galene-client.md
Rama master · réplica de lectura del repositorio original.
# 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 simpler example client can be found in the directory `static/example`.
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, fetch the `.status` JSON at the group URL:
```javascript
let r = await fetch(url + ".status");
if(!r.ok) {
throw new Error(`${r.status} ${r.statusText}`);
}
let status = await r.json();
```
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; all streams will
have been closed by the time it is called. 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, or that their attributes have changed; the
user's state can be found in the `users` dictionary. 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(status.endpoint);
```
You typically join a group in the `onconnected` callback:
```javascript
serverConnection.onconnected = function() {
this.join(group, 'join', username, password);
}
```
After the server has replied to the join request, the `onjoined` callback
will trigger. There, you update your user interface and request incoming
streams:
```javascript
serverConnection.onjoined = function(kind, group, perms, status, data, error, message) {
switch(kind) {
case 'join':
this.request({'':['audio','video']});
// then update the UI, possibly taking perms.present into account
break;
case 'change':
// update the UI
break;
case 'redirect':
this.close();
document.location.href = message;
break;
case 'fail':
if(error === 'need-username') {
// the user attempted to login with a token that does not
// specify a username. Display a dialog requesting a username,
// then join again
} else {
// display the friendly error message
}
break;
}
```
## 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.label` field is one of `camera`, `screenshare` or `video`.
After a new stream is created, `ondowntrack` will be called whenever
a track is added.
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, either by
the server or in response to a call to the `close` method. The optional
parameter is true when the stream is being replaced by a new stream; in
that case, the call to `onclose` will be followed with a call to
`onstream` with the same `localId` value.
## 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.label = ...;
stream.onerror = ...;
stream.onstatus = ...;
localStream.getTracks().forEach(t => {
c.pc.addTrack(t, c.stream);
});
```
The `newUpStream` method takes an optional parameter. If this is set to
the `localId` property of an existing stream, then the existing stream
will be closed and the server will be informed that the new stream
replaces the existing stream.
## 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.
# Peer-to-peer file transfer
Galene's client allows users to transfer files during a meeting. The
protocol is peer-to-peer: the clients exchange network parameters and
cryptographic keys through the server (over messages of type
`usermessage`), but all file transfer is performed directly between the
peers.
An in-progress file transfer is represented by a JavaScript object of
class `TransferredFile`. This object implements a finite state automaton
whose current state is encoded as a string in the field `state`. It obeys
the following state transitions:
```
(empty string) ⟶ inviting ⟶ connecting ⟶ connected ⟶ done ⟶ closed
(any state) ⟶ cancelled ⟶ closed
```
## Downloading files
A client that wishes to participate in the file transfer protocol must set
up the `onfiletransfer` callback of the `ServerConnection` object.
```javascript
serverConnection.onfiletransfer = function(transfer) {
...
};
```
This callback will be called whenever a file transfer is initiated, either
by the remote or by the local peer. The callback receives a single value
of class `TransferredFile`. It should start by setting up the `onevent`
callback, which is called whenever the state of the transfer changes and
whenever data is received:
```javascript
transfer.onevent = func(state, data) {
...
};
```
The direction of the file transfer is indicated by the value of the
boolean `this.up`, which is false in the case of a donwload.
The callback may immediately reject the file transfer by either throwing
an exception or by calling `transfer.cancel` and returning. If the file
transfer is not immediately rejected, the callback should set up an
`onevent` callback on the `TransferredFile` object:
```javascript
transfer.onevent = func(state, data) {
...
};
```
It must then arrange for either `transfer.receive` or `transfer.cancel` to
be called, for example from an `onclick` callback.
The `onevent` callback will then be repeatedly called, which can be used
e.g. to present a progress bar to the user. Eventually, the `onevent`
callback will be called with `state` equal to either `cancelled` or
`done`; in the latter case, the transferred data is passed as a `Blob` in
the `data` parameter of the callback.
## Uploading files
A file upload is initiated by calling the `sendFile` method of the class
`ServerConnection`.
```javascript
serverConnection.sendFile(userid, file);
```
The `userid` parameter is the id of the remote peer. The `file` parameter
is an object of kind `File`, typically obtained from an `HTMLInputElement`
with type `file`.
The `onfiletransfer` callback is then called (with `this.up` set to true),
and the transfer proceeds analogously to a file download, except that no
data is passed to the `onevent` callback at the end of the transfer.
— Juliusz Chroboczek <https://www.irif.fr/~jch/>