WebRTC PeerConnection Example

Abstract	WebRTC PeerConnection Example
Authors	Walter Fan
Status	WIP
Updated	2024-08-21

Overview

在 windows 平台上编译 webrtc library 后会生成一些例子程序,其中的例子 peerconnection client and server 用来研究 WebRTC 源码, 是一份绝佳的餐前开味小菜

测试步骤:

1. start peer connection server

2. start peer connection client 1 - Alice

3. start peer connection client 2 - Bob

4. Alice click “connect” to localhost:8888

5. Bob click “connect” to localhost:8888

6. Alice select the peer of Bob, and connect

PeerConnection Server

It is a signal server that listen port 8888

main class

CRCC Cards

Class	Responsibility	Collaborators	Comments
ListeningSocket	The server socket. Accepts connections and generates DataSocket instances for each new connection	parent: SocketBase - methods: Create(), Close()	methods: Listen(port), Accept()
DataSocket	Represents an HTTP server socket what can read/send data	parent: SocketBase	methods: OnDataAvailable(socket), send(string)
ChannelMember	Represents a single peer connected to the server	DataSocket	methods: NotifyOfOtherMember(other), ForwardRequestTopPeer(), QueryResponse
PeerChannel	Manages all currently connected peers	property: vector<ChannelMember*> Members	methods: Lookup(ds), AddMember(ds), isTargetRequest

main flow

1. listen port 8888

2. Accept new connection as a DataSocket

3. add the DataSocket’s socket handle into a select array

4. select for interesting events: read event to callback DataSocket.OnDataAvailable

5. if it is a new peer, call AddMember

6. if it is a existed peer, lookup and forward message to that peer

PeerConnection Client

main class of pc client

CRCC Cards

Class	Responsibility	Collaborators	Comments
MainWnd	the main window class of pc Client for UI render and event handler	parent: MainWindow, collaborators: VideoRenderer for local/remote stream	the Observer is Conductor
PeerConnectionClient	Message handler for signin, signout, hangup and message send/read	PeerConnectionClientObserver(OnSignedIn, OnDisconnected)	the Observer is Conductor
Conductor	create/delete PC, add/remove MediaStreamTracks, SDP negotiation, ICE candidate handle	parent:PeerConnectionObserver,webrtc::CreateSessionDescriptionObserver,PeerConnectionClientObserver,MainWndCallback	handle message from MainWnd and PeerConnectionClient, and operate the two objects

• UI

enum UI {
   CONNECT_TO_SERVER,
   LIST_PEERS,
   STREAMING,
};

• PeerConnection Client State

 enum State {
    NOT_CONNECTED,
    RESOLVING,
    SIGNING_IN,
    CONNECTED,
    SIGNING_OUT_WAITING,
    SIGNING_OUT,
};

• Conductor callbacks

 enum CallbackID {
    MEDIA_CHANNELS_INITIALIZED = 1,
    PEER_CONNECTION_CLOSED,
    SEND_MESSAGE_TO_PEER,
    NEW_TRACK_ADDED,
    TRACK_REMOVED,
};

main flow of pc client

1. conect the signal server - peerconnection server

2. start to login into the signal server

3. get the peer list and connect to a peer

4. capture media stream track from local camera

5. create offer and do SDP negotiation

6. collect ICE candidate and do connection checking

7. send local media stream via the peer connection

8. got remote media stream and render

FAQ

What’s PhysicalSocketServer

PhysicalSocketServer is A socket server that provides the real sockets of the underlying OS.

It may contains select(), epoll() or WSAWaitForMultipleEvents loop

And the SocketServer Provides the ability to wait for activity on a set of sockets.

The Thread class provides a nice wrapper on a socket server.

The server is also a socket factory.

The sockets it creates will be notified of asynchronous I/O from this server’s Wait method.

What’s AutoSocketServerThread

AutoSocketServerThread automatically installs itself at construction and uninstalls at destruction. If a Thread object is already associated with the current OS thread, it is temporarily disassociated and restored by the destructor.

The rtc::Thread is extended from webrtc::TaskQueueBase

TaskQueueBase means Asynchronously executes tasks in a way that guarantees that they’re executedin FIFO order and that tasks never overlap.

Tasks may always execute on the same worker thread and they may not.

To DCHECK that tasks are executing on a known task queue, use IsCurrent().

What is the difference between wWinMain and WinMain?

The only difference between WinMain and wWinMain is the command line string and you should use wWinMain in Unicode applications (and all applications created these days should use Unicode). You can of course manually call GetCommandLineW() in WinMain and parse it yourself if you really want to.