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Re: [SelectorsAPI] Analysis of Context-Rooted Queries (was: Thoughts on querySelectorAll)

From: Maciej Stachowiak <mjs@apple.com>
Date: Mon, 5 May 2008 14:49:20 -0700
Cc: John Resig <jresig@mozilla.com>, public-webapi@w3.org
Message-Id: <E7D0B315-5147-40C5-A322-4C09F6F69A36@apple.com>
To: Lachlan Hunt <lachlan.hunt@lachy.id.au>


On May 5, 2008, at 2:13 PM, Lachlan Hunt wrote:

>
> *Solution 5*
>
>  Define the methods to behave as if an implicit :scope selector
>  and descendant combinator were prepended to each selector in
>  the group.
>
> This would work by taking the selector and appending the equivalent  
> of ":scope " to the beginning of it, and then parsing as  
> usual.  :scope could be implemented in any way the UA wanted here  
> since it's not actually used by the author.


> While this looks promising on the surface, the real problem lies in  
> the implementation of the selector parsing. This requires signifant  
> changes to the grammar of selectors, and thus alteration of the  
> selector engines in browsers.
>
> Consider the following selector:
>
> ">strong, >em"
>
> The expectation would be for this to become:
>
> ":scope >strong, :scope >em"
>
> The question is how to parse and convert it.

One possibility is to make the scoped API (whether that is  
querySelector on elements or some new method) only accept a single  
selector, not a group of selectors. This would remove any parsing  
difficulties. Also, if commas are only ever used to separate the  
selectors in a group in Selector syntax, then prepending ":scope " and  
inserting " :scope " after every comma before parsing would work. I  
believe this is currently true of Selectors, though it may change in  
the future. I would guess the only likely place for commas to appear  
would be in parameters to function-like pseudo-classes, in which case  
the algorithm above but excluding commas nested inside parentheses  
would be more future-proof. Detail of the algorithm:

1. Initialize nesting level to 0
2. Initialize the output string to the empty string
3. While characters in the input string remain:
     3.a. read the current character
     3.b. if the current character is:
        ",": if the nesting level is 0, append ", :scope " to the  
output string
        "(": increase the nesting level by 1, and append "(" to the  
output string
        ")": decrease the nesting level by 1, and append ")" to the  
output string
        anything else: append the current character to the output string
      3.c. advance to the next character
4. return the output string


Regards,
Maciej
Received on Monday, 5 May 2008 21:49:58 GMT

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