[Chromoting] Add SessionPlugin in JingleSession

remoting/protocol/session_plugin.h

+// 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 {
+
+// An interface to attach data on an outgoing message or handle an incoming of
+// message. The plugin data are designed to be optional, so this class won't
+// impact a session generation.

[Sergey Ulanov, 2016/12/23 00:14:52]

Suggest rewording:
// 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.

[Hzj_jie, 2016/12/24 00:09:55]

Done.

+class SessionPlugin {
+ public:
+  SessionPlugin();
+  virtual ~SessionPlugin();
+
+  // 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> OnOutgoing(

[Sergey Ulanov, 2016/12/23 00:14:52]

Maybe call it GetNextAttachment() or GetNextMessage()?

[Hzj_jie, 2016/12/24 00:09:55]

Done.

+      Session::State state,
+      JingleMessage::ActionType action) = 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 OnIncoming(Session::State state,

[Sergey Ulanov, 2016/12/23 00:14:52]

OnIncomingMessage

[Hzj_jie, 2016/12/24 00:09:55]

Done.

+                          JingleMessage::ActionType action,
+                          const buzz::XmlElement* attachments) = 0;
+};
+
+}  // namespace protocol
+}  // namespace remoting
+
+#endif  // REMOTING_PROTOCOL_SESSION_PLUGIN_H_