Create some handy twig global variables and functions to assist with theming

domain/domain.services.yml

@@ -39,3 +39,8 @@ services:
   domain.loader:
     class: Drupal\domain\DomainLoader
     arguments: ['@config.typed', '@config.factory']
+  domain.twig_extension:
+    class: Drupal\domain\TwigExtension
+    arguments: ['@domain.negotiator']
+    tags:
+      - {name: twig.extension}

domain/src/TwigExtension.php

@@ -0,0 +1,63 @@
+<?php
+
+namespace Drupal\domain;
+
+/**
+ * Class TwigExtension
+ *
+ * @package Drupal\domain
+ */
+class TwigExtension extends \Twig_Extension {
+
+  /**
+   * @var \Drupal\domain\DomainNegotiatorInterface
+   */
+  protected $domainNegotiator;
+
+  /**
+   * TwigExtension constructor.
+   *
+   * @param \Drupal\domain\DomainNegotiatorInterface $domainNegotiator
+   *   Domain negotiator to work out the current domain.
+   */
+  public function __construct(DomainNegotiatorInterface $domainNegotiator) {
+    $this->domainNegotiator = $domainNegotiator;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getGlobals() {
+    if ($domain = $this->domainNegotiator->getActiveDomain()) {
+      return [
+        'domain_active' => $domain,
+        'domain_active_id' => $domain->id(),
+      ];
+    }
+    else {
+      return [];
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFunctions() {
+    return [
+      new \Twig_SimpleFunction('is_domain', [$this, 'isDomain']),
+    ];
+  }
+
+  /**
+   * Check of the current domain id.
+   *
+   * @param $domain_id
+   *   Id of the domain to check.
+   *
+   * @return bool
+   *   if the id passed in a the current domain.
+   */
+  public function isDomain($domain_id) {
+    return $this->domainNegotiator->getActiveId() == $domain_id;
+  }
+}