HandlerStack example

$resource_fetcher->fetchResource(Argument::any())
      ->willReturn($resource);
    $this->container->set('media.oembed.resource_fetcher', $resource_fetcher->reveal());

    // The source plugin will try to fetch the remote thumbnail, so mock the     // HTTP client to ensure that request returns a response with some valid     // image data.     $data = Utils::tryFopen($this->getDrupalRoot() . '/core/misc/druplicon.png', 'r');
    $response = new Response(200, $thumbnail_headers, Utils::streamFor($data));
    $handler = new MockHandler([$response]);
    $client = new Client([
      'handler' => new HandlerStack($handler),
    ]);
    $this->container->set('http_client', $client);

    $media_type = $this->createMediaType('oembed:video');
    $source = $media_type->getSource();

    // Add some HTML to the global site slogan, and use the site:slogan token in     // the thumbnail path, in order to prove that the final thumbnail path is     // stripped of HTML tags, and XML entities are decoded.     $this->config('system.site')
      ->set('slogan', '<h1>this&amp;that</h1>')