Hi List,
TLDR; should a CSS reference like -fx-base convert all styles that use
this value (or derive from it) become AUTHOR level styles (higher
priority than setters) ?
Long version:
In JavaFX 21, I did a fix (see #1072) to solve a problem where a CSS
value could be reset on an unrelated control.
This happened when the CSS engine encountered a stylable that is
overridden by the user (with a setter), and decided NOT to proceed with
the full CSS value calculation (as it could not override the user
setting if that CSS value had lower priority). However, not proceeding
with the calculation meant that a "SKIP" was stored in a shared cache
which was incorrect. This is because when this "SKIP" is later
encountered for an unrelated control (the cache entries are shared for
controls with the same styles at the same level), they could get their
values reset because they were assumed to be unstyled.
However, this fix has exposed what seems to be a deeper bug or perhaps
an unfortunate default:
JavaFX has a special feature where you can refer to certain other styles
by using a reference (which is resolved, recursively, to a final
value). This does not seem to be a CSS standard, but is a feature only
FX has.
It works by saying something like:
-fx-base: RED;
And then using it like this:
-fx-text-fill: -fx-base;
This feature works accross stylesheets of different origins, so an
AUTHOR stylesheet can specify -fx-base, and when a USER_AGENT refers to
-fx-base, the value comes from the AUTHOR stylesheet.
JavaFX then changes the origin of the style to the highest priority
encountered while resolving the reference. This means that Modena can
specify "-fx-text-fill: -fx-base", and when "-fx-base" is then part of
the AUTHOR style sheet, that ALL Modena styles that use -fx-base will be
considered AUTHOR level styles, as per this comment:
// The origin of this parsed value is the greatest of
// any of the resolved reference. If a resolved reference
// comes from an inline style, for example, then the value
// calculated from the resolved lookup should have inline
// as its origin. Otherwise, an inline style could be
// stored in shared cache.
I feel that this is a really unfortunate choice. The style after all
was specified by Modena, only its value came from another (higher
priority) style sheet. I think a more logical choice would have been to
not change the priority at all, unless a "-fx-text-fill" is explicitly
made part of the AUTHOR stylesheet.
A consequence of this (and which is much more visible after the fix) is
that creating a Label with a setTextFill(Color.YELLOW) in its
constructor will only result in a yellow text fill if the AUTHOR
stylesheet did not override any of the Modena colors involved in
calculating the Modena -fx-text-fill default. Overriding -fx-base in any
way will result in the text fill of the label to be overridden (as the
reference came from an AUTHOR stylesheet, which trumps a setter which is
of a lower style origin).
The comment also alludes to a potential problem. If an inline style
would specify "-fx-base", but would be treated as if it came from Modena
(USER_AGENT level), then this value could get stored in the cache as
everything except INLINE styles can be cached. However, I feel that the
changing of style origin level was then probably done to solve a CACHING
problem, instead of what made logical sense for users. If we agree that
a resolved reference should not change the style origin level, then this
would need to be addressed, by perhaps marking such a calculated value
as uncacheable, instead of overloading the meaning of style origin.
I'd like to hear your thoughts, and also how to proceed. JavaFX
versions before 21 seemingly allowed overriding reference without much
consequence because if the user overrode the value manually, the cache
entry would be set to "SKIP". Now that this is no longer the case,
JavaFX more aggressively overrides user set values if they happen to use
a referenced value. See code below.
--John
.root {
-fx-base: #ff0000;
}
packageapp;
importjavafx.application.Application;
importjavafx.scene.Scene;
importjavafx.scene.control.Label;
importjavafx.scene.paint.Color;
importjavafx.stage.Stage;
publicclassTestApp extendsApplication {
publicstaticvoidmain(String[] args) {
launch(args);
}
@Override
publicvoidstart(Stage primaryStage) {
Scene scene = newScene(newMyLabel());
// See the difference with/without -fx-base in the stylesheet
scene.getStylesheets().add(TestApp.class.getResource("/style.css").toExternalForm());
primaryStage.setScene(scene);
primaryStage.show();
}
}
classMyLabel extendsLabel {
publicMyLabel() {
setTextFill(Color.YELLOW);
setText("Hello world");
}
}
