trait ContextAwarePluginAssignmentTrait

trait ContextAwarePluginAssignmentTrait {
    
    /**
     * Ensures the t() method is available.
     *
     * @see \Drupal\Core\StringTranslation\StringTranslationTrait
     */
    protected abstract function t($string, array $args = [], array $options = []);
    
    /**
     * Wraps the context handler.
     *
     * @return \Drupal\Core\Plugin\Context\ContextHandlerInterface
     */
    protected function contextHandler() {
        return \Drupal::service('context.handler');
    }
    
    /**
     * Builds a form element for assigning a context to a given slot.
     *
     * @param \Drupal\Core\Plugin\ContextAwarePluginInterface $plugin
     *   The context-aware plugin.
     * @param \Drupal\Component\Plugin\Context\ContextInterface[] $contexts
     *   An array of contexts.
     *
     * @return array
     *   A form element for assigning context.
     */
    protected function addContextAssignmentElement(ContextAwarePluginInterface $plugin, array $contexts) {
        $element = [];
        foreach ($plugin->getContextDefinitions() as $context_slot => $definition) {
            $valid_contexts = $this->contextHandler()
                ->getMatchingContexts($contexts, $definition);
            $options = [];
            foreach ($valid_contexts as $context_id => $context) {
                $element['#tree'] = TRUE;
                $options[$context_id] = $context->getContextDefinition()
                    ->getLabel();
                $element[$context_slot] = [
                    '#type' => 'value',
                    '#value' => $context_id,
                ];
            }
            // Show the context selector only if there is more than 1 option to choose
            // from. Also, show if there is a single option but the plugin does not
            // require a context.
            if (count($options) > 1 || count($options) == 1 && !$definition->isRequired()) {
                $assignments = $plugin->getContextMapping();
                $element[$context_slot] = [
                    '#title' => $definition->getLabel() ?: $this->t('Select a @context value:', [
                        '@context' => $context_slot,
                    ]),
                    '#type' => 'select',
                    '#options' => $options,
                    '#required' => $definition->isRequired(),
                    '#default_value' => !empty($assignments[$context_slot]) ? $assignments[$context_slot] : '',
                    '#description' => $definition->getDescription(),
                ];
                if (!$definition->isRequired()) {
                    $element[$context_slot]['#empty_value'] = '';
                }
            }
        }
        return $element;
    }

}