[Chromoting] Add SessionPlugin in JingleSession

remoting/protocol/session_plugin.h

Index: remoting/protocol/session_plugin.h
diff --git a/remoting/protocol/session_plugin.h b/remoting/protocol/session_plugin.h
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+++ b/remoting/protocol/session_plugin.h
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+// Copyright 2016 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 REMOTING_PROTOCOL_SESSION_PLUGIN_H_
+#define REMOTING_PROTOCOL_SESSION_PLUGIN_H_
+
+#include <memory>
+
+#include "base/callback.h"
+#include "remoting/protocol/jingle_messages.h"
+#include "remoting/protocol/session.h"
+#include "third_party/webrtc/libjingle/xmllite/xmlelement.h"
+
+namespace remoting {
+namespace protocol {
+
+// Interface for Session plugins. Plugins allow to send and receive optional
+// information that is not essential for session handshake. Messages generated
+// by the plugins on one end of a connection are attached to the session
+// handshake messages and passed to the plugins on the other end. Plugins are
+// optional, i.e. Session doesn't need any plugins to connect successfully.
+class SessionPlugin {
+ public:
+  SessionPlugin() = default;
+  virtual ~SessionPlugin() = default;
+
+  // Returns an XmlElement if the SessionPlugin requires to attach some data
+  // into the outgoing message. This function will be called after other
+  // components have finished generating messages, and have set the state and
+  // action. So |state| is the current session state, |action| is the current
+  // ActionType in message.
+  virtual std::unique_ptr<buzz::XmlElement> GetNextMessage(
+      Session::State state) = 0;
+
+  // Handles messages in |attachments|. This function will be called before
+  // other components have handled the incoming message. So |state| is the last
+  // session state, |action| is the current ActionType in message.
+  virtual void OnIncomingMessage(Session::State state,
+                                 const buzz::XmlElement& attachments) = 0;
+};
+
+}  // namespace protocol
+}  // namespace remoting
+
+#endif  // REMOTING_PROTOCOL_SESSION_PLUGIN_H_