ContextHandler.php

<?php
/**
 * @file
 * Contains \Drupal\block_page\ContextHandler.php.
 */

namespace Drupal\block_page;

use Drupal\Component\Plugin\PluginManagerInterface;
use Drupal\Core\TypedData\DataDefinitionInterface;
use Drupal\Core\TypedData\DataDefinition;
use Drupal\Core\TypedData\TypedDataManager;

class ContextHandler {

    /**
     * @var \Drupal\Core\TypedData\TypedDataManager
     */
    protected $typed_data;

    public function __construct(TypedDataManager $typed_data) {
        $this->typed_data = $typed_data;
    }

    public function checkRequirements(array $contexts, array $requirements) {
        $results = array();
        foreach ($requirements as $name => $requirement) {
            if (!$requirement instanceof DataDefinitionInterface) {
                // @todo make a custom exception class.
                throw new \Exception(sprintf('The %s requirement must be an instance of DataDefinitionInterface.', $name));
            }
            if ($requirement->isRequired()) {
                foreach ($contexts as $context) {
                    if (!$context instanceof DataDefinitionInterface) {
                        // @todo make a custom exception class.
                        throw new \Exception('An illegal context was passed, all contexts must be an instance of DataDefinitionInterface.');
                    }
                    if ($requirement->getDataType() == $context->getDataType()) {
                        foreach ($requirement->getConstraints() as $constraint_name => $constraint) {
                            if ($context->getConstraint($constraint_name) != $constraint) {
                                continue 2;
                            }
                        }
                    }
                    $results[$name] = TRUE;
                }
            }
            else {
                $results[$name] = TRUE;
            }
        }
        return count($requirements) == count($results);
    }

    public function getAvailablePlugins(array $contexts, PluginManagerInterface $manager) {
        $plugins = $manager->getDefinitions();
        $available_plugins = array();
        foreach ($plugins as $plugin_id => $plugin) {
            if (isset($plugin['context'])) {
                $plugin_contexts = $plugin['context'];
                $requirements = array();
                foreach ($plugin_contexts as $context_id => $plugin_context) {
                    $definition = $this->typed_data->getDefinition($plugin_context['type']);
                    if (isset($plugin_context['constraints'])) {
                        if (!isset($definition['constraints'])) {
                            $definition['constraints'] = $plugin_context['constraints'];
                        }
                        else {
                            $definition['constraints'] += $plugin_context['constraints'];
                        }
                    }
                    if (!isset($definition['required'])) {
                        $definition['required'] = TRUE;
                    }
                    $requirements[$context_id] = new DataDefinition($definition);
                }
                if ($this->checkRequirements($contexts, $requirements)) {
                    $available_plugins[$plugin_id] = $plugin;
                }
            }
            else {
                $available_plugins[$plugin_id] = $plugin;
            }
        }
        return $available_plugins;
    }

    public function getValidContexts(array $contexts, DataDefinitionInterface $definition) {
        $valid = array();
        foreach ($contexts as $id => $context) {
            if (!$context instanceof DataDefinitionInterface) {
                // @todo make a custom exception class.
                throw new \Exception('An illegal context was passed, all contexts must be an instance of DataDefinitionInterface.');
            }
            if ($definition->getDataType() == $context->getDataType()) {
                foreach ($definition->getConstraints() as $constraint_name => $constraint) {
                    if ($context->getConstraint($constraint_name) != $constraint) {
                        continue 2;
                    }
                }
            }
            $valid[$id] = $context;
        }
        return $valid;
    }

}