Example of how to interpose yourself in a message stream.

ipc/ipc_channel_proxy.cc

 // 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.
 
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/message_loop.h"
 #include "ipc/ipc_channel_proxy.h"
 #include "ipc/ipc_logging.h"
 #include "ipc/ipc_message_utils.h"
@@ skipping 263 matching lines @@
   if (listener_)
     listener_->OnChannelError();
 }
 
 //-----------------------------------------------------------------------------
 
 ChannelProxy::ChannelProxy(const IPC::ChannelHandle& channel_handle,
                            Channel::Mode mode,
                            Channel::Listener* listener,
                            MessageLoop* ipc_thread)
-    : context_(new Context(listener, ipc_thread)) {
+    : context_(new Context(listener, ipc_thread)),
+      outgoing_message_filter_(NULL) {
   Init(channel_handle, mode, ipc_thread, true);
 }
 
 ChannelProxy::ChannelProxy(const IPC::ChannelHandle& channel_handle,
                            Channel::Mode mode,
                            MessageLoop* ipc_thread,
                            Context* context,
                            bool create_pipe_now)
-    : context_(context) {
+    : context_(context),
+      outgoing_message_filter_(NULL) {
   Init(channel_handle, mode, ipc_thread, create_pipe_now);
 }
 
 ChannelProxy::~ChannelProxy() {
   Close();
 }
 
 void ChannelProxy::Init(const IPC::ChannelHandle& channel_handle,
                         Channel::Mode mode, MessageLoop* ipc_thread_loop,
                         bool create_pipe_now) {
@@ skipping 29 matching lines @@
   // possible that the channel could be closed while it is receiving messages!
   context_->Clear();
 
   if (context_->ipc_message_loop()) {
     context_->ipc_message_loop()->PostTask(FROM_HERE, NewRunnableMethod(
         context_.get(), &Context::OnChannelClosed));
   }
 }
 
 bool ChannelProxy::Send(Message* message) {
+  if (outgoing_message_filter())
+    message = outgoing_message_filter()->Rewrite(message);
+
 #ifdef IPC_MESSAGE_LOG_ENABLED
   Logging::GetInstance()->OnSendMessage(message, context_->channel_id());
 #endif
 
   context_->ipc_message_loop()->PostTask(FROM_HERE,
                                          new SendTask(context_.get(), message));
   return true;
 }
 
 void ChannelProxy::AddFilter(MessageFilter* filter) {
@@ skipping 27 matching lines @@
   Channel *channel = context_.get()->channel_.get();
   // Channel must have been created first.
   DCHECK(channel) << context_.get()->channel_id_;
   return channel->GetClientEuid(client_euid);
 }
 #endif
 
 //-----------------------------------------------------------------------------
 
 }  // namespace IPC