Lazy initialization for ProcessesEventRouter.

chrome/browser/extensions/event_router.h

Index: chrome/browser/extensions/event_router.h
diff --git a/chrome/browser/extensions/event_router.h b/chrome/browser/extensions/event_router.h
index cbfb1b80abaa432e118121b2660ffba3b6f55416..bcd59d44cdbaba698d7318534f4dcfdc9022978f 100644
--- a/chrome/browser/extensions/event_router.h
+++ b/chrome/browser/extensions/event_router.h
@@ -47,6 +47,16 @@ class EventRouter : public content::NotificationObserver,
     USER_GESTURE_NOT_ENABLED = 2,
   };
 
+  // Observers register interest in events with a particular name and are
+  // notified when a listener is added or removed for that |event_name|.
+  class Observer {
+   public:
+    // Called when a listener is added.
+    virtual void OnListenerAdded(const std::string& event_name) {}
+    // Called when a listener is removed.
+    virtual void OnListenerRemoved(const std::string& event_name) {}
+  };
+
   // Sends an event via ipc_sender to the given extension. Can be called on any
   // thread.
   static void DispatchEvent(IPC::Sender* ipc_sender,
@@ -73,6 +83,12 @@ class EventRouter : public content::NotificationObserver,
 
   EventListenerMap& listeners() { return listeners_; }
 
+  // Registers an observer, interested in listeners that receive the event
+  // |event_name|, to be notified when such a listener is added or removed.
+  // There can currently be only one observer for each distinct |event_name|.
+  void RegisterObserver(Observer* observer,

[Aaron Boodman, 2012/11/07 04:08:20]

Do we not need an UnregisterObserver? And if so, I wonder if we need something
like base/observer_list.h but in map form so that we can remove during
iteration.

[Yoyo Zhou, 2012/11/07 22:53:36]

After thinking about this, I do think UnregisterObserver is a good idea. I'll
make it of the form UnregisterObserver(observer) to unregister from all events.

Actually, I originally wanted to only support the use case of starting up event
routers lazily, so that the observers would get removed once OnListenerAdded was
fired the first time, but that turns out not to be enough. But for that case,
the goop class should unregister itself.

And yeah, ObserverList doesn't fit because it's not a map.

+                        const std::string& event_name);
+
   // Add or remove the extension as having a lazy background page that listens
   // to the event. The difference from the above methods is that these will be
   // remembered even after the process goes away. We use this list to decide
@@ -253,6 +269,9 @@ class EventRouter : public content::NotificationObserver,
 
   EventListenerMap listeners_;
 
+  typedef std::map<std::string, Observer*> ObserverMap;
+  ObserverMap observers_;
+
   // True if we should dispatch the event signalling that Chrome was updated
   // upon loading an extension.
   bool dispatch_chrome_updated_event_;