Make BrowserMessageFilter support dispatching messages on different threads.

chrome/browser/browser_message_filter.h

 // Copyright (c) 2010 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 CHROME_BROWSER_BROWSER_IO_MESSAGE_FILTER_H_
-#define CHROME_BROWSER_BROWSER_IO_MESSAGE_FILTER_H_
+#ifndef CHROME_BROWSER_BROWSER_MESSAGE_FILTER_H_
+#define CHROME_BROWSER_BROWSER_MESSAGE_FILTER_H_
 #pragma once
 
 #include "base/process.h"
+#include "chrome/browser/browser_thread.h"
 #include "ipc/ipc_channel_proxy.h"
 
-// Base class for message filters in the browser process that reside on the IO
-// thread.
-class BrowserIOMessageFilter : public IPC::ChannelProxy::MessageFilter,
-                               public IPC::Message::Sender {
+// Base class for message filters in the browser process.  You can receive and
+// send messages on any thread.
+class BrowserMessageFilter : public IPC::ChannelProxy::MessageFilter,
+                             public IPC::Message::Sender {
  public:
-  BrowserIOMessageFilter();
-  virtual ~BrowserIOMessageFilter();
+  BrowserMessageFilter();
+  virtual ~BrowserMessageFilter();
 
   // IPC::ChannelProxy::MessageFilter methods.  If you override them, make sure
-  // to call them as well.
+  // to call them as well.  These are always called on the IO thread.
   virtual void OnFilterAdded(IPC::Channel* channel);
   virtual void OnChannelClosing();
   virtual void OnChannelConnected(int32 peer_pid);
+  // DON'T OVERRIDE THIS!  Override the other version below.
+  virtual bool OnMessageReceived(const IPC::Message& message);
 
-  // IPC::Message::Sender implementation:
-  virtual bool Send(IPC::Message* msg);
+  // IPC::Message::Sender implementation.  Can be called on any thread.  Can't
+  // send sync messages (since we don't want to block the browser on any other
+  // process).
+  virtual bool Send(IPC::Message* message);
+
+  // This allows the message handler to be called on a different thread for the
+  // given message.  If you want to handle the message on a different thread,
+  // change |thread| to the thread id.  Otherwise leave it alone.

[brettw, 2010/12/09 23:44:12]

Can you give a little more detail here? I think what I'm reading is that when
this gets called, *thread will tell me what thread I'm on, and if I like that I
handle the message and leave it alone, and if I don't like that, I change
*thread and don't handle the message?

I'm now realizing I'm not supposed to process the message. It would be more
clear (if this works) to call this BrowserThread::ID
GetThreadForMessage(Message& message);

Of if you need it to "default to the current one" maybe it should be
OverrideThreadForMessage with the current signature?

[jam, 2010/12/10 01:03:53]

Thanks, OverrideThreadForMessage is clearer.  switched.

+  virtual void HandleMessageOnThread(const IPC::Message& message,
+                                     BrowserThread::ID* thread);
+
+  // Override this to receive messages.
+  // Your function will normally be called on the IO thread.  However, if your
+  // HandleMessageOnThread modifies the thread used to dispatch the message,
+  // your function will be called on the requested thread.
+  virtual bool OnMessageReceived(const IPC::Message& message,
+                                 bool* message_was_ok) = 0;
 
  protected:
+  // Can be called on any thread, after OnChannelConnected is called.
   base::ProcessHandle peer_handle() { return peer_handle_; }
 
   // Call this if a message couldn't be deserialized.  This kills the renderer.
+  // Can be called on any thread.
   void BadMessageReceived(uint32 msg_type);
 
  private:
+  // Used for dispatching a message to the derived class.

[brettw, 2010/12/09 23:44:12]

Google comment style: "Dispatches a message to the derived class."

+  bool DispatchMessage(const IPC::Message& message);
+
   IPC::Channel* channel_;
   base::ProcessHandle peer_handle_;
 };
 
-#endif  // CHROME_BROWSER_BROWSER_IO_MESSAGE_FILTER_H_
+#endif  // CHROME_BROWSER_BROWSER_MESSAGE_FILTER_H_