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galene/README.FRONTEND
Juliusz Chroboczek f53276b89e Simplify the protocol and the protocol interface.
Split the id field into id and source, where source indicates the sender
of the message and id the entity being sent.  Remove the label request,
just use the offerer's username.  Maintain the username within the
ServerConnection, this removes a parameter from some methods.
2021-01-03 12:17:30 +01:00

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# Writing a new frontend
The frontend is written in JavaScript and is split into two files:
- `protocol.js` contains the low-level functions that interact with the
server;
- `galene.js` contains the user interface.
A new frontend may either implement Galène's client-server protocol from
scratch, or it may use the functionality of `protocol.js`. This document
documents the latter approach.
## Data structures
The class `ServerConnection` encapsulates a connection to the server as
well as all the associated streams. Unless your frontend communicates
with multiple servers, it will probably create just a single instance of
this class.
The class `Stream` encapsulates a set of related audio and video tracks
(for example, an audio track from a microphone and a video track from
a webcam). A stream is said to go *up* when it carries data from the
client to the server, and *down* otherwise. Streams going up are created
by the client (your frontend), streams going down are created by the server.
## Connecting to the server
First, create a `ServerConnection` and set up all the callbacks:
```javascript
let sc = new ServerConnection()
serverConnection.onconnected = ...;
serverConnection.onclose = ...;
serverConnection.onusermessage = ...;
serverConnection.onjoined = ...;
serverConnection.onuser = ...;
serverConnection.onchat = ...;
serverConnection.onclearchat = ...;
serverConnection.ondownstream = ...;
```
The `onconnected` callback is called when we connect to the server. The
`onclose` callback is called when the socket is closed; you should use it
to close all your up streams (down streams will be closed by the server).
The `onusermessage` callback indicates an application-specific message,
either from another user or from the server; the field `kind` indicates
the kind of message.
Once you have joined a group (see below), the remaining callbacks may
trigger. The `onuser` callback is used to indicate that a user has joined
or left the current group. The `onchat` callback indicates that a chat
message has been posted to the group, and `onclearchat` indicates that the
chat history has been cleared. Finally, `ondownstream` is called when the
server pushes a stream to the client; see the section below about streams.
You may now connect to the server.
```javascript
serverConnection.connect(`wss://${location.host}/ws`);
```
You typically join a group and request media in the `onconnected` callback:
```javascript
serverConnection.onconnected = function() {
this.join(group, 'join', username, password);
this.request('everything');
}
```
You should not attempt to push a stream to the server until it has granted
you the `present` permission through the `onjoined` callback.
## Sending and receiving chat messages
Once you have joined a group, you send chat messages with the `chat`
method of the `ServerConnection` class. No permission is needed to do that.
```javascript
serverConnection.chat(username, '', id, 'Hi!');
```
You receive chat messages in the `onchat` callback. The server may
request that you clear your chat window, in that case the `onclearchat`
callback will trigger.
## Other messages
The `usermessage` method of the `ServerConnection` is similar to the
`chat` method, but it sends an application-specific message. Just like
chat messages, application-specific messages are not interpreted by the
server; unlike chat messages, they are not kept in the chat history.
The `useraction` method is used to ask the server to act on a remote user
(kick it, change its permissions, etc.); similarly, the `groupaction`
class requests an action to be performed on the current group. Most
actions require either the `Op` or the `Record` permission.
## Accepting incoming video streams
When the server pushes a stream to the client, the `ondownstream` callback
will trigger; you should set up the stream's callbacks here.
```javascript
serverConnection.ondownstream = function(stream) {
stream.onclose = ...;
stream.onerror = ...;
stream.ondowntrack = ...;
stream.onstatus = ...;
}
```
The `stream.labels` dictionary maps each track's id to one of `audio`,
`video` or `screenshare`. Since `stream.labels` is already available at
this point, you may set up an `audio` or `video` component straight away,
or you may choose to wait until the `ondowntrack` callback is called.
After a new stream is created, `ondowntrack` will be called whenever
a track is added. If the `MediaStream` passed to `ondowntrack` differs
from the one previously received, then the stream has been torn down and
recreated, and you must drop all previously received tracks; in practice,
it is enough to set the `srcObject` property of the video component to the
new stream.
The `onstatus` callback is invoked whenever the client library detects
a change in the status of the stream; states `connected` and `complete`
indicate a functioning stream; other states indicate that the stream is
not working right now but might recover in the future.
The `onclose` callback is called when the stream is destroyed by the
server.
## Pushing outgoing video streams
If you have the `present` permission, you may use the `newUpStream` method
to push a stream to the server. Given a `MediaStream` called `localStream`
(as obtained from `getUserMedia` or `getDisplayMedia`).
```javascript
let stream = serverConnection.newUpStream();
stream.onerror = ...;
stream.onabort = ...;
stream.onstatus = ...;
localStream.getTracks().forEach(t => {
c.labels[t.id] = t.kind;
c.pc.addTrack(t, c.stream);
});
```
See above for information about setting up the `labels` dictionary.
## Stream statistics
Some statistics about streams are made available by calling the
`setStatsInterval` method and setting the `onstats` callback. These
include the data rate for streams in the up direction, and the average
audio energy (the square of the volume) for streams in the down direction.
--- Juliusz Chroboczek <https://www.irif.fr/~jch/>