Introduce content::ChildMessageFilter.

content/child/child_message_filter.h

Index: content/child/child_message_filter.h
diff --git a/content/child/child_message_filter.h b/content/child/child_message_filter.h
new file mode 100644
index 0000000000000000000000000000000000000000..1086d05e5d2c40f4bd16774c4971d058cba73fd6
--- /dev/null
+++ b/content/child/child_message_filter.h
@@ -0,0 +1,55 @@
+// Copyright 2013 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_CHILD_CHILD_MESSAGE_FILTER_H_
+#define CONTENT_CHILD_CHILD_MESSAGE_FILTER_H_
+
+#include "ipc/ipc_channel_proxy.h"
+
+namespace content {
+
+class ThreadSafeSender;
+
+// Convenience base class for implementing IPC MessageFilters that run on the
+// child thread and can dispatch messages to arbitrary task runners.
+class ChildMessageFilter : public IPC::ChannelProxy::MessageFilter,
+                           public IPC::Sender {
+ public:
+  ChildMessageFilter();
+
+  // IPC::ChannelProxy::MessageFilter overrides.
+  //
+  // DON'T OVERRIDE THIS! Implement HandleMessage() instead.

[jam, 2013/09/25 20:58:58]

in retrospect, I should have made BrowserMessageFilter have an inner class that
is an IPC::ChannelProxy::MessageFilter, and hid that implementation detail from
child classes. That would remove this issue of OnMessageReceived confusion

[scherkus (not reviewing), 2013/09/25 23:07:15]

Done.

I find it ends up being a biiiittt confusing for subclasses since they're no
longer forced to implement any methods, but maybe that's OK.

+  virtual bool OnMessageReceived(const IPC::Message& message) OVERRIDE;
+
+  // IPC::Sender implementation.
+  //
+  // Can be called on any thread.
+  virtual bool Send(IPC::Message* message) OVERRIDE;
+
+  // Override to have HandleMessage() run on a TaskRunner of your choosing.
+  // Returns NULL by default, which runs HandleMessage() on the child thread.
+  virtual base::TaskRunner* OverrideTaskRunnerForMessage(
+      const IPC::Message& message);
+
+  // Subclasses implement to handle IPC messages returning true if the message
+  // was handled, false otherwise.
+  //
+  // XXX naming is hard ... unlike in BrowserMessageFilter we don't
+  // have |message_is_ok| to differentiate the OnMessageReceived() methods.
+  virtual bool HandleMessage(const IPC::Message& message) = 0;
+
+ protected:
+  friend class base::RefCountedThreadSafe<ChildMessageFilter>;
+  virtual ~ChildMessageFilter();
+
+ private:
+  scoped_refptr<ThreadSafeSender> thread_safe_sender_;
+
+  DISALLOW_COPY_AND_ASSIGN(ChildMessageFilter);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_CHILD_CHILD_MESSAGE_FILTER_H_