Example of how to interpose yourself in a message stream.

ipc/ipc_channel_proxy.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 IPC_IPC_CHANNEL_PROXY_H__
 #define IPC_IPC_CHANNEL_PROXY_H__
 #pragma once
 
 #include <vector>
 
@@ skipping 81 matching lines @@
     // on etc.
     virtual void OnDestruct() const;
   };
 
   struct MessageFilterTraits {
     static void Destruct(const MessageFilter* filter) {
       filter->OnDestruct();
     }
   };
 
+  // Interface for a filter to be imposed on outgoing messages which can
+  // re-write the message.  Used mainly for testing.
+  class OutgoingMessageFilter {
+   public:
+    // Returns a re-written message, freeing the original, or simply the
+    // original unchanged if no rewrite indicated.
+    virtual Message *Rewrite(Message *message) = 0;
+  };
+
   // Initializes a channel proxy.  The channel_handle and mode parameters are
   // passed directly to the underlying IPC::Channel.  The listener is called on
   // the thread that creates the ChannelProxy.  The filter's OnMessageReceived
   // method is called on the thread where the IPC::Channel is running.  The
   // filter may be null if the consumer is not interested in handling messages
   // on the background thread.  Any message not handled by the filter will be
   // dispatched to the listener.  The given message loop indicates where the
   // IPC::Channel should be created.
   ChannelProxy(const IPC::ChannelHandle& channel_handle,
                Channel::Mode mode,
@@ skipping 21 matching lines @@
   // Ordinarily, messages sent to the ChannelProxy are routed to the matching
   // listener on the worker thread.  This API allows code to intercept messages
   // before they are sent to the worker thread.
   // If you call this before the target process is launched, then you're
   // guaranteed to not miss any messages.  But if you call this anytime after,
   // then some messages might be missed since the filter is added internally on
   // the IO thread.
   void AddFilter(MessageFilter* filter);
   void RemoveFilter(MessageFilter* filter);
 
+  void set_outgoing_message_filter(OutgoingMessageFilter* filter) {
+    outgoing_message_filter_ = filter;
+  }
+
   // Called to clear the pointer to the IPC message loop when it's going away.
   void ClearIPCMessageLoop();
 
 #if defined(OS_POSIX)
   // Calls through to the underlying channel's methods.
   int GetClientFileDescriptor() const;
   bool GetClientEuid(uid_t* client_euid) const;
 #endif  // defined(OS_POSIX)
 
  protected:
@@ skipping 75 matching lines @@
 
     // Holds filters between the AddFilter call on the listerner thread and the
     // IPC thread when they're added to filters_.
     std::vector<scoped_refptr<MessageFilter> > pending_filters_;
     // Lock for pending_filters_.
     base::Lock pending_filters_lock_;
   };
 
   Context* context() { return context_; }
 
+  OutgoingMessageFilter* outgoing_message_filter() {
+    return outgoing_message_filter_;
+  }
+
  private:
   friend class SendTask;
 
   void Init(const IPC::ChannelHandle& channel_handle, Channel::Mode mode,
             MessageLoop* ipc_thread_loop, bool create_pipe_now);
 
   // By maintaining this indirection (ref-counted) to our internal state, we
   // can safely be destroyed while the background thread continues to do stuff
   // that involves this data.
   scoped_refptr<Context> context_;
+
+  OutgoingMessageFilter* outgoing_message_filter_;
 };
 
 }  // namespace IPC
 
 #endif  // IPC_IPC_CHANNEL_PROXY_H__