On 20/10/11 12:39 AM, Timmy Willison wrote:
>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 <slightly...@google.com
<mailto:slightly...@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 <http://root.id> = root.id
<http://root.id>||("qUnique"+(ctr++));
query = "#"+root.id
<http://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.
It was definitely not a design flaw in QSA. As Alex's sample code shows
it is possible to get findAll() behavior using QSA. To do the reverse
would involve calling document.findAll() then filtering for nodes that
are descendants of the invoking node.
Clearly JS libs aren't going to switch from implied :scope to explicit
:scope.
But if findAll() is implemented they can advertise that avoiding
non-standard pseudo selectors gives virtually native performance (on
supporting platforms). I imagine this would be almost equivalent to
deprecating them, which would be a win.
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.
