Re: [geometry] DOMQuad is weird

> On Oct 19, 2016, at 11:37 AM, Rik Cabanier <cabanier@gmail.com> wrote:
> 
> On Tue, Oct 18, 2016 at 11:29 AM, Simon Fraser <smfr@me.com <mailto:smfr@me.com>> wrote:
> DOMQuad seems like the odd one out of the Geometry interfaces. It doesn’t have a ReadOnly variant, and, unlike DOMRect, its points are 3D DOMPoints (with x,y,w,z). In the WebKit implementation, we never have a need to store quads in 3D space; they only ever exist in the plane of an element. We do have code that tracks an accumulated matrix along with a planar quad when mapping through 3D rendering contexts, but the quads are only ever used once you flatten into the plane of an element.
> 
> Also, I think [SameObject] readonly attribute DOMPoint cause unwanted behavior for JS authors.
> 
> Consider:
> 
> var quad = DOMQuad.fromQuad({ p1:{ x:11, y:12 }, p2:{ x:22, y:23 }, p3:{ x:33, y:34 }, p4:{ x:44, y:45 } });
> 
> var firstPoint = quad.p1;
> firstPoint.x = 9999;
> 
> if (quad.p1.x == 9999)
>     console.log(“what the hey”); <— this happens
> 
> I think it’s very counterintuitive for JS authors to have the points be “live” if you pass them to other parts of the code and then modify them.
> 
> I don't understand why that would be counter-intuitive. Isn't this normal JavaScript behavior?

As a naive JavaScript author, I would consider a point to be a primitive type that's copied by value.

Dean points out that the primitive types are actually DOMPoint.x and DOMPoint.y, so what I wrote above is "expected" JS behavior.

> Having them be non-live would be "magic" because a DOMQuad is supposed to be a simple detached object.

Maybe DOMQuad should be entirely read-only, or should return copies of points (getP1(), getP2() etc).

Simon