setBuild example

$block_manager = $this->prophesize(BlockManagerInterface::class);
    $block_manager->createInstance('some_block_id', ['id' => 'some_block_id'])->willReturn($existing_block->reveal());

    // Imitate an event subscriber by setting a resulting build on the event.     $event_dispatcher = $this->prophesize(EventDispatcherInterface::class);
    $event_dispatcher
      ->dispatch(Argument::type(SectionComponentBuildRenderArrayEvent::class), LayoutBuilderEvents::SECTION_COMPONENT_BUILD_RENDER_ARRAY)
      ->shouldBeCalled()
      ->will(function D$args) {
        /** @var \Drupal\layout_builder\Event\SectionComponentBuildRenderArrayEvent $event */
        $event = $args[0];
        $event->setBuild(['#markup' => $event->getPlugin()->getPluginId()]);
        return $event;
      });

    $layout_plugin = $this->prophesize(LayoutInterface::class);
    $layout_plugin->build(Argument::type('array'))->willReturnArgument(0);

    $layout_manager = $this->prophesize(LayoutPluginManagerInterface::class);
    $layout_manager->createInstance('layout_onecol', [])->willReturn($layout_plugin->reveal());

    $container = new ContainerBuilder();
    $container->set('plugin.manager.block', $block_manager->reveal());
    

        // @todo Use new label methods so         // data-layout-content-preview-placeholder-label doesn't have to use         // preview fallback in https://www.drupal.org/node/2025649.         $build['#attributes']['data-layout-content-preview-placeholder-label'] = $preview_fallback_string;

        if ($is_content_empty && $is_placeholder_ready) {
          $build['content']['#markup'] = $this->t('Placeholder for the @preview_fallback', ['@preview_fallback' => $block->getPreviewFallbackString()]);
        }
      }

      $event->setBuild($build);
    }
  }

}