Adds the basic Tooltip Text Formatter (no javascript).

cdesai_tff.module

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+<?php
+
+use Drupal\Core\Template\Attribute;
+
+/**
+ * @file
+ *  Implements hook functions needed by cdesai_tff module.
+ */
+
+/**
+ * Implements hook_theme().
+ */
+function cdesai_tff_theme($existing, $type, $theme, $path) {
+  return [
+    'cdesai_tff_tooltip_formatter' => [
+      'template' => 'cdesai-tff-tooltip-formatter',
+      'variables' => [
+        'value' => NULL,
+        'attributes' => [],
+      ],
+    ],
+  ];
+}
+
+/**
+ * Preprocessor for cdesai_tff_tooltip_formatter theme.
+ */
+function template_preprocess_cdesai_tff_tooltip_formatter(&$variables) {
+  //$attributes = new Attribute($variables['attributes']);
+}

src/Plugin/Field/FieldFormatter/TooltipTextFormatter.php

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+<?php
+
+
+namespace Drupal\cdesai_tff\Plugin\Field\FieldFormatter;
+
+use Drupal\cdesai_tff\DesaiTextFormatterBase;
+use Drupal\Core\Field\FieldItemInterface;
+use Drupal\Core\Field\FieldItemListInterface;
+
+/**
+ * Implementation of custom Tooltip text formatter.
+ *
+ * @FieldFormatter(
+ *   id = "cdesai_tff_tooltip",
+ *   label = @Translation("Custom Tooltip Formatter"),
+ *   field_types = {
+ *     "string",
+ *     "text"
+ *   }
+ * )
+ */
+class TooltipTextFormatter extends DesaiTextFormatterBase {
+
+  /**
+   * @inheritDoc
+   */
+  function getCssClassId() {
+    return 'cdesai-tff-tooltip';
+  }
+
+  /**
+   * @inheritDoc
+   */
+  public function viewElements(FieldItemListInterface $items, $langcode) {
+    $elements = [];
+    foreach ($items as $delta => $item) {
+      $elements[$delta] = [
+        '#theme' => 'cdesai_tff_tooltip_formatter',
+        '#value' => $this->getValue($item),
+        '#attributes' => [
+          'class' => [$this->getCssClassId()],
+        ],
+      ];
+    }
+
+    return $elements;
+  }
+
+  /**
+   * @inheritDoc
+   */
+  function getValue(FieldItemInterface $item) {
+    return $item->getString();
+  }
+
+}
+

templates/cdesai-tff-tooltip-formatter.html.twig

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+<div{{ attributes }}>
+  {{ value }}
+</div>