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Re: QSA, the problem with ":scope", and naming

From: Timmy Willison <timmywillisn@gmail.com>
Date: Wed, 19 Oct 2011 09:39:52 -0400
Message-ID: <CAEWpY1q678tVtMNJzWsoQAQVEAkZw+M_+dNKeA=fVUos1W+wHw@mail.gmail.com>
To: Alex Russell <slightlyoff@google.com>
Cc: Matt Shulman <mattsh@google.com>, Webapps WG <public-webapps@w3.org>, Yehuda Katz <wycats@gmail.com>, John Resig <jeresig@gmail.com>, Paul Irish <paulirish@google.com>, Lachlan Hunt <lachlan.hunt@lachy.id.au>
>From the perspective of building a selector engine, I think all selector
engines need something like .findAll, and not something like :scope.

On Tue, Oct 18, 2011 at 8:00 PM, Alex Russell <slightlyoff@google.com>wrote:
> No need to wait. We had something nearly identical for this in Dojo
> using an ID prefix hack. It looked something like this:
>        (function(){
>                var ctr = 0;
>                query = function(query, root){
>                        root = root||document;
>                        var rootIsDoc = (root.nodeType == 9);
>                        var doc = rootIsDoc ? root :
> (root.ownerDocment||document);
>                        if(!rootIsDoc || (">~+".indexOf(query.charAt(0)) >=
> 0)){
>                                // Generate an ID prefix for the selector
>                                root.id = root.id||("qUnique"+(ctr++));
>                                query = "#"+root.id+" "+query;
>                        }
>                        return Array.prototype.slice.call(
>                                doc.querySelectorAll(query)
>                        );
>                };
>        })();
> This is exactly the same dance that ":scope" does.
Sizzle and Slick do the same thing. As far as I can tell, nwmatcher doesn't
deal with it.  We can't just add :scope to all selections (for many reasons)
and adding just before QSA would require the same logic that Alex has
demonstrated above.

All of the selector engines do predictions at loadtime on whether QSA will
work.  They continue differently beyond that, but one thing every library
has in common is a try/catch around the call to QSA that falls back to
manual parsing if it throws an exception (intentionally avoiding the need
for complete parsing before calling QSA).  The point is it is a
misconception that selector engines parse selectors before delegating to
QSA. The number of things libraries want to do before getting to the QSA
call is very minimal.  The one that hurts us all the most is this need for
scoping and ':scope' would simply never be used in a selector engine, since
the id trick already works everywhere.  The case Alex wrote above is pretty
much the only case where the selector is parsed beyond checking for tag
only, id only, or class only and it is due to what all of the js libraries
has considered a design flaw in QSA.  A method like findAll would fix that,
leaving as much parsing as possible in the hands of the browser.

PS - I should say I don't necessarily think the name 'findAll' would work. I
agree it should be short.  The equivalent of querySelector would be find and
in library land 'find' selects more than one thing, but I'm not as concerned
about the name.
Received on Thursday, 20 October 2011 09:05:03 UTC

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