third_party_settings' => [],
+      ];
+      return $this->formatterPluginManager->createInstance($formatter_id, $configuration);
+    }, array_combine(array_keys($formatters), array_keys($formatters)));
+    $filtered_formatter_instances = array_filter($formatter_instances, function (FormatterInterface $formatter) {
+      return $formatter->isApplicable($this->fieldDefinition);
+    });
+    $options = array_map(function (FormatterInterface $formatter) {
+      return $formatter->getPluginDefinition()['label'];
+    }, $filtered_formatter_instances);
+    return $options;
+  }
+
+  /**
+   * Ajax submit callback for formatter type change.
+   *
+   * @param array $form
+   *   The form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The form state.
+   *
+   * @return array
+   *   The replaced form substructure.
+   */
+  public static function onFormatterTypeChange(array $form, FormStateInterface $form_state) {
+    return $form['fields'][$form_state->getStorage()['plugin_settings_edit']]['plugin']['settings_edit_form']['settings']['settings'];
+  }
+  /**
+   * Rebuilds the form on select submit.
+   *
+   * @param array $form
+   *   The form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The form state.
+   */
+  public static function rebuildSubmit(array $form, FormStateInterface $form_state) {
+    $form_state->setRebuild(TRUE);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function settingsForm(array $form, FormStateInterface $form_state) {
+    // Name of the field this formatter is currently displaying.
+    $field_name = $this->fieldDefinition->getName();
+    $triggering_element = $form_state->getTriggeringElement();
+
+    $field_storage = $this->fieldDefinition->getFieldStorageDefinition();
+    $formatter_options = $this->getAvailableFormatterOptions($field_storage);
+    if ($field_name) {
+      // Form state is not updated as long just select elements are triggered.
+      $formatter_type = $this->getSetting('type');
+      if ($triggering_element['#name'] == "fields[$field_name][settings_edit_form][settings][field_name]") {
+        $formatter_type = key($formatter_options);
+      }
+      else if ($triggering_element['#name'] == "fields[$field_name][settings_edit_form][settings][type]") {
+        // If triggered element is formatter set correct formatter type.
+        $formatter_type = $triggering_element['#value'];
+      }
+
+      $form['type'] = [
+        '#type' => 'select',
+        '#title' => $this->t('Formatter'),
+        '#options' => $formatter_options,
+        '#default_value' => $formatter_type,
+        // Note: We cannot use ::foo syntax, because the form is the entity form
+        // display.
+        '#ajax' => [
+          'callback' => [static::class, 'onFormatterTypeChange'],
+          'wrapper' => 'field-formatter-settings-ajax',
+          'method' => 'replace',
+        ],
+        '#submit' => [[static::class, 'rebuildSubmit']],
+        '#executes_submit_callback' => TRUE,
+      ];
+
+      $options = [
+        'field_definition' => $this->getFieldDefinition($field_storage),
+        'configuration' => [
+          'type' => $formatter_type,
+          'settings' => $this->getSetting('settings'),
+          'label' => '',
+          'weight' => 0,
+        ],
+        'view_mode' => '_custom',
+      ];
+
+      // Get the formatter settings form.
+      $settings_form = ['#value' => []];
+      if ($formatter = $this->formatterPluginManager->getInstance($options)) {
+        $settings_form = $formatter->settingsForm($form, $form_state);
+      }
+      $form['settings'] = $settings_form;
+      $form['settings']['#prefix'] = '<div id="field-formatter-settings-ajax">';
+      $form['settings']['#suffix'] = '</div>';
+    }
+
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function settingsSummary() {
+    $summary = parent::settingsSummary();
+
+    if ($type = $this->getSetting('type')) {
+      $summary[] = $this->t('Formatter %type used.', ['%type' => $type]);
+    }
+    else {
+      $summary[] = $this->t('Formatter not configured.');
+    }
+
+    return $summary;
+  }
+
+  /**
+   * Returns a view display object used to render the actual content of the
+   * field.
+   * @param string $bundle_id
+   *   The bundle ID.
+   *
+   * @return \Drupal\Core\Entity\Display\EntityViewDisplayInterface
+   */
+  protected function getViewDisplay($bundle_id) {
+    if (!isset($this->viewDisplay[$bundle_id])) {
+
+      $display = EntityViewDisplay::create([
+        'targetEntityType' => $this->fieldDefinition->getTargetEntityTypeId(),
+        'bundle' => $bundle_id,
+        'status' => TRUE,
+      ]);
+      $display->setComponent($this->fieldDefinition->getName(), [
+        'type' => $this->getSetting('type'),
+        'settings' => $this->getSetting('settings'),
+      ]);
+      $this->viewDisplay[$bundle_id] = $display;
+    }
+    return $this->viewDisplay[$bundle_id];
+  }
+
+  /**
+   * Returns the wrapped field output.
+   */
+  protected function getFieldOutput(FieldItemListInterface $items, $langcode) {
+    /** @var \Drupal\Core\Entity\FieldableEntityInterface $entity */
+    $entity = $items->getEntity();
+
+    $build = $this->getViewDisplay($entity->bundle())->build($entity);
+    return isset($build[$this->fieldDefinition->getName()]) ? $build[$this->fieldDefinition->getName()] : [];
+  }
+
+}

tests/src/Kernel/Plugin/Field/FieldFormatter/FieldLinkTest.php

@@ -0,0 +1,55 @@
+<?php
+
+namespace Drupal\Tests\field_formatter\Kernel\Plugin\Field\FieldFormatter;
+
+use Drupal\entity_test\Entity\EntityTest;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * @coversDefaultClass \Drupal\field_formatter\Plugin\Field\FieldFormatter\FieldLink
+ * @group field_formatter
+ */
+class FieldLinkTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['entity_test', 'field_formatter', 'user'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $this->installEntitySchema('entity_test');
+    $this->installEntitySchema('user');
+  }
+
+  /**
+   * @covers ::viewElements
+   * @covers ::getFieldOutput
+   * @covers ::getViewDisplay
+   */
+  public function testFieldLinkFormatter() {
+    $entity = EntityTest::create([
+      'name' => 'test name',
+    ]);
+    $entity->save();
+
+    $build = $entity->name->view([
+      'type' => 'field_link',
+      'settings' => [
+        'type' => 'string',
+      ],
+    ]);
+
+    $output = \Drupal::service('renderer')->renderRoot($build);
+    $this->setRawContent($output);
+
+    $href = (string) $this->cssSelect('a')[0]->attributes()['href'];
+    $this->assertEquals($href, $entity->toUrl()->toString(TRUE)->getGeneratedUrl());
+    $this->assertText('test name');
+  }
+
+}