Yep, thanks for that. I somehow missed the bit where it says, very
clearly, that attributes MUST NOT be dictionaries.
I was looking for something to give someone else who doesn't like
explanations and prefers reading from the scripture. Your explanation
matches my understanding.
BTW, there are some people who consider the fact that a dictionary
becomes outside the control of the browser to be a feature. That is
partly why WebRTC is how it is.
On Mon, Feb 15, 2016 at 10:11 AM, Cameron McCormack <c...@mcc.id.au> wrote:
> Martin Thomson:
>> I know that this is not good practice, but is there something written
>> down somewhere explaining why?
>
> I don’t know if it’s written down somewhere. Using dictionary types for
> IDL attributes is forbidden by the spec, because it would mean that a
> new copy of the object would need to be returned each time the property
> were accessed. This is the case for sequence<T> types too, where you
> can much more obviously encourage wasteful object creation:
>
> interface A {
> attribute sequence<double> values;
> };
>
> for (var i = 0; i < myA.values.length; i++) {
> …
> }
>
> That would create a new JS Array each time around the loop. With
> dictionaries you might access a couple of properties from the return
> value of the getter property and have similar issues:
>
> dictionary D {
> double x;
> doubly y;
> };
>
> interface A {
> attribute D d;
> };
>
> Math.sqrt(myA.d.x * myA.d.x + myA.d.y * myA.d.y);
>
> This would create four copies of the JS object for the dictionary.
>
> Another point is that these sequence and dictionary objects can’t be
> monitored for changes, so for example you couldn’t write a spec that
> required the browser to do something when you assign to myA.d.x since
> that’s just a plain data property on the object returned from d. So for
> APIs where you do want to notice property value changes like this,
> you’ll need to use interfaces, and for array-ish things we’ve now got
> FrozenArray<T>, which is an array reference type (as opposed to
> sequence’s (and dictionaries’) pass-by-value behaviour).
>
> We don’t currently have a type that means “reference to an object that
> has a particular shape defined by a dictionary”. So for now if you
> really want an API that allows
>
> myA.d = { x: 1, y: 2 };
>
> where the A object either immediately, or later, inspects the values on
> the object, then you have to use “object” as the type and invoke the
> type conversions or do the JS property getting in the spec yourself.
>
> --
> Cameron McCormack ≝ http://mcc.id.au/
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