Issue #3183523: How to display unread messages in a view ?

src/Plugin/views/filter/PrivateMessageThreadIsUnread.php

+<?php
+
+/**
+ * @file
+ * Definition of Drupal\private_message\Plugin\views\filter\PrivateMessageThreadIsUnread.
+ */
+
+namespace Drupal\private_message\Plugin\views\filter;
+
+use Drupal\Core\Session\AccountInterface;
+use Drupal\views\Plugin\views\filter\FilterPluginBase;
+use Drupal\views\Plugin\ViewsHandlerManager;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Filters threads by the fact they are unread.
+ *
+ * @ingroup views_filter_handlers
+ *
+ * @ViewsFilter("private_message_thread_is_unread")
+ */
+class PrivateMessageThreadIsUnread extends FilterPluginBase {
+
+  /**
+   * The current user.
+   *
+   * @var \Drupal\Core\Session\AccountInterface
+   */
+  protected $currentUser;
+
+  /**
+   * Views Handler Plugin Manager.
+   *
+   * @var \Drupal\views\Plugin\ViewsHandlerManager
+   */
+  protected $joinHandler;
+
+  /**
+   * Creates an instance of PrivateMessageThreadCleanHistory.
+   */
+  public function __construct(
+    array $configuration,
+    $plugin_id,
+    $plugin_definition,
+    AccountInterface $current_user,
+    ViewsHandlerManager $join_handler
+  ) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+    $this->currentUser = $current_user;
+    $this->joinHandler = $join_handler;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    return new static(
+      $configuration,
+      $plugin_id,
+      $plugin_definition,
+      $container->get('current_user'),
+      $container->get('plugin.manager.views.join')
+    );
+  }
+
+  /**
+   * Override the views query.
+   */
+  public function query() {
+    $current_user_id = $this->currentUser->id();
+
+    $definition = [
+      'table' => 'pm_thread_history',
+      'field' => 'thread_id',
+      'left_table' => 'private_message_threads',
+      'left_field' => 'id',
+      'operator' => '=',
+      'extra' => 'pm_thread_history.access_timestamp < private_message_threads.updated',
+    ];
+
+    $join = $this->joinHandler->createInstance('standard', $definition);
+    $this->query->addRelationship('pm_thread_history', $join, 'access_timestamp');
+    $this->query->addWhere(NULL, 'pm_thread_history.uid', $current_user_id);
+
+    if (!empty($this->value)) {
+      parent::query();
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function adminSummary() {}
+
+  /**
+   * {@inheritdoc}
+   */
+  public function canExpose() {
+    return FALSE;
+  }
+
+}