diff --git a/tests/src/Kernel/Controller/ViewerControllerTest.php b/tests/src/Kernel/Controller/ViewerControllerTest.php
new file mode 100644
index 00000000..13b67349
--- /dev/null
+++ b/tests/src/Kernel/Controller/ViewerControllerTest.php
@@ -0,0 +1,355 @@
+<?php
+
+/*
+ * Copyright the Collabora Online contributors.
+ *
+ * SPDX-License-Identifier: MPL-2.0
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\collabora_online\Kernel\Controller;
+
+use ColinODell\PsrTestLogger\TestLogger;
+use Drupal\collabora_online\Cool\CollaboraDiscoveryFetcherInterface;
+use Drupal\collabora_online\Exception\CollaboraNotAvailableException;
+use Drupal\collabora_online\Jwt\JwtTranscoderInterface;
+use Drupal\Core\Utility\Error;
+use Drupal\file\Entity\File;
+use Drupal\file\FileInterface;
+use Drupal\media\MediaInterface;
+use Drupal\Tests\collabora_online\Kernel\CollaboraKernelTestBase;
+use Drupal\user\UserInterface;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpFoundation\Response;
+
+/**
+ * @coversDefaultClass \Drupal\collabora_online\Controller\ViewerController
+ */
+class ViewerControllerTest extends CollaboraKernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'collabora_online_test',
+  ];
+
+  /**
+   * The user with access to perform operations.
+   *
+   * @var \Drupal\user\UserInterface
+   */
+  protected UserInterface $user;
+
+  /**
+   * The media where to perform operations.
+   *
+   * @var \Drupal\media\MediaInterface
+   */
+  protected MediaInterface $media;
+
+  /**
+   * The source file.
+   *
+   * @var \Drupal\file\FileInterface
+   */
+  protected FileInterface $file;
+
+  /**
+   * The test logger channel.
+   *
+   * @var \ColinODell\PsrTestLogger\TestLogger
+   */
+  protected TestLogger $logger;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $fetcher = $this->createMock(CollaboraDiscoveryFetcherInterface::class);
+    $file = dirname(__DIR__, 3) . '/fixtures/discovery.mimetypes.xml';
+    $xml = file_get_contents($file);
+    $fetcher->method('getDiscoveryXml')->willReturn($xml);
+    $this->container->set(CollaboraDiscoveryFetcherInterface::class, $fetcher);
+
+    $this->logger = new TestLogger();
+    \Drupal::service('logger.factory')->addLogger($this->logger);
+
+    $collabora_settings = \Drupal::configFactory()->getEditable('collabora_online.settings');
+    $cool = $collabora_settings->get('cool');
+    $cool['key_id'] = 'collabora';
+    $collabora_settings->set('cool', $cool)->save();
+
+    $this->media = $this->createMediaEntity('document');
+    $this->user = $this->createUser([
+      'access content',
+      'preview document in collabora',
+      'edit any document in collabora',
+    ]);
+    $fid = $this->media->getSource()->getSourceFieldValue($this->media);
+    $this->file = File::load($fid);
+
+    $this->setCurrentUser($this->user);
+  }
+
+  /**
+   * Tests successful requests.
+   *
+   * @covers ::editor
+   */
+  public function testEditor(): void {
+    // Requests to view the editor.
+    $request = $this->createRequest('view');
+    $this->assertResponseOk($request);
+    $request = $this->createRequest('view', write: TRUE);
+    $this->assertResponseOk($request);
+
+    // Requests to edit the editor.
+    $request = $this->createRequest('edit', write: TRUE);
+    $this->assertResponseOk($request);
+    $request = $this->createRequest('edit');
+    $this->assertResponseOk($request);
+  }
+
+  /**
+   * Tests requests with unavailable Collabora.
+   *
+   * @covers ::editor
+   */
+  public function testEditorCollaboraUnavailable(): void {
+    // Collabora is not available on tests, we need to restore the service.
+    $fetcher = $this->createMock(CollaboraDiscoveryFetcherInterface::class);
+    $this->container->set(CollaboraDiscoveryFetcherInterface::class, $fetcher);
+
+    // Requests to view the editor.
+    $request = $this->createRequest('view');
+    $this->assertBadRequestResponse(
+      'The Collabora Online editor/viewer is not available.',
+      $request,
+      "Collabora Online is not available.<br>\n" . Error::DEFAULT_ERROR_MESSAGE,
+    );
+
+    // Requests to edit the editor.
+    $request = $this->createRequest('edit');
+    $this->assertBadRequestResponse(
+      'The Collabora Online editor/viewer is not available.',
+      $request,
+      "Collabora Online is not available.<br>\n" . Error::DEFAULT_ERROR_MESSAGE,
+    );
+  }
+
+  /**
+   * Tests requests with a scheme not matching the Collabora client URL.
+   *
+   * @covers ::editor
+   */
+  public function testEditorMismatchScheme(): void {
+    // Requests to view the editor.
+    $request = $this->createRequest('view', 'https');
+    $this->assertBadRequestResponse(
+      'Viewer error: Protocol mismatch.',
+      $request,
+      "The current request uses 'https' url scheme, but the Collabora client url is 'http'.",
+    );
+
+    // Requests to edit the editor.
+    $request = $this->createRequest('edit', 'https');
+    $this->assertBadRequestResponse(
+      'Viewer error: Protocol mismatch.',
+      $request,
+      "The current request uses 'https' url scheme, but the Collabora client url is 'http'.",
+    );
+  }
+
+  /**
+   * Tests requests with a no viewer avaliable after request.
+   *
+   * @covers ::editor
+   */
+  public function testEditorNoViewer(): void {
+    $request = $this->createRequest('view');
+
+    // Mock transcoder to force fail.
+    $transcoder = $this->createMock(jwtTranscoderInterface::class);
+    $transcoder->method('encode')->willThrowException(new CollaboraNotAvailableException());
+    $this->container->set(jwtTranscoderInterface::class, $transcoder);
+    $this->expectException(CollaboraNotAvailableException::class);
+
+    // Requests to view the editor.
+    $this->assertBadRequestResponse(
+      'The Collabora Online editor/viewer is not available.',
+      $request,
+      "Cannot show the viewer/editor.<br>\n" . Error::DEFAULT_ERROR_MESSAGE,
+    );
+
+    // Requests to edit the editor.
+    $request = $this->createRequest('edit');
+    $this->assertBadRequestResponse(
+      'The Collabora Online editor/viewer is not available.',
+      $request,
+      [
+        'message' => "Cannot show the viewer/editor.<br>\n" . Error::DEFAULT_ERROR_MESSAGE,
+      ]
+    );
+  }
+
+  /**
+   * Creates a view/edit request.
+   *
+   * @param string $action
+   *   E.g. 'view' or 'edit'.
+   * @param string $scheme
+   *   The protocol used for the request.
+   * @param int|null $media_id
+   *   Media entity id, if different from the default.
+   * @param int|null $user_id
+   *   User id, if different from the default.
+   * @param bool $write
+   *   TRUE if write access is requested.
+   * @param array $token_payload
+   *   Explicit token payload values.
+   *   This can be used to cause a bad token.
+   *
+   * @return \Symfony\Component\HttpFoundation\Request
+   *   The request.
+   */
+  protected function createRequest(
+    string $action,
+    string $scheme = 'http',
+    ?int $media_id = NULL,
+    ?int $user_id = NULL,
+    bool $write = FALSE,
+    array $token_payload = [],
+  ): Request {
+    $media_id ??= (int) $this->media->id();
+    $user_id ??= (int) $this->user->id();
+    $uri = "$scheme://localhost/cool/$action/$media_id";
+    $token = $this->createAccessToken($media_id, $user_id, $write, $token_payload);
+    $parameters = [
+      'media' => $media_id,
+      'edit' => $write,
+      'access_token' => $token,
+    ];
+    return Request::create($uri, 'GET', $parameters);
+  }
+
+  /**
+   * Retrieves an encoded access token.
+   *
+   * @param int|null $fid
+   *   The file id.
+   * @param int|null $uid
+   *   The user id.
+   * @param bool $write
+   *   The write permission.
+   * @param array $payload
+   *   Explicit payload values.
+   *   This can be used to cause a bad token.
+   *
+   * @return string
+   *   The enconded token.
+   */
+  protected function createAccessToken(?int $fid = NULL, ?int $uid = NULL, bool $write = FALSE, array $payload = []): string {
+    /** @var \Drupal\collabora_online\Jwt\JwtTranscoderInterface $transcoder */
+    $transcoder = \Drupal::service(JwtTranscoderInterface::class);
+    $expire_timestamp = gettimeofday(TRUE) + 1000;
+    $payload += [
+      'fid' => (string) ($fid ?? $this->media->id()),
+      'uid' => (string) ($uid ?? $this->user->id()),
+      'wri' => $write,
+      'exp' => $expire_timestamp,
+    ];
+    return $transcoder->encode($payload, $expire_timestamp);
+  }
+
+  /**
+   * Asserts an sucessful response given a request.
+   *
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The request to perform.
+   * @param string $message
+   *   Message to distinguish this from other assertions.
+   */
+  protected function assertResponseOk(Request $request, string $message = ''): void {
+    $response = $this->handleRequest($request);
+
+    $this->assertEquals(Response::HTTP_OK, $response->getStatusCode(), $message);
+    $this->assertStringContainsString('iframe', $response->getContent(), $message);
+    $this->assertEquals('', $response->headers->get('Content-Type'), $message);
+  }
+
+  /**
+   * Asserts an bad request response given a request.
+   *
+   * @param string $expected_content
+   *   The expected content.
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The request to perform.
+   * @param array $expected_log
+   *   The expected log entry.
+   * @param string $message
+   *   Message to distinguish this from other assertions.
+   */
+  protected function assertBadRequestResponse(string $expected_content, Request $request, string $expected_log = '', string $message = ''): void {
+    $this->assertResponse(
+      Response::HTTP_BAD_REQUEST,
+      $expected_content,
+      'text/plain',
+      $request,
+      $message,
+    );
+
+    if ($expected_log) {
+      $this->assertTrue(
+        $this->logger->hasRecord($expected_log),
+        sprintf('The logger does not contain a record like: "%s".', $expected_log)
+      );
+      $this->logger->reset();
+    }
+  }
+
+  /**
+   * Asserts status code and content in a response given a request.
+   *
+   * @param int $expected_code
+   *   The expected response status code.
+   * @param string $expected_content
+   *   The expected response content.
+   * @param string $expected_content_type
+   *   The type of content of the response.
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The request to perform.
+   * @param string $message
+   *   Message to distinguish this from other assertions.
+   */
+  protected function assertResponse(int $expected_code, string $expected_content, string $expected_content_type, Request $request, string $message = ''): void {
+    $response = $this->handleRequest($request);
+
+    $this->assertEquals($expected_code, $response->getStatusCode(), $message);
+    $this->assertEquals($expected_content, $response->getContent(), $message);
+    $this->assertEquals($expected_content_type, $response->headers->get('Content-Type'), $message);
+  }
+
+  /**
+   * Handles a request and gets the response.
+   *
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   Incoming request.
+   *
+   * @return \Symfony\Component\HttpFoundation\Response
+   *   The response.
+   */
+  protected function handleRequest(Request $request): Response {
+    /** @var \Symfony\Component\HttpKernel\HttpKernelInterface $kernel */
+    $kernel = \Drupal::service('http_kernel');
+    return $kernel->handle($request);
+  }
+
+}