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Re: The synchronous load event is trouble

From: Erik Dahlstrom <ed@opera.com>
Date: Mon, 21 Dec 2009 09:44:36 +0100
To: "Henri Sivonen" <hsivonen@iki.fi>, www-svg <www-svg@w3.org>
Cc: "Boris Zbarsky" <bzbarsky@mit.edu>
Message-ID: <op.u4948m0lgeuyw5@localhost>
Hi Henri,

On Thu, 26 Nov 2009 13:13:32 +0100, Henri Sivonen <hsivonen@iki.fi> wrote:

> SVG 1.2 Tiny says about the 'load' event:
>> The event is triggered at the point at which the user agent finishes  
>> loading the element and any dependent resources (such as images, style  
>> sheets, or scripts). In the case the element references a script, the  
>> event will be raised only after an attempt to interpret the script has  
>> been made. Dependent resources that fail to load will not prevent this  
>> event from firing if the element that referenced them is still in the  
>> document tree unless they are designated as externalResourcesRequired.  
>> The event is independent of the means by which the element was added to  
>> DOM tree.
>
>
> SVG 1.1 Full says about the event the is handled by the 'onload' event  
> handler:
>> The event is triggered at the point at which the user agent has fully  
>> parsed the element and its descendants and is ready to act  
>> appropriately upon that element, such as being ready to render the  
>> element to the target device. Referenced external resources that are  
>> required must be loaded, parsed and ready to render before the event is  
>> triggered. Optional external resources are not required to be ready for  
>> the event to be triggered.
...
> I think the definition of the event has at least the following three  
> problems:
>
>  1) The firing of the event is synchronous with the parse. This is  
> highly unusual. I'm not aware of any other standards-based feature  
> requiring an event to be dispatched synchronously with the parse. Gecko  
> has some non-stardards-based analogous behaviors, but, as I understand  
> it, those behaviors cause scripts to run either from the task queue or  
> from a deferred execution mechanism that isn't fully synchronous but  
> isn't the task queue, either. Having scripts run as the side effect of  
> the parse is generally trouble. It's impossible to eliminate </script>  
> running scripts as a side effect of the parse, but I think we should get  
> rid of all the other cases, so that parsing can be optimized with the  
> assumption that only </script> may cause synchronous script execution  
> (which requires the parser to go into a state that's safe for script  
> execution).

I agree that it would be good to dispatch the 'load' event asynchronously,  
to not require running scripts directly while parsing (</script> being the  
exception).

>  2) The event is defined to be fired at a particular point relative to  
> parse but also relative to network activity. Scripts already have to  
> block the parser for legacy reasons. However, all blocking of the parser  
> due to external resources is bad for performance, so it would be  
> desirable to allow styles to load asynchronously with the parse. As  
> written, the spec text seems to establish </svg> as a point where the  
> parser potentially has to yield to the event loop if there are pending  
> style sheet loads. (I don't know how cross-resource 'use' works, but it  
> seems trouble if those don't attach asynchronously.)

The spec doesn't explicitly list what the "optional external resources"  
can be, but I don't think it intends to include external stylesheets. I  
think that "optional external resources" for example means 'image'  
elements that have a 'type' attribute value that the UA doesn't support  
(in which case it's allowed to not load that particular external resource).

I don't think the intention of the spec is to block the parser while  
waiting for stylesheets to load.

>  3) As I understand it, a load event dispatched to an <svg> element  
> embedded in an HTML document would bubble to the HTML <body> element and  
> up. JavaScript libraries written for text/html may legitimately assume  
> that the event handled by the 'onload' event handler is dispatched once  
> per document life cycle. It seems to me that adding some embedded SVG to  
> an HTML page has the potential of severely confusing pre-existing  
> JavaScript code that is listening for the traditional HTML load event.

'load' doesn't bubble, so this doesn't apply.

> Considering the above, I propose removing the current SVG 'load' event  
> behavior and then adding the following:
>
>  A) If the root element is the <svg> element, dispatch the 'load' event  
> to that element the same way the 'load' element is dispatched to the  
> <body> element when <html> is the root element. (I.e. wait for external  
> resources, then dispatch the event asynchronously from the task queue.)

Sounds good to me, would need to be tweaked a bit to fit into the svg  
spec.  Though I don't really see how the current wording forbids this  
behaviour I agree that it may need to be clarified. Reasoning: even if the  
event is triggered at some particular point in time this doesn't  
necessarily prevent the UA from delaying script execution until a later  
time as long as the order of the events (or order of event processing) is  
identical to what would have been the case otherwise.

> If that's not enough and existing SVG content relies on the 'load' event  
> being dispatched to each nested <svg> element also (I don't know what  
> the legacy constraints are), then also add the following:
>
>  B) When the end tag of a non-root <svg> element is parsed, dispatch a  
> *non-bubbling* 'load' event to that element asynchronously from the task  
> queue without waiting for external resources.

I think it would be bad to dispatch 'load' for something that isn't loaded  
fully. It would be better to queue the dispatch until all resources have  
loaded for the root element (since it's not uncommon for the nested  
content to be dependent on that too given that the nested elements are  
still in the same document) and then dispatch the 'load' event(s) for all  
the nested <svg> elements.

There's an open issue, ISSUE-2254 [1], for considering changes for how  
'load' events are handled in SVG for the next version, and this has  
significant overlap. I've added this thread to the issue to be  
discussed/resolved at the same time.

Cheers
/Erik

[1] http://www.w3.org/Graphics/SVG/WG/track/issues/2254

-- 
Erik Dahlstrom, Core Technology Developer, Opera Software
Co-Chair, W3C SVG Working Group
Personal blog: http://my.opera.com/macdev_ed
Received on Monday, 21 December 2009 08:42:46 GMT

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