Used a helper to get the default site slug.

application/config/module.config.php

@@ -412,6 +412,7 @@
             'blockLayout' => Service\ViewHelper\BlockLayoutFactory::class,
             'blockThumbnailTypeSelect' => Service\ViewHelper\BlockThumbnailTypeSelectFactory::class,
             'dataType' => Service\ViewHelper\DataTypeFactory::class,
+            'defaultSiteSlug' => Service\ViewHelper\DefaultSiteSlugFactory::class,
             'i18n' => Service\ViewHelper\I18nFactory::class,
             'logger' => Service\ViewHelper\LoggerFactory::class,
             'media' => Service\ViewHelper\MediaFactory::class,

application/src/Service/ViewHelper/DefaultSiteSlugFactory.php

@@ -0,0 +1,33 @@
+<?php
+namespace Omeka\Service\ViewHelper;
+
+use Interop\Container\ContainerInterface;
+use Omeka\View\Helper\DefaultSiteSlug;
+use Zend\ServiceManager\Factory\FactoryInterface;
+
+/**
+ * Service factory to get the default site slug, or the first site slug.
+ *
+ * @todo Store the default site as slug instead of id?
+ * @todo Set a setting for the default site of the user?
+ */
+class DefaultSiteSlugFactory implements FactoryInterface
+{
+    /**
+     * Create and return the DefaultSiteSlug view helper.
+     *
+     * @return DefaultSiteSlug
+     */
+    public function __invoke(ContainerInterface $services, $requestedName, array $options = null)
+    {
+        $defaultSiteId = $services->get('Omeka\Settings')->get('default_site');
+        $api = $services->get('Omeka\ApiManager');
+        if ($defaultSiteId) {
+            $slugs = $api->search('sites', ['id' => $defaultSiteId], ['returnScalar' => 'slug'])->getContent();
+        } else {
+            $slugs = $api->search('sites', ['limit' => 1], ['returnScalar' => 'slug'])->getContent();
+        }
+        $defaultSiteSlug = (string) reset($slugs);
+        return new DefaultSiteSlug($defaultSiteSlug);
+    }
+}

application/src/Service/ViewHelper/PublicResourceUrlFactory.php

@@ -13,21 +13,13 @@
 class PublicResourceUrlFactory implements FactoryInterface
 {
     /**
-     * Create and return the PublicResourceUrlFactory view helper
+     * Create and return the PublicResourceUrl view helper
      *
-     * @return PublicResourceUrlFactory
+     * @return PublicResourceUrl
      */
     public function __invoke(ContainerInterface $services, $requestedName, array $options = null)
     {
-        // Get the slug for the default site, else the first one.
-        $defaultSiteId = $services->get('Omeka\Settings')->get('default_site');
-        $api = $services->get('Omeka\ApiManager');
-        if ($defaultSiteId) {
-            $slugs = $api->search('sites', ['id' => $defaultSiteId], ['returnScalar' => 'slug'])->getContent();
-        } else {
-            $slugs = $api->search('sites', ['limit' => 1], ['returnScalar' => 'slug'])->getContent();
-        }
-        $defaultSiteSlug = reset($slugs);
-        return new PublicResourceUrl($defaultSiteSlug);
+        $defaultSiteSlug = $services->get('ViewHelperManager')->get('defaultSiteSlug');
+        return new PublicResourceUrl($defaultSiteSlug());
     }
 }

application/src/View/Helper/DefaultSiteSlug.php

@@ -0,0 +1,35 @@
+<?php
+namespace Omeka\View\Helper;
+
+use Zend\View\Helper\AbstractHelper;
+
+/**
+ * View helper to get the default site slug, or the first one.
+ */
+class DefaultSiteSlug extends AbstractHelper
+{
+    /**
+     * @var string
+     */
+    protected $defaultSiteSlug;
+
+    /**
+     * Construct the helper.
+     *
+     * @param string|null $defaultSiteSlug
+     */
+    public function __construct($defaultSiteSlug)
+    {
+        $this->defaultSiteSlug = $defaultSiteSlug;
+    }
+
+    /**
+     * Return the default site slug, or the first one.
+     *
+     * @return string|null
+     */
+    public function __invoke()
+    {
+        return $this->defaultSiteSlug;
+    }
+}