Issue #3378704 by fgm,Wim Leers: Add config schema for formatter settings validation.

config/schema/image_delta_formatter.schema.yml

+field.formatter.settings.image_delta_formatter:
+  type: mapping
+  label: 'Image Delta Formatter: image settings'
+  mapping:
+    deltas:
+      type: sequence
+      label: 'Deltas'
+      sequence:
+        type: integer
+        label: 'deltas'
+        constraints:
+          Range:
+            min: 0
+    deltas_reversed:
+      type: boolean
+      label: 'Reversed'
+    image_link:
+      type: string
+      label: 'Link image to'
+    image_style:
+      type: string
+      label: 'Image style'
+    image_loading:
+      type: mapping
+      label: 'Image loading settings'
+      mapping:
+        attribute:
+          type: string
+          label: 'Loading attribute'
+
+field.formatter.settings.media_delta_formatter:
+  type: mapping
+  label: 'Image Delta Formatter: media settings'
+  mapping:
+    deltas:
+      type: sequence
+      label: 'Deltas'
+      sequence:
+        type: integer
+        label: 'deltas'
+        constraints:
+          Range:
+            min: 0
+    deltas_reversed:
+      type: boolean
+      label: 'Reversed'
+    image_style:
+      type: string
+      label: 'Image style'
+    image_link:
+      type: string
+      label: 'Image link'
+    image_loading:
+      type: mapping
+      label: 'Image loading'
+      mapping:
+        attribute:
+          type: string
+          label: 'Attribute: lazy or eager'

src/OptionalPlugin/Field/FieldFormatter/ResponsiveImageDeltaFormatter.php

@@ -4,6 +4,8 @@ declare(strict_types = 1);
 
 namespace Drupal\image_delta_formatter\OptionalPlugin\Field\FieldFormatter;
 
+use Drupal\Core\Field\Attribute\FieldFormatter;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
 use Drupal\image_delta_formatter\Plugin\Field\FieldFormatter\ImageDeltaTrait;
 use Drupal\responsive_image\Plugin\Field\FieldFormatter\ResponsiveImageFormatter;
 

src/Plugin/Field/FieldFormatter/ImageDeltaFormatter.php

@@ -4,6 +4,8 @@ declare(strict_types = 1);
 
 namespace Drupal\image_delta_formatter\Plugin\Field\FieldFormatter;
 
+use Drupal\Core\Field\Attribute\FieldFormatter;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
 use Drupal\image\Plugin\Field\FieldFormatter\ImageFormatter;
 
 /**
@@ -18,6 +20,14 @@ use Drupal\image\Plugin\Field\FieldFormatter\ImageFormatter;
  *   }
  * )
  */
+#[FieldFormatter(
+  id: 'image_delta_formatter',
+  label: new TranslatableMarkup('Image delta'),
+  description: new TranslatableMarkup('Display specific deltas of an image field.'),
+  field_types: [
+    'image',
+  ],
+)]
 class ImageDeltaFormatter extends ImageFormatter {
 
   use ImageDeltaTrait;

src/Plugin/Field/FieldFormatter/ImageDeltaTrait.php

@@ -20,7 +20,7 @@ trait ImageDeltaTrait {
    */
   public static function defaultSettings(): array {
     return [
-      'deltas' => '0',
+        'deltas' => [],
         'deltas_reversed' => FALSE,
       ] + parent::defaultSettings();
   }
@@ -39,20 +39,25 @@ trait ImageDeltaTrait {
   public function settingsForm(array $form, FormStateInterface $form_state): array {
     $element = parent::settingsForm($form, $form_state);
 
+    $deltas = $this->getSetting('deltas');
+    if (is_array($deltas)) {
+      $deltas = implode(', ', $deltas);
+    }
     $element['deltas'] = [
-      '#type' => 'textfield',
-      '#title' => $this->t('Delta'),
+      '#default_value' => $deltas,
       '#description' => $this->t('Enter a delta, or a comma-separated list of deltas that should be shown. For example: 0, 1, 4.'),
-      '#size' => 10,
-      '#default_value' => $this->getSetting('deltas'),
+      '#element_validate' => [[$this, 'validateDeltas']],
       '#required' => TRUE,
+      '#size' => 10,
+      '#title' => $this->t('Delta'),
+      '#type' => 'textfield',
       '#weight' => -20,
     ];
     $element['deltas_reversed'] = [
-      '#title' => $this->t('Reversed'),
-      '#type' => 'checkbox',
       '#default_value' => $this->getSetting('deltas_reversed'),
       '#description' => $this->t('Start from the last values.'),
+      '#title' => $this->t('Reversed'),
+      '#type' => 'checkbox',
       '#weight' => -10,
     ];
 
@@ -66,17 +71,63 @@ trait ImageDeltaTrait {
     $settings = $this->getSettings();
     $summary = parent::settingsSummary();
 
-    $deltas = '' . ($settings['deltas'] ?? '');
-    $count = count(explode(',', $deltas));
-    $args = ['@deltas' => trim($deltas)];
-    $delta_summary = $settings['deltas_reversed']
-      ? $this->formatPlural($count, 'Delta: @deltas (reversed, no effect).', 'Deltas: @deltas (reversed).', $args)
-      : $this->formatPlural($count, 'Delta: @deltas', 'Deltas: @deltas', $args);
+    ['deltas' => $deltas, 'deltas_reversed' => $reversed] = $settings;
+    // Handle legacy config format.
+    if (is_scalar($deltas)) {
+      $deltas = $this->deltasFromString($deltas);
+    }
+    $count = count($deltas);
+    $args = [
+      '@deltas' => trim(implode(', ', $deltas)),
+    ];
+    $delta_summary = $reversed
+      ? $this->formatPlural($count,
+        'Delta: @deltas (reversed, no effect).', 'Deltas: @deltas (reversed).',
+        $args)
+      : $this->formatPlural($count,
+        'Delta: @deltas', 'Deltas: @deltas',
+        $args);
+
     $summary[] = $delta_summary;
 
     return $summary;
   }
 
+  /**
+   * Convert a deltas string to a clean deltas array.
+   *
+   * @param scalar $s
+   *   A scalar describing the list of deltas.
+   *
+   * @return int[]
+   *   The deduplicated, unsorted, integer deltas found in the source string.
+   */
+  protected static function deltasFromString($s): array {
+    $split = explode(',', '' . $s);
+    $trimmed = array_map(fn($v) => (int) trim($v), $split);
+    $unique = array_unique($trimmed);
+    // Avoid non-sequential index keys that could result from input like "0,,1".
+    $normalized = array_values($unique);
+    return $normalized;
+  }
+
+  /**
+   * Element validate handler for deltas.
+   *
+   * @param array<string,mixed> $element
+   *   The deltas elements.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The form state.
+   * @param array<string,mixed> $complete_form
+   *   The complete form.
+   */
+  public function validateDeltas(array &$element, FormStateInterface $form_state, array &$complete_form): void {
+    /** @var string $value */
+    $value = $element['#value'] ?? '';
+    $deltas = $this->deltasFromString($value);
+    $form_state->setValueForElement($element, $deltas);
+  }
+
   /**
    * Returns the referenced entities for display.
    *
@@ -93,18 +144,24 @@ trait ImageDeltaTrait {
   protected function getEntitiesToView(EntityReferenceFieldItemListInterface $items, $langcode): array {
     $files = parent::getEntitiesToView($items, $langcode);
     // Prepare an array of selected deltas from the entered string.
-    /** @var string $raw_deltas */
-    $raw_deltas = $this->getSetting('deltas');
-    if (mb_strpos($raw_deltas, ',')) {
-      $deltas = explode(',', $raw_deltas);
+    /** @var string|array $rawDeltas */
+    $rawDeltas = $this->getSetting('deltas');
+    if (is_scalar($rawDeltas)) {
+      $rawDeltas = '' . $rawDeltas;
+      if (mb_strpos($rawDeltas, ',')) {
+        $deltas = explode(',', $rawDeltas);
         $deltas = array_map('trim', $deltas);
         $deltas = array_map('intval', $deltas);
       }
       else {
         /** @var int $delta */
-      $delta = (int) trim($raw_deltas);
+        $delta = (int) trim($rawDeltas);
         $deltas = [$delta];
       }
+    }
+    else {
+      $deltas = $rawDeltas;
+    }
 
     foreach (array_keys($files) as $delta) {
       if (!in_array($delta, $deltas)) {