# 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/>