[webkit-dev] Multiple inheritance in the DOM

[Dirk Schulze]

On Jul 25, 2012, at 2:33 PM, Adam Barth wrote:

> Eric Seidel points out that SVG uses multiple inheritance in its DOM
> interfaces.  However, the situation there is a bit different.
> Although SVGSVGElement implements SVGLocatable, there aren't any
> interfaces with methods that return SVGLocatable, which means we don't
> need to implement toJS(SVGLocatable*).
SVG 2 will use WebIDL. Therefore we also reorganize our inheritance behavior. Cameron, editor of WebIDL and SVG WG member, will update SVG 2 ED soon.

Greetings,
Dirk

> 
> He also points out that Node inherits from EventTarget, which already
> contains a virtual interfaceName() function similar to that used by
> Event.  That pushes us further towards using a common DOMInterface
> base class because introducing Region::interfaceName would mean that
> Element would see both EventTarget::interfaceName and
> Region::interfaceName.
> 
> Adam
> 
> 
> On Wed, Jul 25, 2012 at 2:00 PM, Adam Barth <abarth at webkit.org> wrote:
>> The CSS Regions specification [1] defines a CSSOM interface named
>> Region, which can be mixed into interfaces for other objets that can
>> be CSS regions.  That means that Region introduces a form of multiple
>> inheritance into the DOM.  For example, Element implements Region but
>> Node does not implement Region.
>> 
>> There's a patch up for review that implements Region using C++
>> multiple inheritance [2]:
>> 
>> - class Element : public ContainerNode {
>> + class Element : public ContainerNode, public CSSRegion {
>> 
>> One difficulty in implementing this feature how to determine the
>> correct JavaScript wrapper return for a given Region object.
>> Specifically, toJS(Region*) needs to return a JavaScript wrapper for
>> an Element if the Region pointer actually points to an Element
>> instance.
>> 
>> We've faced a similar problem elsewhere in the DOM when implementing
>> normal single inheritance.  For example, there are many subclass of
>> Event and toJS(Event*) needs to return a wrapper for the appropriate
>> subtype.  To solve the same problem, CSSRule has a m_type member
>> variable and a bevy of isFoo() functions [3].
>> 
>> A) Should we push back on the folks writing the CSS Regions
>> specification to avoid using multiple inheritance?  As far as I know,
>> this is the only instance of multiple inheritance in the platform.
>> Historically, EventTarget used multiple inheritance, but that's been
>> fixed in DOM4 [4].
>> 
>> B) If CSS Regions continues to require multiple inheritance, should we
>> build another one-off RTTI replacement to implement toJS(Region*), or
>> should we improve our bindings to implement this aspect of WebIDL more
>> completely?
>> 
>> One approach to implementing toJS in a systematic way is to introduce
>> a base class DOMInterface along these lines:
>> 
>> class DOMInterface {
>> public:
>>    virtual const AtomicString& primaryInterfaceName() = 0;
>> }
>> 
>> That returns the name of the primary interface (i.e., as defined by
>> WebIDL [5]).  When implementing toJS, we'd then call
>> primaryInterfaceName to determine which kind of wrapper to use.
>> 
>> One downside of this approach is that it introduces a near-universal
>> base class along the lines of IUnknown [6] or nsISupports [7].  I
>> don't think any of us want WebKit to grow an implementation of
>> XPCOM...
>> 
>> I welcome any thoughts you have on this topic.
>> 
>> Thanks,
>> Adam
>> 
>> [1] http://dev.w3.org/csswg/css3-regions/
>> [2] https://bugs.webkit.org/show_bug.cgi?id=91076
>> [3] http://trac.webkit.org/browser/trunk/Source/WebCore/css/CSSRule.h?rev=123653#L65
>> [4] http://www.w3.org/TR/dom/#node
>> [5] http://www.w3.org/TR/WebIDL/#dfn-primary-interface
>> [6] http://msdn.microsoft.com/en-us/library/windows/desktop/ms680509(v=vs.85).aspx
>> [7] https://developer.mozilla.org/en/XPCOM_Interface_Reference/nsISupports
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