[Webkit-unassigned] [Bug 15427] New: much of the CSS system should be autogenerated

[bugzilla-daemon at webkit.org]

http://bugs.webkit.org/show_bug.cgi?id=15427

           Summary: much of the CSS system should be autogenerated
           Product: WebKit
           Version: 522+ (nightly)
          Platform: Macintosh
        OS/Version: Mac OS X 10.4
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: eric at webkit.org
                CC: hyatt at apple.com, darin at apple.com, sam at webkit.org


much of the CSS system should be autogenerated

CSSParser::parseValue
CSSStyleSelector::applyProperty
CSSComputedStyleDeclaration::getPropertyCSSValue

Could all be autogenerated.  Instead of using a switch, a function table keyed
off the autogenerated CSS_VAL_ defines.  The actual main body for each
parseValue, applyProperty, getPropertyCSSValue function would be completely
autogenerated in each case.   Most sub-functions (for simple values, like
enums) could be autogenrated, but this would provide an easy override point for
more complicated value/property handling.  An example subfunction which could
be autogenerated:

PassRefPtr<CSSValue>
CSSComputedStyleDeclarationFunctions::getTextAnchor(RenderStyle* style, ...) {
    if (svgStyle->textAnchor() == TA_START)
        return new CSSPrimitiveValue(CSS_VAL_START);
    else if (svgStyle->textAnchor() == TA_MIDDLE)
        return new CSSPrimitiveValue(CSS_VAL_MIDDLE);
    else
        return new CSSPrimitiveValue(CSS_VAL_END);
}

Also, due to bug 15422, we no longer have very good compile-time checking (on
SVG enabled builds) for all the necessary switch statements for CSS handling. 
This would add that back by either auto-generating all the needed subfunctions,
or leaving them empty (and thus causing a compile failure).

We could even make CSSValueKeywords.in autogenerated from some more complicated
file.

One possible source file:

CSSPropertiesAndValues.in:
stroke-miter-limit: <number>
alignment-baseline: [prefix: AB_] auto | baseline | before-edge |
text-before-edge | middle | central | after-edge | text-after-edge |
ideographic | alphabetic | hanging | mathematical | inherit
fill # custom
stroke # custom

That's obviously not a complete specification above, but some things to note:
1.  inherit could be explicit or assumed depending on what's more common
2.  some sort of custom value handling *by default*
3. automatic enum handling (note the use of a prefix for specifying how to
convert enums for the Impl)
4. handles different value types (just like the CSS grammar does)
5. Note that CSS_VAL defines become automatic and implicit.  Only if your
overriding behavior do you ever know they exist.  They become an implementation
detail of the CSS system hidden from most coders.

I'd be curious to hear others thoughts.  I think it'd be relatively
straightforward to break the existing switch statements into subfunctions and
autogenerate the parent function today.  Then slowly we could add enum support,
and other more advanced sub-function support.


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