Re: [webcomponents] Inheritance in Custom Elements (Was Proposal for Cross Origin Use Case and Declarative Syntax)

On Sun, Dec 8, 2013 at 7:25 PM, Ryosuke Niwa <> wrote:

> On Dec 7, 2013, at 4:38 PM, Dominic Cooney <> wrote:
> On Fri, Dec 6, 2013 at 3:34 PM, Ryosuke Niwa <> wrote:
>> On Dec 5, 2013, at 10:09 PM, Hajime Morrita <> wrote:
>> > On inheritance around HTMLElement family, there seems to be a confusion
>> between interface side inheritance and implementation side inheritance.
>> Right.  Differentiating the two is very important.
>> > For Custom Elements, the inheritance is not only about interface, but
>> also about implementation. The implementation is more complex and flux in
>> detail, thus worth being shared between different elements. Actually, the
>> implementation of built-in HTML elements, which are usually done in C++,
>> uses inheritance heavily, at least Blink and (I believe) WebKit.
>> The reason we can use inheritance heavily is because we have control over
>> both parent and child classes, and there are careful distinctions between
>> public, protected, and private member functions and member variables in
>> those classes.
>> Unfortunately, custom elements can't easily inherit from these builtin
>> HTML elements because builtin elements do not provide "protected"
>> functions, and they don't expose necessary hooks to override internal
>> member functions to modify behaviors.
> Built-in HTML elements have lots of hooks to modify their behaviour (for
> example, HTMLVideoElement's autoplay attribute.) The analogy is extending a
> Java class which has private and public final members, but no protected or
> public non-final ones.
> If someone were to make proposals about adding more hooks to the web
> platform to enable more subtyping use cases (for example, a protocol for
> participating in form submission) I would look forward to working with them
> on those proposals.
> <>
> The problem here is that until such hooks are well defined, inheriting
> from builtin elements doesn't make any sense. So let's not support that
> until such proposal is made and implemented.

You assert that inheriting from built-in elements does not make any sense.
You seem to base this on the claim that hooks (the example being form
submission protocol hooks) are not well defined. Whether hooks are well
defined or not doesn't sufficiently support your assertion, because:

1. Not all subtyping relies on hooks. For example, subtyping a built-in
element can add additional API to it. This may make sense. For example, a
social network "endorse" button has a number of additional properties. Yet
it is (or could be) a button.

2. The premise that hooks are not defined is also incorrect. Many hooks are
already defined. Here is one example: the INPUT element has input and
change events and a value property. These are sufficient to observe, filter
and change the value of the INPUT element.

In mentioning hooks, was merely observing that we could define more hooks
to enable even more use cases.

> In addition, consider how inheriting "views" work in AppKit, UIKit, .net
> and MFC. When a view T inherits from another view S, T doesn't simply put S
> into a subregion of T; S intrudes into T's view and modifies what's been
> displayed or places extra content after or before T draws itself.

I can not discern a specific point you are making here. You are drawing an
analogy to various UI frameworks but you have not completed the analogy. I
think if you completed the analogy it would end up being a claim about
Shadow DOM. You may have confused extending an element and having Shadow
DOM. As Rafael Weinstein points out here: <>,
whether an element has Shadow DOM is orthogonal to whether it is a Custom
Element or not. I also disagree with your implied argument about Shadow DOM.

> I have a hard time understanding how this bizarre mixture of inheritance
> and composition is considered as the minimal subset of features while
> adding one optional argument to document.register to eliminate the most
> common boilerplate is considered as "building a framework" and "bundling"
> orthogonal features. Where did use cases and developer ergonomics go?

The use cases were discussed on this list in 2010.

As for ergonomics, this is subjective, but I spent a day last week building
a Polymer app and the ergonomics were splendid. I have played with X-Tags
in the past and they're great too.


Received on Monday, 9 December 2013 00:42:55 UTC