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Re: [selectors-api] Summary of Feature Requests for v2

From: Sean Hogan <shogun70@westnet.com.au>
Date: Fri, 25 Sep 2009 11:09:49 +1000
Message-ID: <4ABC185D.6080205@westnet.com.au>
To: Boris Zbarsky <bzbarsky@MIT.EDU>
CC: public-webapps <public-webapps@w3.org>
Boris Zbarsky wrote:
> On 9/24/09 6:45 PM, Sean Hogan wrote:
>> That is surprising. Does the CSS engine do the same? If the CSS engine
>> doesn't store the parsed selector then it probably doesn't matter for JS
>> calls either.
>
> In Gecko the CSS engine stores the parsed selector.  In addition, it 
> stores the selectors in various bins in a data structure to make 
> matching faster.  In practice this means that you don't have to 
> actually match most nodes against most selectors when computing the 
> set of rules that match a given node.  This makes sense because you're 
> guaranteed that every time a node is inserted into the DOM you will 
> have to match it against every single one of those selectors. 
> https://developer.mozilla.org/en/Writing_Efficient_CSS has a 
> description of the setup.  I believe Webkit has something similar.  
> Again, I can't speak to Trident or Presto.
>
> In the querySelector(All) case, the browser has no way to know that 
> the selector will ever be reused.  In practice, the native 
> implementations were enough faster than what they were replacing, even 
> without any particularly fancy optimizations, that simplicity was 
> judged more important than squeezing every bit of performance out.  At 
> least in Gecko's case.  If we get to the point where they're being a 
> bottleneck again, that will likely be revisited.
>
>> Take a event-delegation system that uses matchesSelector.
>> Every event that it handles will walk the event path trying
>> element.matchesSelector with every registered handler.
>> e.g. There are twenty registered click handlers and a click event occurs
>> on an element ten levels deep. There could be 20 * 10 = 200 calls to
>> matchesSelector. Or 400 if the system simulates capture phase as well.
>
> 200 calls would equate to ~1ms of selector parsing time in the the 
> case of Gecko.  For a click event, that's not terrible.

No. It will be negligible compared to everything else that has to be done.

>
>> Or take a framework that adds enhancements to HTML elements based on
>> selectors.
>> The framework wants to handle dynamic insertion to / removal from the
>> page, so every DOMNodeInserted / DOMNodeRemoved (or equivalent) it will
>> call querySelectorAll for all registered enhancements to see if there is
>> any work to do.
>
> This could be much more of a problem.  I'd want be interested in what 
> the actual performance is like in this situation.  Remember, the 
> selector-parsing time was just the overhead; the real time usage is 
> walking the DOM and doing the matching.  For matchesSelector this is 
> much less significant, of course, but for querySelectorAll it's likely 
> to be the dominating factor (gut feeling; if someone wants to measure 
> that would be welcome).
>
> I also wonder how well XBL or something like that would handle cases 
> like this...  This setup (matching every node in a subtree against a 
> set of selectors) is really not that well served by any of the APIs 
> described here.  It's much closer to the CSS use case and would 
> benefit from similar optimizations.
>

XBL (and standard DOM implementations) is what we want.

> Note that I don't have anything against exposing "parsed selector" 
> objects in JS.  I don't think it would be that difficult to implement 
> it.  I'm just not sure whether the added complexity is really needed, 
> and whether it's the best solution for the use cases.  Maybe it is; 
> I'm just gathering data.  This is not exactly my area of expertise.
>
> -Boris
>

Thanks for that perspective. My main concern was that we don't create 
parsed selectors in JS (at least not for performance reasons).
Received on Friday, 25 September 2009 01:10:37 GMT

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