cc: Adding a BeginFrameObserverMultiplexer.

cc/scheduler/begin_frame_source.h

Index: cc/scheduler/begin_frame_source.h
diff --git a/cc/scheduler/begin_frame_source.h b/cc/scheduler/begin_frame_source.h
index d845dacd8e417505e27b3663246ae38c53234285..a1b7c20b0afecda7fe8b11996898662a1cfe409c 100644
--- a/cc/scheduler/begin_frame_source.h
+++ b/cc/scheduler/begin_frame_source.h
@@ -86,6 +86,37 @@ class CC_EXPORT BeginFrameObserverMixIn : public BeginFrameObserver {
   int64_t dropped_begin_frame_args_;
 };
 
+// A "virtual" frame observer which forwards any received BeginFrame messages
+// to all registered observers. Also provides a callback allowing detection of
+// observers existing.
+class CC_EXPORT BeginFrameObserverMultiplexer : public BeginFrameObserverMixIn {

[simonhong, 2015/03/25 02:03:22]

As dana said, how about subclassing BeginFrameObserver directly?
I think OnBeginFrameMixInDelegate() can be replaced with
BeginFrameObserver::OnBeginFrame().

[mithro-old, 2015/03/25 02:34:47]

Implementing BeginFrameObserver means duplicating the code inside the
BeginFrameObserverMixIn. Having two places which implement identical logic
substantially increases the risk of them getting out of sync leading to subtle
bugs which are hard to debug.

Using inheritance here should makes sure you don't forget to maintain the
BeginFrameObserver contract by not updating the LastBeginFrameArgs output.

An alternative to using inheritance here would be to make the code in
BFSOberverMixIn some type of "helper library" but we would need to be vigilant
in making sure it always used.

+ public:
+  using HasObserversCallback = base::Callback<void(bool has_observers)>;
+
+  static scoped_ptr<BeginFrameObserverMultiplexer> Create();
+  ~BeginFrameObserverMultiplexer() override;
+
+  void AddObserver(BeginFrameObserver* obs);
+  void RemoveObserver(BeginFrameObserver* obs);
+  bool HasObservers();
+  void SetHasObserverCallback(const HasObserversCallback& cb);
+
+  void AsValueInto(base::trace_event::TracedValue* dict) const override;
+
+ protected:
+  BeginFrameObserverMultiplexer();
+
+  bool last_callback_state_;
+  HasObserversCallback has_observers_callback_;
+  void CallCallbackOnChange();
+  void CallCallback(bool has_observers);
+
+  // BeginFrameObserverMixIn
+  bool OnBeginFrameMixInDelegate(const BeginFrameArgs& args) override;
+
+  ObserverList<BeginFrameObserver> observer_list_;
+};
+
 // Interface for a class which produces BeginFrame calls to a
 // BeginFrameObserver.
 //