public function build() {
    $route_name = $this->routeMatch->getRouteName();
    if ($route_name !== 'acm_checkout.form') {
      return [];
    }

    // Load the CheckoutFlow plugin.
    $config = \Drupal::config('acm_checkout.settings');
    $checkout_flow_plugin = $config->get('checkout_flow_plugin') ?: 'multistep_default';
    $plugin_manager = \Drupal::service('plugin.manager.acm_checkout_flow');
    $checkout_flow = $plugin_manager->createInstance($checkout_flow_plugin, []);

    // Build the steps sent to the template.
    $steps = [];
    $visible_steps = array_filter($checkout_flow->getVisibleSteps(), function ($step_definition) {
      return (!isset($step_definition['hide_from_progress']));
    });
    $visible_step_ids = array_keys($visible_steps);
    $current_step_id = $checkout_flow->getStepId();
    $current_step_index = array_search($current_step_id, $visible_step_ids);

    // Get last step completed in the cart.
    $cart = $this->cartStorage;
    $cart_step_id = $cart->getCheckoutStep();
    $cart_step_index = array_search($cart_step_id, $visible_step_ids);

    $index = 0;
    foreach ($visible_steps as $step_id => $step_definition) {
      $is_link = FALSE;
      $completed = FALSE;
      if ($index < $current_step_index) {
        $position = 'previous';
      }
      elseif ($index == $current_step_index) {
        $position = 'current';
      }
      else {
        $position = 'next';
      }

      $label = $step_definition['label'];
      // Add a class if this step has been completed already.
      if ($index < $cart_step_index) {
        $completed = TRUE;
      }
      // Set the label to a link if this step has already been completed so that
      // the progress bar can be used as a sort of navigation.
      if ($index <= $cart_step_index && $index !== $current_step_index) {
        $is_link = TRUE;
        $label = Link::createFromRoute($label, 'acm_checkout.form', [
          'step' => $step_id,
        ])->toString();
      }

      $steps[] = [
        'id' => $step_id,
        'label' => $label,
        'position' => $position,
        'completed' => $completed,
        'is_link' => $is_link,
      ];

      $index++;
    }

    return [
      '#attached' => [
        'library' => ['acm_checkout/checkout_progress'],
      ],
      '#theme' => 'acm_checkout_progress',
      '#steps' => $steps,
    ];
  }