$plugin->setContextValue()

gistfile1.txt

  /**
   * Implements \Drupal\Component\Plugin\ContextAwarePluginInterface::setContextValue().
   */
  public function setContextValue($key, $value) {
    $context_definition = $this->getContextDefinition($key);
    $this->context[$key] = new Context($context_definition);
    $this->context[$key]->setValue($value);
    
    if ($this->context[$key]->validate($value)->count() > 0) {
      throw new PluginException("The provided value for $key is not valid.");
    }
    return $this;
  }