Initial commit

composer.json

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+{
+	"name": "drupal/simple_message",
+	"description": "Displays a simple message.",
+	"type": "drupal-module"
+}

config/schema/simple_message.schema.yml

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+simple_message.config:
+  type: config_object
+  label: Simple Message
+  mapping:
+    message:
+      type: text

simple_message.info.yml

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+name: Simple Message
+description: Displays a simple message.
+core_version_requirement: ^8 || ^9
+type: module
+package: Custom

simple_message.services.yml

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+services:
+  Drupal\Core\Config\ConfigFactoryInterface:
+    alias: config.factory
+
+  Drupal\Core\Messenger\MessengerInterface:
+    alias: messenger
+
+  Drupal\Core\Routing\AdminContext:
+    alias: router.admin_context
+
+  Drupal\simple_message\DisplaySimpleMessage:
+    autowire: true
+    tags:
+      - { name: event_subscriber }

src/DisplaySimpleMessage.php

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+<?php
+
+namespace Drupal\simple_message;
+
+use Drupal\Core\Routing\AdminContext;
+use Drupal\Core\Messenger\MessengerInterface;
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Symfony\Component\HttpKernel\KernelEvents;
+use Symfony\Component\HttpKernel\Event\GetResponseEvent;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+final class DisplaySimpleMessage implements EventSubscriberInterface {
+
+  use StringTranslationTrait;
+
+  private $messenger;
+  private $adminContext;
+  private $config;
+
+  public function __construct(
+    MessengerInterface $messenger,
+    AdminContext $adminContext,
+    ConfigFactoryInterface $configFactory
+  ) {
+    $this->messenger = $messenger;
+    $this->adminContext = $adminContext;
+    $this->config = $configFactory->get('simple_message.config');
+  }
+
+  public function displayMessage(GetResponseEvent $event) {
+    if ($this->adminContext->isAdminRoute()) {
+      return;
+    }
+
+    if ($message = $this->config->get('message')) {
+      $this->messenger->addMessage($this->t($message));
+    }
+  }
+
+  /**
+   * @inheritDoc
+   */
+  public static function getSubscribedEvents() {
+    $events[KernelEvents::REQUEST][] = ['displayMessage'];
+
+    return $events;
+  }
+
+}