Hixie's Natural Log

2004-04-28 15:45 UTC Server-sent DOM events

Someone suggested to me recently that it would be quite cool if there was a way in which servers could dispatch DOM events straight into a Web page so that script in the page could then react to them, updating the page and so forth.

At the moment pages fake this by opening an iframe to a page that the server keeps open, slowly trickling script blocks into it, and having the browser execute them as it finds them.

This is pretty ugly, but it works. So here is a proposal for a cleaner, declarative way to do this.

To specify an event source in an HTML document authors use a new (empty) element event-source, with an attribute src="" that takes a URI to open as a stream and, if the data found at that URI is of the appropriate type, treat as an event source.

The event stream MIME type is application/x-dom-event-stream.

The event stream is always be encoded as UTF-8. Line are always be terminated by a single U+000A line feed character.

The event stream format is (in pseudo-BNF):

<stream>  ::= <event>*
<event>   ::= [ <comment> | <field> ]+ <newline>
<comment> ::= ';' <data> <newline>
<field>   ::= <name> [ ':' <space>? <data> ]? <newline>
<name>    ::= one or more UNICODE characters other than ':' and U+000A
<data>    ::= zero or more UNICODE characters other than U+000A
<space>   ::= a single U+0020 character (' ')
<newline> ::= a single U+000A character

The stream is parsed by reading everything line by line, in blocks separated by blank lines (blank lines are those consisting of just a single lone line feed character). Comment lines (those starting with the character ';' not proceeded by any other characters) are ignored.

For each non-blank, non-comment line, the field name is first taken. This is everything on the line up to but not including the first colon (':') or the line feed, whichever comes first. Then, if there was a colon, the data for that line is taken. This is everything after the colon, ignoring a single space after the colon if there is one, up to the end of the line. If there was no colon the data is the empty string.


Field name: Field data
This is a blank field
1. These two lines: have the same data
2. These two lines:have the same data
1. But these two lines:  do not
2. But these two lines: do not

If a field name occurs multiple times, the data values for those lines are concatenated with a newline between them.

For example, the following:

Test: Line 1
Foo:  Bar
Test: Line 2

...is treated as having two fields, one called Test with the value Line 1\nLine 2 (where \n represents a newline), and one called Foo with the value Bar.

(Since any random stream of characters matches the above format, there is no need to define any error handling.)

Once the fields have been parsed, they are interpreted as follows (these are case sensitive exact comparisons):

Once a blank line is reached, an event of the appropriate type is synthesized and dispatched to the appropriate node as described by the fields above. No event is dispatched until a blank line has been received.

The RemoteEvent interface is defined as follows:

interface RemoteEvent : Event {
  readonly attribute DOMString       data;
  void               initRemoteEvent(in DOMString typeArg,
                                     in boolean canBubbleArg,
                                     in boolean cancelableArg,
                                     in DOMString dataArg);
  void               initRemoteEventNS(in DOMString namespaceURI,
                                       in DOMString type,
                                       in boolean canBubbleArg,
                                       in boolean cancelableArg,
                                       in DOMString dataArg);

Events that use the RemoteEvent interface never have any default action associated with them.

The event-source element may also have an onevent="" attribute. If present, the attribute is treated as script representing an event handler registered as non-capture listener of events with name event and the namespace uuid:755e2d2d-a836-4539-83f4-16b51156341f or null, that are targetted at or bubble through the element.

The following event description, once followed by a blank line:

Event: stock change
data: YHOO
data: -2
data: 10

...would cause an event stock change with the interface RemoteEvent to be dispatched on the event-source element, which would then bubble up the DOM, and whose data attribute would contain the string YHOO\n-2\n10 (where \n again represents a newline).

This could be used as follows:

<event-source src="http://stocks.example.com/ticker.php" id="stock">
<script type="text/javascript">
document.getElementById('stock').addEventListener('stock change',
  function () {
    var data = event.data.split(' ');
    updateStocks(data[0], data[1], data[2]);
  }, false);

...where updateStocks is a function defined as:

function updateStocks(symbol, delta, value) { ... }

...or some such.

What do you think? Let me know.

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