Example of how to interpose yourself in a message stream.

ipc/ipc_sync_channel.h

Index: ipc/ipc_sync_channel.h
===================================================================
--- ipc/ipc_sync_channel.h	(revision 83227)
+++ ipc/ipc_sync_channel.h	(working copy)
@@ -25,6 +25,7 @@
 
 class SyncMessage;
 class MessageReplyDeserializer;
+class OutgoingMessageFilter;
 
 // This is similar to ChannelProxy, with the added feature of supporting sending
 // synchronous messages.
@@ -90,6 +91,11 @@
   // of some particular channels to not re-enter in such cases.
   void SetRestrictDispatchToSameChannel(bool value);
 
+  // Sets this channel to rewrite its outgoing messages.
+  void set_outgoing_message_filter(OutgoingMessageFilter *filter) {
+    outgoing_message_filter_ = filter;
+  }
+
  protected:
   class ReceivedSyncMsgQueue;
   friend class ReceivedSyncMsgQueue;
@@ -188,6 +194,7 @@
   static void WaitForReplyWithNestedMessageLoop(SyncContext* context);
 
   bool sync_messages_with_no_timeout_allowed_;
+  OutgoingMessageFilter *outgoing_message_filter_;

[jam, 2011/04/28 23:43:46]

nit: * then space

 
   // Used to signal events between the IPC and listener threads.
   base::WaitableEventWatcher dispatch_watcher_;
@@ -195,6 +202,15 @@
   DISALLOW_COPY_AND_ASSIGN(SyncChannel);
 };
 
+// Interface for a filter to be imposed on outgoing messages which can
+// re-write the message.  Used mainly for testing.
+class OutgoingMessageFilter {

[jam, 2011/04/28 23:43:46]

since this doesn't have to do with sync messages, i think it's better to put
this in IPC::ChannelProxy.  then this interface can be put right after
IPC::ChannelProxy::MessageFilter.

+ public:
+  // Returns a re-written message, freeing the original, or simply the
+  // original unchanged if no rewrite indicated.
+  virtual Message *Rewrite(Message *message) = 0;
+};
+
 }  // namespace IPC
 
 #endif  // IPC_IPC_SYNC_SENDER_H__