Webxdc Developer Reference

This document gives a quick overview about the Webxdc specification, It is meant for both, developing Webxdc apps and developing Webxdc implementations.

The Webxdc guidebook shows more detailed information when developing Webxdc apps.

Webxdc File Format

a Webxdc is a ZIP-file with the extension .xdc
the ZIP-file must use the default compression methods as of RFC 1950, this is "Deflate" or "Store"
the ZIP-file must contain at least the file index.html
if the Webxdc is started, index.html is opened in a restricted webview that allow accessing resources only from the ZIP-file

Webxdc API

There are some additional APIs available once webxdc.js is included (the file will be provided by the concrete implementations, no need to add webxdc.js to your ZIP-file):

<script src="webxdc.js"></script>

sendUpdate()

window.webxdc.sendUpdate(update, descr);

A Webxdc is usually shared in a chat and run independently on each peer. To get a shared state, the peers use sendUpdate() to send updates to each other.

update: an object with the following properties:
- update.payload: any javascript primitive, array or object.
- update.info: optional, short, informational message that will be added to the chat, eg. "Alice voted" or "Bob scored 123 in MyGame". usually only one line of text is shown and if there are series of info messages, older ones may be dropped. use this option sparingly to not spam the chat.
- update.document: optional, name of the document in edit, must not be used eg. in games where the Webxdc does not create documents
- update.summary: optional, short text, shown beside Webxdc icon; it is recommended to use some aggregated value, eg. "8 votes", "Highscore: 123"
descr: short, human-readable description what this update is about. this is shown eg. as a fallback text in an email program.

All peers, including the sending one, will receive the update by the callback given to setUpdateListener().

There are situations where the user cannot send messages to a chat, eg. if the webxdc instance comes as a contact request or if the user has left a group. In these cases, you can still call sendUpdate(), however, the update won't be sent to other peers and you won't get the update by setUpdateListener().

setUpdateListener()

let promise = window.webxdc.setUpdateListener((update) => {}, serial);

With setUpdateListener() you define a callback that receives the updates sent by sendUpdate(). The callback is called for updates sent by you or other peers. The serial specifies the last serial that you know about (defaults to 0). The returned promise resolves when the listener has processed all the update messages known at the time when setUpdateListener was called.

Each update which is passed to the callback comes with the following properties:

update.payload: equals the payload given to sendUpdate()
update.serial: the serial number of this update. Serials are larger 0 and newer serials have higher numbers. There may be gaps in the serials and it is not guaranteed that the next serial is exactly incremented by one.
update.max_serial: the maximum serial currently known. If max_serial equals serial this update is the last update (until new network messages arrive).
update.info: optional, short, informational message (see sendUpdate())
update.document: optional, document name as set by the sender, (see sendUpdate()), implementations show the document name eg. beside the app icon or in the title bar
update.summary: optional, short text, shown beside icon (see sendUpdate())

selfAddr

window.webxdc.selfAddr

Property with the peer's own address. This is esp. useful if you want to differ between different peers - just send the address along with the payload, and, if needed, compare the payload addresses against selfAddr() later on.

selfName

window.webxdc.selfName

Property with the peer's own name. This is name chosen by the user in their settings, if there is nothing set, that defaults to the peer's address.

manifest.toml

If the ZIP-file contains a manifest.toml in its root directory, some basic information are read and used from there.

the manifest.toml has the following format

name = "My Name"
source_code_url = "https://example.org/orga/repo"

name - The name of the Webxdc. If no name is set or if there is no manifest, the filename is used as the Webxdc name.
source_code_url - Optional URL where the source code of the Webxdc and maybe other information can be found. UI may make the url accessible via a "Help" menu in the Webxdc window.

Webxdc Icon

If the ZIP-root contains an icon.png or icon.jpg, these files are used as the icon for the Webxdc. The icon should be a square at reasonable width/height; round corners etc. will be added by the implementations as needed. If no icon is set, a default icon will be used.

Other APIs and Tags Usage Hints

localStorage, sessionStorage, indexedDB are okay to be used
visibilitychange-events are okay to be used
window.navigator.language is okay to be used, on desktop it is the system language
<a href="localfile.html"> and other internal links are okay to be used
<a href="mailto:addr@example.org?body=...">- mailto links are okay to be used
<meta name="viewport" ...> usage is okay to be used and useful esp. different webviews have different defaults

Discouraged Things

document.cookie is known not to work on desktop and iOS use localStorage instead
unload-, beforeunload- and pagehide-events are known not to work on iOS and are flaky on other systems (also partly discouraged by mozilla) use visibilitychange instead
<title> and document.title is ignored by Webxdc; use the name property from manifest.toml instead
newest js features may not work on all webviews, you may want to transpile your code down to an older js version eg. with https://babeljs.io
<a href="https://example.org/foo"> and other external links are blocked by definition; instead, embed content or use mailto: link to offer a way for contact
<input type="file"> is discouraged currently; this may change in future

Webxdc Examples

The following example shows an input field and every input is show on all peers.

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8"/>
    <script src="webxdc.js"></script>
  </head>
  <body>
    <input id="input" type="text"/>
    <a href="" onclick="sendMsg(); return false;">Send</a>
    <p id="output"></p>
    <script>
    
      function sendMsg() {
        msg = document.getElementById("input").value;
        window.webxdc.sendUpdate({payload: msg}, 'Someone typed "'+msg+'".');
      }
    
      function receiveUpdate(update) {
        document.getElementById('output').innerHTML += update.payload + "<br>";
      }
    
      window.webxdc.setUpdateListener(receiveUpdate, 0);
    </script>
  </body>
</html>

More examples at github.com/webxdc and topic #webxdc

github.com/webxdc/hello offers an Webxdc Tool that can be used in many browsers without any installation needed. You can also use that repository as a template for your own Webxdc - just clone and start adapting things to your need.