Issue #3495996 by d34dman: Define all Plural Form Rules as Plugins as mentioned in Gettext Manual

config/schema/string_plural_form.schema.yml

+# Schema for the configuration files of the String Plural Forms module.
+string_plural_form.settings:
+  type: config_object
+  label: 'String Plural Forms settings'
+  mapping:
+    rules_map:
+      type: sequence
+      label: 'Rules'
+      orderby: key
+      sequence:
+        type: string
+        label: 'Langcode - Rule mapping'

src/Form/SettingsForm.php

+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\string_plural_form\Form;
+
+use Drupal\Core\Config\TypedConfigManagerInterface;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\Language\LanguageManagerInterface;
+use Drupal\string_plural_form\StringPluralFormManager;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Form\ConfigFormBase;
+use Drupal\Core\Form\FormStateInterface;
+
+/**
+ * Configure String Plural Forms settings for this site.
+ */
+final class SettingsForm extends ConfigFormBase {
+
+  public function __construct(
+    ConfigFactoryInterface $config_factory,
+    TypedConfigManagerInterface $typedConfigManager,
+    private LanguageManagerInterface $languageManager,
+    private StringPluralFormManager $stringPluralFormManager,
+  ) {
+    parent::__construct($config_factory, $typedConfigManager);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('config.factory'),
+      $container->get('config.typed'),
+      $container->get('language_manager'),
+      $container->get('plugin.manager.string_plural_form')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId(): string {
+    return 'string_plural_form_settings';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getEditableConfigNames(): array {
+    return ['string_plural_form.settings'];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state): array {
+
+    $languages = $this->languageManager->getLanguages(LanguageInterface::STATE_CONFIGURABLE);
+    $rules = $this->stringPluralFormManager->getDefinitions();
+    $options = [];
+    foreach ($rules as $rule) {
+      $options[$rule['id']] = $rule['admin_label'];
+    }
+    $saved_rules = $this->config('string_plural_form.settings')->get('rules');
+    $rules_map = [];
+    foreach ($languages as $language) {
+      $rules_map[$language->getId()] = NULL;
+      if ($saved_rules && $saved_rules[$language->getId()]) {
+        $rules_map[$language->getId()] = $saved_rules[$language->getId()];
+      }
+    }
+    $form['rules'] = [
+      '#type' => 'container',
+      '#tree' => TRUE,
+    ];
+    foreach ($languages as $language) {
+      $form['rules'][$language->getId()] = [
+        '#type' => 'select',
+        '#title' => sprintf("%s (%s)", $language->getName(), $language->getId()),
+        '#default_value' => $rules_map[$language->getId()],
+        '#options' => $options,
+        '#required' => TRUE,
+      ];
+    }
+    return parent::buildForm($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state): void {
+    $rules = $form_state->getValue('rules');
+    $this->config('string_plural_form.settings')
+      ->set('rules', $rules)
+      ->save();
+    // @codingStandardsIgnoreLine
+    if (\Drupal::hasService('locale.plural.formula')) {
+      foreach ($rules as $langcode => $rule_id) {
+        /** @var \Drupal\string_plural_form\StringPluralFormPluginInterface $rule */
+        $rule = $this->stringPluralFormManager->createInstance($rule_id);
+        // @codingStandardsIgnoreLine
+        \Drupal::service('locale.plural.formula')
+          ->setPluralFormula($langcode, $rule->getGettextNplurals(), $rule->generateLocalePluralIndex());
+      }
+    }
+    parent::submitForm($form, $form_state);
+  }
+
+}

string_plural_form.links.menu.yml

+string_plural_form.settings:
+  title: String Plural Form
+  route_name: string_plural_form.settings
+  weight: 10

string_plural_form.routing.yml

+string_plural_form.settings:
+  path: '/admin/config/regional/language/string-plural-form'
+  defaults:
+    _title: 'String Plural Form'
+    _form: 'Drupal\string_plural_form\Form\SettingsForm'
+  requirements:
+    _permission: 'administer site configuration'