getThemeHook example

$layout_definition = $this->layoutPluginManager->getDefinition('theme_a_provided_layout');
    $this->assertSame('theme_a_provided_layout', $layout_definition->id());
    $this->assertSame('2 column layout', (string) $layout_definition->getLabel());
    $this->assertSame('Columns: 2', (string) $layout_definition->getCategory());
    $this->assertSame('A theme provided layout', (string) $layout_definition->getDescription());
    $this->assertInstanceOf(TranslatableMarkup::class, $layout_definition->getLabel());
    $this->assertInstanceOf(TranslatableMarkup::class, $layout_definition->getCategory());
    $this->assertInstanceOf(TranslatableMarkup::class, $layout_definition->getDescription());
    $this->assertSame('twocol', $layout_definition->getTemplate());
    $this->assertSame('themes/theme_a/templates', $layout_definition->getPath());
    $this->assertSame('theme_a/twocol', $layout_definition->getLibrary());
    $this->assertSame('twocol', $layout_definition->getThemeHook());
    $this->assertSame('themes/theme_a/templates', $layout_definition->getTemplatePath());
    $this->assertSame('theme_a', $layout_definition->getProvider());
    $this->assertSame('right', $layout_definition->getDefaultRegion());
    $this->assertSame(LayoutDefault::class, $layout_definition->getClass());
    $expected_regions = [
      'left' => [
        'label' => new TranslatableMarkup('Left region', [], ['context' => 'layout_region']),
      ],
      'right' => [
        'label' => new TranslatableMarkup('Right region', [], ['context' => 'layout_region']),
      ],
    ];

public function build(array $regions) {
    // Ensure $build only contains defined regions and in the order defined.     $build = [];
    foreach ($this->getPluginDefinition()->getRegionNames() as $region_name) {
      if (array_key_exists($region_name, $regions)) {
        $build[$region_name] = $regions[$region_name];
      }
    }
    $build['#in_preview'] = $this->inPreview;
    $build['#settings'] = $this->getConfiguration();
    $build['#layout'] = $this->pluginDefinition;
    $build['#theme'] = $this->pluginDefinition->getThemeHook();
    if ($library = $this->pluginDefinition->getLibrary()) {
      $build['#attached']['library'][] = $library;
    }
    return $build;
  }

  /** * {@inheritdoc} */
  public function getConfiguration() {
    return $this->configuration;
  }

  public function getThemeImplementations() {
    $hooks = [];
    $hooks['layout'] = [
      'render element' => 'content',
    ];
    /** @var \Drupal\Core\Layout\LayoutDefinition[] $definitions */
    $definitions = $this->getDefinitions();
    foreach ($definitions as $definition) {
      if ($template = $definition->getTemplate()) {
        $hooks[$definition->getThemeHook()] = [
          'render element' => 'content',
          'base hook' => 'layout',
          'template' => $template,
          'path' => $definition->getTemplatePath(),
        ];
      }
    }
    return $hooks;
  }

  /** * {@inheritdoc} */