[webkit-dev] Multiple inheritance in the DOM

[Adam Barth]

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*).

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