class Context

1. 8.x drupal/core/lib/Drupal/Core/Plugin/Context/Context.php \Drupal\Core\Plugin\Context\Context
2. 8.x drupal/core/lib/Drupal/Component/Plugin/Context/Context.php \Drupal\Component\Plugin\Context\Context

ContextAwarePluginBase.php in drupal/core/lib/Drupal/Core/Plugin/ContextAwarePluginBase.php

Contains \Drupal\Core\Plugin\ContextAwarePluginBase

Page::buildOptionsForm in drupal/core/modules/views/lib/Drupal/views/Plugin/views/display/Page.php

Overrides \Drupal\views\Plugin\views\display\callbackPluginBase::buildOptionsForm().

views.display.schema.yml in drupal/core/modules/views/config/schema/views.display.schema.yml

drupal/core/modules/views/config/schema/views.display.schema.yml

class Context extends ComponentContext {

  /**
   * Overrides \Drupal\Component\Plugin\Context\Context::getContextValue().
   */
  public function getContextValue() {
    $typed_value = parent::getContextValue();

    // If the typed data is complex, pass it on as typed data. Else pass on its
    // plain value, such that e.g. a string will be directly returned as PHP
    // string.
    $is_complex = $typed_value instanceof ComplexDataInterface;
    if (!$is_complex && $typed_value instanceof ListInterface) {
      $is_complex = $typed_value[0] instanceof ComplexDataInterface;
    }

    // @todo We won't need the getType == entity check once #1868004 lands.
    if ($typed_value instanceof TypedDataInterface && (!$is_complex || $typed_value instanceof EntityWrapper)) {
      return $typed_value
        ->getValue();
    }
    return $typed_value;
  }

  /**
   * Implements \Drupal\Component\Plugin\Context\ContextInterface::setContextValue().
   */
  public function setContextValue($value) {

    // Make sure the value set is a typed data object.
    if (!empty($this->contextDefinition['type']) && !$value instanceof TypedDataInterface) {
      $value = \Drupal::typedData()
        ->create($this->contextDefinition, $value);
    }
    parent::setContextValue($value);
  }

  /**
   * Gets the context value as typed data object.
   *
   * parent::getContextValue() does not do all the processing required to
   * return plain value of a TypedData object. This class overrides that method
   * to return the appropriate values from TypedData objects, but the object
   * itself can be useful as well, so this method is provided to allow for
   * access to the TypedData object. Since parent::getContextValue() already
   * does all the processing we need, we simply proxy to it here.
   *
   * @return \Drupal\Core\TypedData\TypedDataInterface
   */
  public function getTypedContext() {
    return parent::getContextValue();
  }

  /**
   * Implements \Drupal\Component\Plugin\Context\ContextInterface::getConstraints().
   */
  public function getConstraints() {
    if (!empty($this->contextDefinition['type'])) {

      // If we do have typed data, leverage it for getting constraints.
      return $this
        ->getTypedContext()
        ->getConstraints();
    }
    return parent::getConstraints();
  }

  /**
   * Overrides \Drupal\Component\Plugin\Context\Context::getConstraints().
   */
  public function validate() {
    $validator = Validation::createValidatorBuilder()
      ->setTranslator(new DrupalTranslator())
      ->getValidator();

    // @todo We won't need to special case "entity" here once #1868004 lands.
    if (!empty($this->contextDefinition['type']) && $this->contextDefinition['type'] == 'entity') {
      $value = $this
        ->getTypedContext();
    }
    else {
      $value = $this
        ->getContextValue();
    }
    return $validator
      ->validateValue($value, $this
      ->getConstraints());
  }

}