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 <[email protected]> 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 > [email protected] > http://lists.webkit.org/mailman/listinfo/webkit-dev _______________________________________________ webkit-dev mailing list [email protected] http://lists.webkit.org/mailman/listinfo/webkit-dev
