Make GetDisplayedNotifications asynchronous.

chrome/browser/notifications/notification_display_service.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_NOTIFICATIONS_NOTIFICATION_DISPLAY_SERVICE_H_
 #define CHROME_BROWSER_NOTIFICATIONS_NOTIFICATION_DISPLAY_SERVICE_H_
 
+#include <memory>
 #include <set>
 #include <string>
 
+#include "base/callback_forward.h"
 #include "base/macros.h"
 #include "chrome/browser/notifications/notification_common.h"
 #include "components/keyed_service/core/keyed_service.h"
 
 class Notification;
 class Profile;
 
 // Profile-bound service that enables notifications to be displayed and
 // interacted with on the user's screen, orthogonal of whether this
 // functionality is provided by the browser or by the operating system. An
 // instance can be retrieved through the NotificationDisplayServiceFactory.
 //
 // TODO(peter): Add a NotificationHandler mechanism for registering listeners.
-// TODO(miguelg): Remove the SupportsNotificationCenter method.
 class NotificationDisplayService : public KeyedService {
  public:
+  using DisplayedNotificationsCallback =
+      base::Callback<void(std::unique_ptr<std::set<std::string>>,
+                          bool /* supports_synchronization */)>;
   NotificationDisplayService() {}
   ~NotificationDisplayService() override {}
 
   // Displays the |notification| identified by |notification_id|.
   virtual void Display(NotificationCommon::Type notification_type,
                        const std::string& notification_id,
                        const Notification& notification) = 0;
 
   // Closes the notification identified by |notification_id|.
   virtual void Close(NotificationCommon::Type notification_type,
                      const std::string& notification_id) = 0;
 
-  // Returns whether the implementation can retrieve a list of currently visible
-  // notifications and stores them in |*notification_ids| when possible.
-  virtual bool GetDisplayed(std::set<std::string>* notifications) const = 0;
+  // Writes the ids of all currently displaying notifications and
+  // invokes |callback| with the result once known.
+  virtual void GetDisplayed(
+      const DisplayedNotificationsCallback& callback) const = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(NotificationDisplayService);
 };
 
 #endif  // CHROME_BROWSER_NOTIFICATIONS_NOTIFICATION_DISPLAY_SERVICE_H_