class Context

class Context implements ContextInterface {
    
    /**
     * The value of the context.
     *
     * @var mixed
     */
    protected $contextValue;
    
    /**
     * The definition to which a context must conform.
     *
     * @var \Drupal\Component\Plugin\Context\ContextDefinitionInterface
     */
    protected $contextDefinition;
    
    /**
     * Create a context object.
     *
     * @param \Drupal\Component\Plugin\Context\ContextDefinitionInterface $context_definition
     *   The context definition.
     * @param mixed|null $context_value
     *   The value of the context.
     */
    public function __construct(ContextDefinitionInterface $context_definition, $context_value = NULL) {
        $this->contextDefinition = $context_definition;
        $this->contextValue = $context_value;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getContextValue() {
        // Support optional contexts.
        if (!isset($this->contextValue)) {
            $definition = $this->getContextDefinition();
            $default_value = $definition->getDefaultValue();
            if (!isset($default_value) && $definition->isRequired()) {
                $type = $definition->getDataType();
                throw new ContextException(sprintf("The %s context is required and not present.", $type));
            }
            // Keep the default value here so that subsequent calls don't have to look
            // it up again.
            $this->contextValue = $default_value;
        }
        return $this->contextValue;
    }
    
    /**
     * {@inheritdoc}
     */
    public function hasContextValue() {
        return $this->contextValue !== NULL || $this->getContextDefinition()
            ->getDefaultValue() !== NULL;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getContextDefinition() {
        return $this->contextDefinition;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getConstraints() {
        if (empty($this->contextDefinition['class'])) {
            throw new ContextException("An error was encountered while trying to validate the context.");
        }
        return [
            new Type($this->contextDefinition['class']),
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function validate() {
        $validator = Validation::createValidatorBuilder()->getValidator();
        return $validator->validateValue($this->getContextValue(), $this->getConstraints());
    }

}