Attrs are never observed unless added with `@attr` decorator

[kristoforsalmin]

Hi folks,

While I was testing the attributes — trying to work my way around #207 — I found another subtle detail. Manually initialized attributes are never observed.

Catalyst's guide provides the following example for vanilla users: https://github.com/github/catalyst/blob/main/docs/_guide/attrs.md#what-about-without-decorators.

Please notice the second argument there defineObservedAttributes(HelloWorldElement, ['foo']) and compare it with the actual function declaration:

https://github.com/github/catalyst/blob/a8fb3ba59a4ac258d0fe64a50ecd3b6997f5f016/src/attr.ts#L99

It doesn't expect the second argument (arguments aren't used either).

Now, internally defineObservedAttributes() pulls attribute names from getAttrNames(), which, in turn, pulls them from attrs (WeakMap). The only way to populate attrs is to use @attr decorator:

https://github.com/github/catalyst/blob/a8fb3ba59a4ac258d0fe64a50ecd3b6997f5f016/src/attr.ts#L15

Maybe I'm missing something, but it seems like there's no way to specify observed attributes without decorators. In any case, it'd be very nice to clarify that 🙂

Thank you!

[kristoforsalmin]

Using @attr as a function (like in tests) is the best workaround I found so far:

import {attr, controller} from '@github/catalyst'

class HelloWorldElement extends HTMLElement {
  foo = 1
}

attr(HelloWorldElement.prototype, 'foo');

controller(HelloWorldElement);

Attributes are correctly initialized (solving #207) and being observed... This can actually be a recommended way to add attributes without decorators?.. A bit verbose, but no more than calling initializeAttrs() and defineObservedAttributes() manually. With a tiny wrapper on user's end it could look like defineAttrs(HelloWorldElement, ['foo']).

Thoughts?