class TestDisplayVariant

class TestDisplayVariant extends VariantBase implements PageVariantInterface, ContextAwareVariantInterface {
    
    /**
     * The render array representing the main page content.
     *
     * @var array
     */
    protected $mainContent = [];
    
    /**
     * The page title: a string (plain title) or a render array (formatted title).
     *
     * @var string|array
     */
    protected $title = '';
    
    /**
     * An array of collected contexts.
     *
     * This is only used on runtime, and is not stored.
     *
     * @var \Drupal\Component\Plugin\Context\ContextInterface[]
     */
    protected $contexts = [];
    
    /**
     * Gets the contexts.
     *
     * @return \Drupal\Component\Plugin\Context\ContextInterface[]
     *   An array of set contexts, keyed by context name.
     */
    public function getContexts() {
        return $this->contexts;
    }
    
    /**
     * Sets the contexts.
     *
     * @param \Drupal\Component\Plugin\Context\ContextInterface[] $contexts
     *   An array of contexts, keyed by context name.
     *
     * @return $this
     */
    public function setContexts(array $contexts) {
        $this->contexts = $contexts;
        return $this;
    }
    
    /**
     * {@inheritdoc}
     */
    public function setMainContent(array $main_content) {
        assert(!empty($this->getConfiguration()['required_configuration']), 'Ensure that ::setMainContent() is called with the variant configuration');
        $this->mainContent = $main_content;
        return $this;
    }
    
    /**
     * {@inheritdoc}
     */
    public function setTitle($title) {
        assert(!empty($this->getConfiguration()['required_configuration']), 'Ensure that ::setTitle() is called with the variant configuration');
        $this->title = $title;
        return $this;
    }
    
    /**
     * {@inheritdoc}
     */
    public function build() {
        $config = $this->getConfiguration();
        if (empty($config['required_configuration'])) {
            throw new \Exception('Required configuration is missing!');
        }
        $contexts = $this->getContexts();
        if (!isset($contexts['context'])) {
            throw new \Exception('Required context is missing!');
        }
        $build = [];
        $build['content']['default'] = [
            '#markup' => $config['required_configuration'] . ' ' . $contexts['context']->getContextValue(),
        ];
        CacheableMetadata::createFromObject($this)->applyTo($build);
        return $build;
    }

}