The [`@import`](https://www.w3.org/TR/css-cascade-5/#at-import) rule is
extended to support conditional stylesheet imports:
@import [ <url> | <string> ] <media-query-list>? ;
Conceptually, a conditional import takes all rules of the referenced
stylesheet, literally includes them at the location of the `@import` rule, and
surrounds them with a `@media` rule with the specified `<media-query-list>`.
The implementation of this feature extends media queries with a context-free
evaluation function:
interface MediaQuery {
TriState evaluate(); // returns TRUE / FALSE / UNKNOWN
}
This mode of evaluation uses Kleene's strong trivalent logic to determine
whether the media query will always match (it's a tautology), will never match
(it's a contradiction), or whether it depends on the context or we simply don't
know.
Using this mechanism, a conditional stylesheet import that can never match will
be skipped at runtime. Similarly, a conditional stylesheet import that will
always match doesn't need the `<media-query-list>` at all; its rules will be
pasted verbatim into the main stylesheet.
Note that this mechanism is _not_ meant to be a full-blown theorem solver. It
won't detect that, for example, the following import conditions will always
match:
@import url("stylesheet.css") (width > 1000) or (width <= 1000);
Its purpose is an optimization for "obvious" cases. For example, at the moment
the built-in themes use hard-coded conditions to include user-agent stylesheets
at runtime:
// in PlatformImpl.java:
if (isSupported(ConditionalFeature.INPUT_TOUCH)) {
uaStylesheets.add("com/sun/javafx/scene/control/skin/modena/touch.css");
}
In the future, we might have a way to test for `ConditionalFeature` flags that
achieves the same thing, but with a public API available to all theme authors:
@import url("com/sun/javafx/scene/control/skin/modena/touch.css")
(-fx-supports-conditional-feature: input-touch);
-------------
Commit messages:
- Conditional stylesheet imports
Changes: https://git.openjdk.org/jfx/pull/2031/files
Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2031&range=00
Issue: https://bugs.openjdk.org/browse/JDK-8364149
Stats: 934 lines in 21 files changed: 821 ins; 45 del; 68 mod
Patch: https://git.openjdk.org/jfx/pull/2031.diff
Fetch: git fetch https://git.openjdk.org/jfx.git pull/2031/head:pull/2031
PR: https://git.openjdk.org/jfx/pull/2031
