Example of how to interpose yourself in a message stream.

content/common/child_thread.h

 // Copyright (c) 2011 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 CONTENT_COMMON_CHILD_THREAD_H_
 #define CONTENT_COMMON_CHILD_THREAD_H_
 #pragma once
 
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
 #include "content/common/message_router.h"
 #include "webkit/glue/resource_loader_bridge.h"
 
 class FileSystemDispatcher;
 class MessageLoop;
 class NotificationService;
 class QuotaDispatcher;
 class ResourceDispatcher;
 class SocketStreamDispatcher;
 
 namespace IPC {
 class SyncChannel;
+class OutgoingMessageFilter;
 class SyncMessageFilter;
 }
 
 // The main thread of a child process derives from this class.
 class ChildThread : public IPC::Channel::Listener,
                     public IPC::Message::Sender {
  public:
   // Creates the thread.
   ChildThread();
   // Used for single-process mode.
   explicit ChildThread(const std::string& channel_name);
   virtual ~ChildThread();
 
   // IPC::Message::Sender implementation:
   virtual bool Send(IPC::Message* msg);
 
   // See documentation on MessageRouter for AddRoute and RemoveRoute
   void AddRoute(int32 routing_id, IPC::Channel::Listener* listener);
   void RemoveRoute(int32 routing_id);
 
   IPC::Channel::Listener* ResolveRoute(int32 routing_id);
 
+  // Adds a filter to rewrite outgoing messages (used mainly for testing).

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

how about just making channel() public, it seems that should have been done from
the start.  that way don't need to add methods to wrap the channel one.

+  void SetOutgoingMessageFilter(IPC::OutgoingMessageFilter* filter);
+
   // Creates a ResourceLoaderBridge.
   // Tests can override this method if they want a custom loading behavior.
   virtual webkit_glue::ResourceLoaderBridge* CreateBridge(
       const webkit_glue::ResourceLoaderBridge::RequestInfo& request_info);
 
   ResourceDispatcher* resource_dispatcher();
 
   SocketStreamDispatcher* socket_stream_dispatcher() {
     return socket_stream_dispatcher_.get();
   }
@@ skipping 71 matching lines @@
   scoped_ptr<NotificationService> notification_service_;
 
   scoped_ptr<FileSystemDispatcher> file_system_dispatcher_;
 
   scoped_ptr<QuotaDispatcher> quota_dispatcher_;
 
   DISALLOW_COPY_AND_ASSIGN(ChildThread);
 };
 
 #endif  // CONTENT_COMMON_CHILD_THREAD_H_