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 <aba...@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 _______________________________________________ webkit-dev mailing list webkit-dev@lists.webkit.org http://lists.webkit.org/mailman/listinfo/webkit-dev