[webkit-dev] Multiple inheritance in the DOM

[Adam Barth]

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?

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
>> _______________________________________________
>> webkit-dev mailing list
>> webkit-dev at lists.webkit.org
>> http://lists.webkit.org/mailman/listinfo/webkit-dev
>