Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/lib/Drupal/Component/Plugin/Context/Context.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Component\Plugin\Context\Context.
+ */
+
+namespace Drupal\Component\Plugin\Context;
+
+use Drupal\Component\Plugin\Exception\ContextException;
+use Symfony\Component\Validator\Constraints\Type;
+use Symfony\Component\Validator\Validation;
+
+/**
+ * A generic context class for wrapping data a plugin needs to operate.
+ */
+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;
+
+  /**
+   * Sets the contextDefinition for us without needing to call the setter.
+   *
+   * @param \Drupal\Component\Plugin\Context\ContextDefinitionInterface $context_definition
+   *   The context definition.
+   */
+  public function __construct(ContextDefinitionInterface $context_definition) {
+    $this->contextDefinition = $context_definition;
+  }
+
+  /**
+   * Implements \Drupal\Component\Plugin\Context\ContextInterface::setContextValue().
+   */
+  public function setContextValue($value) {
+    $this->contextValue = $value;
+  }
+
+  /**
+   * Implements \Drupal\Component\Plugin\Context\ContextInterface::getContextValue().
+   */
+  public function getContextValue() {
+    // Support optional contexts.
+    if (!isset($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 setContextDefinition(ContextDefinitionInterface $context_definition) {
+    $this->contextDefinition = $context_definition;
+  }
+
+  /**
+   * Implements \Drupal\Component\Plugin\Context\ContextInterface::getContextDefinition().
+   */
+  public function getContextDefinition() {
+    return $this->contextDefinition;
+  }
+
+  /**
+   * Implements \Drupal\Component\Plugin\Context\ContextInterface::getConstraints().
+   */
+  public function getConstraints() {
+    if (empty($this->contextDefinition['class'])) {
+      throw new ContextException("An error was encountered while trying to validate the context.");
+    }
+    return array(new Type($this->contextDefinition['class']));
+  }
+
+  /**
+   * Implements \Drupal\Component\Plugin\Context\ContextInterface::validate().
+   */
+  public function validate() {
+    $validator = Validation::createValidatorBuilder()
+      ->getValidator();
+    return $validator->validateValue($this->getContextValue(), $this->getConstraints());
+  }
+
+}

core/lib/Drupal/Component/Plugin/Context/ContextDefinitionInterface.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Component\Plugin\Context\ContextDefinitionInterface.
+ */
+
+namespace Drupal\Component\Plugin\Context;
+
+/**
+ * Interface for context definitions.
+ *
+ * @todo WARNING: This interface is going to receive some additions as part of
+ * https://www.drupal.org/node/2346999.
+ */
+interface ContextDefinitionInterface {
+
+  /**
+   * Gets a human readable label.
+   *
+   * @return string
+   *   The label.
+   */
+  public function getLabel();
+
+  /**
+   * Sets the human readable label.
+   *
+   * @param string $label
+   *   The label to set.
+   *
+   * @return $this
+   */
+  public function setLabel($label);
+
+  /**
+   * Gets a human readable description.
+   *
+   * @return string|null
+   *   The description, or NULL if no description is available.
+   */
+  public function getDescription();
+
+  /**
+   * Sets the human readable description.
+   *
+   * @param string|null $description
+   *   The description to set.
+   *
+   * @return $this
+   */
+  public function setDescription($description);
+
+  /**
+   * Gets the data type needed by the context.
+   *
+   * If the context is multiple-valued, this represents the type of each value.
+   *
+   * @return string
+   *   The data type.
+   */
+  public function getDataType();
+
+  /**
+   * Sets the data type needed by the context.
+   *
+   * @param string $data_type
+   *   The data type to set.
+   *
+   * @return $this
+   */
+  public function setDataType($data_type);
+
+  /**
+   * Determines whether the data is multi-valued, i.e. a list of data items.
+   *
+   * @return bool
+   *   Whether the data is multi-valued; i.e. a list of data items.
+   */
+  public function isMultiple();
+
+  /**
+   * Sets whether the data is multi-valued.
+   *
+   * @param bool $multiple
+   *   (optional) Whether the data is multi-valued. Defaults to TRUE.
+   *
+   * @return $this
+   */
+  public function setMultiple($multiple = TRUE);
+
+  /**
+   * Determines whether the context is required.
+   *
+   * For required data a non-NULL value is mandatory.
+   *
+   * @return bool
+   *   Whether a data value is required.
+   */
+  public function isRequired();
+
+  /**
+   * Sets whether the data is required.
+   *
+   * @param bool $required
+   *   (optional) Whether the data is multi-valued. Defaults to TRUE.
+   *
+   * @return $this
+   */
+  public function setRequired($required = TRUE);
+
+  /**
+   * Gets the default value for this context definition.
+   *
+   * @return mixed
+   *   The default value or NULL if no default value is set.
+   */
+  public function getDefaultValue();
+
+  /**
+   * Sets the default data value.
+   *
+   * @param mixed $default_value
+   *   The default value to be set or NULL to remove any default value.
+   *
+   * @return $this
+   */
+  public function setDefaultValue($default_value);
+
+  /**
+   * Gets an array of validation constraints.
+   *
+   * @return array
+   *   An array of validation constraint definitions, keyed by constraint name.
+   *   Each constraint definition can be used for instantiating
+   *   \Symfony\Component\Validator\Constraint objects.
+   */
+  public function getConstraints();
+
+  /**
+   * Sets the array of validation constraints.
+   *
+   * NOTE: This will override any previously set constraints. In most cases
+   * ContextDefinitionInterface::addConstraint() should be used instead.
+   *
+   * @param array $constraints
+   *   The array of constraints.
+   *
+   * @return $this
+   *
+   * @see self::addConstraint()
+   */
+  public function setConstraints(array $constraints);
+
+  /**
+   * Adds a validation constraint.
+   *
+   * @param string $constraint_name
+   *   The name of the constraint to add, i.e. its plugin id.
+   * @param array|null $options
+   *   The constraint options as required by the constraint plugin, or NULL.
+   *
+   * @return $this
+   */
+  public function addConstraint($constraint_name, $options = NULL);
+
+  /**
+   * Gets a validation constraint.
+   *
+   * @param string $constraint_name
+   *   The name of the constraint, i.e. its plugin id.
+   *
+   * @return array
+   *   A validation constraint definition which can be used for instantiating a
+   *   \Symfony\Component\Validator\Constraint object.
+   */
+  public function getConstraint($constraint_name);
+
+}

core/lib/Drupal/Component/Plugin/Context/ContextInterface.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Component\Plugin\Context\ContextInterface.
+ */
+
+namespace Drupal\Component\Plugin\Context;
+
+/**
+ * A generic context interface for wrapping data a plugin needs to operate.
+ */
+interface ContextInterface {
+
+  /**
+   * Sets the context value.
+   *
+   * @param mixed $value
+   *   The value of this context, matching the context definition.
+   *
+   * @see \Drupal\Component\Plugin\Context\ContextInterface::setContextDefinition().
+   */
+  public function setContextValue($value);
+
+  /**
+   * Gets the context value.
+   *
+   * @return mixed
+   *   The currently set context value, or NULL if it is not set.
+   */
+  public function getContextValue();
+
+  /**
+   * Sets the definition that the context must conform to.
+   *
+   * @param \Drupal\Component\Plugin\Context\ContextDefinitionInterface $context_definition
+   *   A defining characteristic representation of the context against which
+   *   that context can be validated.
+   */
+  public function setContextDefinition(ContextDefinitionInterface $context_definition);
+
+  /**
+   * Gets the provided definition that the context must conform to.
+   *
+   * @return \Drupal\Component\Plugin\Context\ContextDefinitionInterface
+   *   The defining characteristic representation of the context.
+   */
+  public function getContextDefinition();
+
+  /**
+   * Gets a list of validation constraints.
+   *
+   * @return array
+   *   Array of constraints, each being an instance of
+   *   \Symfony\Component\Validator\Constraint.
+   */
+  public function getConstraints();
+
+  /**
+   * Validates the set context value.
+   *
+   * @return \Symfony\Component\Validator\ConstraintViolationListInterface
+   *   A list of constraint violations. If the list is empty, validation
+   *   succeeded.
+   */
+  public function validate();
+
+}