[webkit-dev] Multiple inheritance in the DOM

[Dirk Schulze]

On Jul 25, 2012, at 3:50 PM, Adam Barth wrote:

> On Wed, Jul 25, 2012 at 3:44 PM, Dirk Schulze <dschulze at adobe.com> wrote:
>> 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.
> 
> Do you anticipate adding properties or functions that return
> interfaces like SVGLocatable?
Here is a Wiki what we plan to do: http://www.w3.org/Graphics/SVG/WG/wiki/Proposals/IDL_interface_reorganization

It might not reflect all changes that we discussed during the SVG WG meeting today.

Greetings,
Dirk

> 
> Adam
> 
> 
>>> 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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