[webkit-dev] Multiple inheritance in the DOM

[Adam Barth]

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