Support private classes

[idoros]

Currently any CSS class declared in a Stylable stylesheet is considered a pseudo-element public API. In some cases a stylesheet author could keep "template" classes by importing them and mixin them in, or use the future -st-filter: deny(*) to block any CSS overrides, but it would be much more helpful to introduce an -st-access: private directive that will simply hide the class from external overrides.

/* gallery.st.css */
.navBtn {}
.internalBox {
    -st-access: private;
}

/* page.st.css */
:import {
    -st-from: './gallery.st.css';
    -st-default: Gallery;
}

Gallery::navBtn {/* style the internal navBtn */}
Gallery::internalBox {/* error: unknown pseudo-element "::internalBox" */}

---

edit: possible values should be:

• public (default)
• private - cannot be accessed as a pseudo element or extended
• protected - cannot be accessed as a pseudo element, but can be styled in the context of a root that directly extends the stylesheet (or extending a stylesheet that extends the original stylesheet).

[brucelawson]

OK, putting on my W3C mailing list hat: what's the use-case?

[idoros]

Simply that Stylable exposes any CSS class as a pseudo-element by default, and there is no way to have simple CSS class that are not accessible from the outside.

[brucelawson]

From a conversation with Ido on Slack (putting it here to remind me when I have another attack of the stupids):

why have a special thing that hides it, when -st-overrride: deny(*) would do the same thing?

idoros [6:12 PM]
denying access to override is not the same as hidding a class
denying overrides will not prevent access to any lower pseudo-elements

bruce [6:13 PM]
BINGO
now I understand the difference

[brucelawson]

Interesting note from @tabatkins on design decisions for CSS that allows component authors to "lock down" styles, while allowing component users to override defaults w3c/csswg-drafts#2290 (comment)

[tomrav]

Closing in favor of #2604