Example of how to interpose yourself in a message stream.

ipc/ipc_sync_channel.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 "ipc/ipc_sync_channel.h"
 
 #include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/message_loop.h"
 #include "base/threading/thread_local.h"
@@ skipping 360 matching lines @@
     const IPC::ChannelHandle& channel_handle,
     Channel::Mode mode,
     Channel::Listener* listener,
     MessageLoop* ipc_message_loop,
     bool create_pipe_now,
     WaitableEvent* shutdown_event)
     : ChannelProxy(
           channel_handle, mode, ipc_message_loop,
           new SyncContext(listener, ipc_message_loop, shutdown_event),
           create_pipe_now),
-      sync_messages_with_no_timeout_allowed_(true) {
+      sync_messages_with_no_timeout_allowed_(true),
+      outgoing_message_filter_(NULL) {
   // Ideally we only want to watch this object when running a nested message
   // loop.  However, we don't know when it exits if there's another nested
   // message loop running under it or not, so we wouldn't know whether to
   // stop or keep watching.  So we always watch it, and create the event as
   // manual reset since the object watcher might otherwise reset the event
   // when we're doing a WaitMany.
   dispatch_watcher_.StartWatching(sync_context()->GetDispatchEvent(), this);
 }
 
 SyncChannel::~SyncChannel() {
 }
 
 void SyncChannel::SetRestrictDispatchToSameChannel(bool value) {
   sync_context()->set_restrict_dispatch(value);
 }
 
 bool SyncChannel::Send(Message* message) {
   return SendWithTimeout(message, base::kNoTimeout);
 }
 
 bool SyncChannel::SendWithTimeout(Message* message, int timeout_ms) {
+  if (outgoing_message_filter_)
+    message = outgoing_message_filter_->Rewrite(message);
+
   if (!message->is_sync()) {
     ChannelProxy::Send(message);
     return true;
   }
 
   // *this* might get deleted in WaitForReply.
   scoped_refptr<SyncContext> context(sync_context());
   if (context->shutdown_event()->IsSignaled()) {
     delete message;
     return false;
@@ skipping 90 matching lines @@
 void SyncChannel::OnWaitableEventSignaled(WaitableEvent* event) {
   DCHECK(event == sync_context()->GetDispatchEvent());
   // The call to DispatchMessages might delete this object, so reregister
   // the object watcher first.
   event->Reset();
   dispatch_watcher_.StartWatching(event, this);
   sync_context()->DispatchMessages();
 }
 
 }  // namespace IPC