Mojo cpp bindings: add support for validating incoming messages.

mojo/public/cpp/bindings/message_filter.h

Index: mojo/public/cpp/bindings/message_filter.h
diff --git a/mojo/public/cpp/bindings/message_filter.h b/mojo/public/cpp/bindings/message_filter.h
new file mode 100644
index 0000000000000000000000000000000000000000..5d9eba2e005206e7a48a606f7da8affbd22d558c
--- /dev/null
+++ b/mojo/public/cpp/bindings/message_filter.h
@@ -0,0 +1,44 @@
+// Copyright 2014 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 MOJO_PUBLIC_CPP_BINDINGS_MESSAGE_FILTER_H_
+#define MOJO_PUBLIC_CPP_BINDINGS_MESSAGE_FILTER_H_
+
+#include "mojo/public/cpp/bindings/message.h"
+#include "mojo/public/cpp/system/macros.h"
+
+namespace mojo {
+
+// This class is the base class for message filters. Subclasses should
+// implement the pure virtual method Accept() inherited from MessageReceiver to
+// process messages and/or forward them to |next_|.
+class MessageFilter : public MessageReceiver {
+ public:
+  // Doesn't take ownership of |next|. Therefore |next| has to stay alive while
+  // this object is alive.
+  explicit MessageFilter(MessageReceiver* next = NULL);
+  virtual ~MessageFilter();
+
+  void set_next(MessageReceiver* next) { next_ = next; }

[darin (slow to review), 2014/05/14 06:51:51]

it might make sense to rename "next" to "sink" because from the perspective of
the filter, it seems as though the message terminates with this receiver--this
sink. it isn't necessarily a next filter.

[yzshen1, 2014/05/14 08:14:03]

Done.

+
+ protected:
+  MessageReceiver* next_;
+
+ private:
+  // MessageReceiver method that shouldn't be reached.
+  virtual bool AcceptWithResponder(Message* message,
+                                   MessageReceiver* responder) MOJO_OVERRIDE;
+};
+
+// A trivial filter that simply forwards every message it receives to |next_|.

[darin (slow to review), 2014/05/14 06:51:51]

nit: maybe call this class MessageForwarder? At first I thought "null filter"
would send things to /dev/null (i.e., ignore incoming messages). You could also
call it a PassThrough{Message}Filter or Forwarding{Message}Filter? I'm not sure
how important it is to have the term "Message" in those names.

[yzshen1, 2014/05/14 08:14:03]

Done. I changed it to PassThroughFilter.

+class NullFilter : public MessageFilter {
+ public:
+  explicit NullFilter(MessageReceiver* next = NULL);
+
+  virtual bool Accept(Message* message) MOJO_OVERRIDE;
+};
+
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_MESSAGE_FILTER_H_