WIP: First sketch of the OG context module.

modules/og_context/config/install/og_context.ogcontextneogation.yml

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+og_context:

modules/og_context/config/schema/og_context_config.schema.yml

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+og_context.og_context_config.*:
+  type: config_entity
+  label: 'OG context config config'
+  mapping:
+    id:
+      type: string
+      label: 'ID'
+    label:
+      type: label
+      label: 'Label'
+    uuid:
+      type: string

modules/og_context/og_context.info.yml

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+name: OG context
+type: module
+description: Get a group from a viewed page.
+core: 8.x
+package: OG
+dependencies:
+  - og_ui

modules/og_context/og_context.links.menu.yml

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+# OG context config menu items definition
+entity.og_context_config.collection:
+  title: 'OG context'
+  route_name: entity.og_context_config.collection
+  description: 'Manage order of OG context plugins.'
+  parent: og_ui.admin_index
+  weight: 99

modules/og_context/og_context.services.yml

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+services:
+  plugin.manager.og.context:
+    class: Drupal\og_context\Plugin\OgContextManager
+    parent: default_plugin_manager

modules/og_context/src/Annotation/OgContext.php

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+<?php
+
+namespace Drupal\og_context\Annotation;
+
+use Drupal\Component\Annotation\Plugin;
+
+/**
+ * Defines a OG context item annotation object.
+ *
+ * @see \Drupal\og_context\Plugin\OgContextManager
+ * @see plugin_api
+ *
+ * @Annotation
+ */
+class OgContext extends Plugin {
+
+
+  /**
+   * The plugin ID.
+   *
+   * @var string
+   */
+  public $id;
+
+  /**
+   * The label of the plugin.
+   *
+   * @var \Drupal\Core\Annotation\Translation
+   *
+   * @ingroup plugin_translatable
+   */
+  public $label;
+
+}

modules/og_context/src/Entity/OgContextConfig.php

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+<?php
+
+namespace Drupal\og_context\Entity;
+
+use Drupal\Core\Config\Entity\ConfigEntityBase;
+
+/**
+ * Defines the OG context config entity.
+ *
+ * @ConfigEntityType(
+ *   id = "og_context_config",
+ *   label = @Translation("OG context config"),
+ *   handlers = {
+ *     "list_builder" = "Drupal\og_context\OgContextConfigListBuilder",
+ *     "route_provider" = {
+ *       "html" = "Drupal\og_context\OgContextConfigHtmlRouteProvider",
+ *     },
+ *   },
+ *   config_prefix = "og_context_config",
+ *   admin_permission = "administer site configuration",
+ *   entity_keys = {
+ *     "id" = "id",
+ *     "label" = "label",
+ *     "uuid" = "uuid"
+ *   },
+ *   links = {
+ *     "collection" = "/admin/config/og/og_context_config"
+ *   }
+ * )
+ */
+class OgContextConfig extends ConfigEntityBase implements OgContextConfigInterface {
+
+  /**
+   * The OG context config ID.
+   *
+   * @var string
+   */
+  protected $id;
+
+  /**
+   * The OG context config label.
+   *
+   * @var string
+   */
+  protected $label;
+
+}

modules/og_context/src/Entity/OgContextConfigInterface.php

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+<?php
+
+namespace Drupal\og_context\Entity;
+
+use Drupal\Core\Config\Entity\ConfigEntityInterface;
+
+/**
+ * Provides an interface for defining OG context config entities.
+ */
+interface OgContextConfigInterface extends ConfigEntityInterface {
+
+  // Add get/set methods for your configuration properties here.
+}

modules/og_context/src/Form/OgContextConfigForm.php

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+<?php
+
+namespace Drupal\og_context\Form;
+
+use Drupal\Core\Entity\EntityForm;
+use Drupal\Core\Form\FormStateInterface;
+
+/**
+ * Class OgContextConfigForm.
+ *
+ * @package Drupal\og_context\Form
+ */
+class OgContextConfigForm extends EntityForm {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function form(array $form, FormStateInterface $form_state) {
+    $form = parent::form($form, $form_state);
+
+    $og_context_config = $this->entity;
+    $form['label'] = array(
+      '#type' => 'textfield',
+      '#title' => $this->t('Label'),
+      '#maxlength' => 255,
+      '#default_value' => $og_context_config->label(),
+      '#description' => $this->t("Label for the OG context config."),
+      '#required' => TRUE,
+    );
+
+    $form['id'] = array(
+      '#type' => 'machine_name',
+      '#default_value' => $og_context_config->id(),
+      '#machine_name' => array(
+        'exists' => '\Drupal\og_context\Entity\OgContextConfig::load',
+      ),
+      '#disabled' => !$og_context_config->isNew(),
+    );
+
+    /* You will need additional form elements for your custom properties. */
+
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function save(array $form, FormStateInterface $form_state) {
+    $og_context_config = $this->entity;
+    $status = $og_context_config->save();
+
+    switch ($status) {
+      case SAVED_NEW:
+        drupal_set_message($this->t('Created the %label OG context config.', [
+          '%label' => $og_context_config->label(),
+        ]));
+        break;
+
+      default:
+        drupal_set_message($this->t('Saved the %label OG context config.', [
+          '%label' => $og_context_config->label(),
+        ]));
+    }
+    $form_state->setRedirectUrl($og_context_config->urlInfo('collection'));
+  }
+
+}

modules/og_context/src/Form/OgContextNeogation.php

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+<?php
+
+namespace Drupal\og_context\Form;
+
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Drupal\Core\Entity\EntityManager;
+
+/**
+ * Class OgContextNeogation.
+ *
+ * @package Drupal\og_context\Form
+ */
+class OgContextNeogation extends FormBase {
+
+  /**
+   * Drupal\Core\Entity\EntityManager definition.
+   *
+   * @var Drupal\Core\Entity\EntityManager
+   */
+  protected $entityManager;
+  public function __construct(
+    EntityManager $entity_manager
+  ) {
+    $this->entityManager = $entity_manager;
+  }
+
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('entity.manager')
+    );
+  }
+
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'og_context_neogation';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+
+  }
+
+}

modules/og_context/src/OgContext.php

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+<?php
+
+namespace Drupal\og_context;
+
+use Drupal\og_context\Plugin\OgContextBase;
+use Drupal\og_context\Plugin\OgContextManager;
+
+class OgContext {
+
+  /**
+   * Get a instance of an OG context plugin.
+   *
+   * @param $plugin
+   *   The plugin ID.
+   *
+   * @return OgContextBase
+   */
+  static public function getPlugin($plugin) {
+    /** @var OgContextManager $service */
+    $service = \Drupal::service('plugin.manager.og.context');
+
+    return $service->createInstance($plugin);
+  }
+
+}

modules/og_context/src/OgContextConfigHtmlRouteProvider.php

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+<?php
+
+namespace Drupal\og_context;
+
+use Drupal\Core\Entity\EntityTypeInterface;
+use Drupal\Core\Entity\Routing\AdminHtmlRouteProvider;
+use Symfony\Component\Routing\Route;
+
+/**
+ * Provides routes for OG context config entities.
+ *
+ * @see Drupal\Core\Entity\Routing\AdminHtmlRouteProvider
+ * @see Drupal\Core\Entity\Routing\DefaultHtmlRouteProvider
+ */
+class OgContextConfigHtmlRouteProvider extends AdminHtmlRouteProvider {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getRoutes(EntityTypeInterface $entity_type) {
+    $collection = parent::getRoutes($entity_type);
+
+    $entity_type_id = $entity_type->id();
+
+    if ($collection_route = $this->getCollectionRoute($entity_type)) {
+      $collection->add("entity.{$entity_type_id}.collection", $collection_route);
+    }
+
+    return $collection;
+  }
+
+  /**
+   * Gets the collection route.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeInterface $entity_type
+   *   The entity type.
+   *
+   * @return \Symfony\Component\Routing\Route|null
+   *   The generated route, if available.
+   */
+  protected function getCollectionRoute(EntityTypeInterface $entity_type) {
+    if ($entity_type->hasLinkTemplate('collection') && $entity_type->hasListBuilderClass()) {
+      $entity_type_id = $entity_type->id();
+      $route = new Route($entity_type->getLinkTemplate('collection'));
+      $route
+        ->setDefaults([
+          '_entity_list' => $entity_type_id,
+          // Make sure this is not a TranslatableMarkup object as the
+          // TitleResolver translates this string again.
+          '_title' => (string) $entity_type->getLabel(),
+        ])
+        ->setRequirement('_permission', $entity_type->getAdminPermission())
+        ->setOption('_admin_route', TRUE);
+
+      return $route;
+    }
+  }
+
+}

modules/og_context/src/OgContextConfigListBuilder.php

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+<?php
+
+namespace Drupal\og_context;
+
+use Drupal\Core\Config\Entity\ConfigEntityListBuilder;
+use Drupal\Core\Entity\EntityInterface;
+
+/**
+ * Provides a listing of OG context config entities.
+ */
+class OgContextConfigListBuilder extends ConfigEntityListBuilder {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildHeader() {
+    $header['label'] = $this->t('OG context config');
+    $header['id'] = $this->t('Machine name');
+    return $header + parent::buildHeader();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildRow(EntityInterface $entity) {
+    $row['label'] = $entity->label();
+    $row['id'] = $entity->id();
+    // You probably want a few more properties here...
+    return $row + parent::buildRow($entity);
+  }
+
+}

modules/og_context/src/Plugin/OgContext/Enitty.php

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+<?php
+
+namespace Drupal\og_context\Plugin\OgContext;
+
+use Drupal\og_context\Plugin\OgContextBase;
+
+/**
+ * @OgContext(
+ *  id = "entity",
+ *  title = "Entity base",
+ *  description = @Translation("Get the group from the current entity.")
+ * )
+ */
+class Entity extends OgContextBase {
+
+}