Make GetDisplayedNotifications asynchronous.

content/public/browser/platform_notification_service.h

Index: content/public/browser/platform_notification_service.h
diff --git a/content/public/browser/platform_notification_service.h b/content/public/browser/platform_notification_service.h
index c1ad9ecd12d03f138b9723274bd68626e4583cad..00e9910017e96662a64a7148df0ccca748b4358a 100644
--- a/content/public/browser/platform_notification_service.h
+++ b/content/public/browser/platform_notification_service.h
@@ -33,6 +33,10 @@ class CONTENT_EXPORT PlatformNotificationService {
  public:
   virtual ~PlatformNotificationService() {}
 
+  using NotificationResultCallback =
+      base::Callback<void(std::unique_ptr<std::set<std::string>>,
+                          bool /* supports synchronization */)>;

[Peter Beverloo, 2017/03/15 18:07:51]

Something to consider: now that the first argument is an std::unique_ptr<>, we
can convey that it's not supported by passing a nullptr. (Since NULL and empty
are now semantically different.) That saves you a fair number of allocations in
places where it's not supported.

[Miguel Garcia, 2017/03/16 14:57:42]

I played with it a little because I thought it was a good idea, the problem I
ran myself into is that the callback and the underlying vector is passed around
many times and in everyone of them you'd have to check for null which is a pain.

+
   // Checks if |origin| has permission to display Web Notifications.
   // This method must only be called on the UI thread.
   virtual blink::mojom::PermissionStatus CheckPermissionOnUIThread(
@@ -78,12 +82,11 @@ class CONTENT_EXPORT PlatformNotificationService {
       BrowserContext* browser_context,
       const std::string& notification_id) = 0;
 
-  // Writes the ids of all currently displaying notifications for the
-  // given |browser_context| to |displayed_notifications|. Returns whether the
-  // platform is able to provide such a set.
-  virtual bool GetDisplayedNotifications(
+  // Retrieves Writes the ids of all currently displaying notifications and

[Peter Beverloo, 2017/03/15 18:07:51]

nit: s/Writes//

[Miguel Garcia, 2017/03/16 14:57:42]

Done.

+  // posts |callback| with the result.
+  virtual void GetDisplayedNotifications(
       BrowserContext* browser_context,
-      std::set<std::string>* displayed_notifications) = 0;
+      const NotificationResultCallback& callback) = 0;
 };
 
 }  // namespace content