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LibWeb: Start implementing the CSS cascade

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The 'C' in "CSS" is for Cascade, so let's actually implement the cascade
in LibWeb. :^)

StyleResolver::resolve_style() now begins by collecting all the matching
CSS rules for the given DOM::Element. Rules are then processed in the
spec's cascade order (instead of in the order we encounter them.)

With this, "!important" is now honored on CSS properties.

After performing the cascade, we do another pass of what the spec calls
"defaulting" where we resolve "inherit" and "initial" values.
I've left a FIXME about supporting correct "initial" values for every
property, since we're currently lacking some coverage there.

Note that this mechanism now resolves every known CSS property. This is
*not* space-efficient and we'll eventually need to come up with some
strategies to reduce memory usage around this. However, this will do
fine until we have more of the engine working correctly. :^)
This commit is contained in:
Andreas Kling 2021-09-21 11:38:18 +02:00
parent c55909f175
commit d965a9552f
4 changed files with 150 additions and 77 deletions
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159
Userland/Libraries/LibWeb/CSS/StyleResolver.cpp
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@ -52,22 +52,26 @@ static StyleSheet& quirks_mode_stylesheet()
}
template<typename Callback>
void StyleResolver::for_each_stylesheet(Callback callback) const
void StyleResolver::for_each_stylesheet(CascadeOrigin cascade_origin, Callback callback) const
{
callback(default_stylesheet());
if (document().in_quirks_mode())
callback(quirks_mode_stylesheet());
for (auto& sheet : document().style_sheets().sheets()) {
callback(sheet);
if (cascade_origin == CascadeOrigin::Any || cascade_origin == CascadeOrigin::UserAgent) {
callback(default_stylesheet());
if (document().in_quirks_mode())
callback(quirks_mode_stylesheet());
}
if (cascade_origin == CascadeOrigin::Any || cascade_origin == CascadeOrigin::Author) {
for (auto& sheet : document().style_sheets().sheets()) {
callback(sheet);
}
}
}
Vector<MatchingRule> StyleResolver::collect_matching_rules(DOM::Element const& element) const
Vector<MatchingRule> StyleResolver::collect_matching_rules(DOM::Element const& element, CascadeOrigin declaration_type) const
{
Vector<MatchingRule> matching_rules;
size_t style_sheet_index = 0;
for_each_stylesheet([&](auto& sheet) {
for_each_stylesheet(declaration_type, [&](auto& sheet) {
size_t rule_index = 0;
static_cast<CSSStyleSheet const&>(sheet).for_each_effective_style_rule([&](auto& rule) {
size_t selector_index = 0;
@ -531,54 +535,129 @@ Optional<StyleProperty> StyleResolver::resolve_custom_property(DOM::Element& ele
return resolved_with_specificity.style;
}
NonnullRefPtr<StyleProperties> StyleResolver::resolve_style(DOM::Element& element) const
struct MatchingDeclarations {
Vector<MatchingRule> user_agent_rules;
Vector<MatchingRule> author_rules;
};
void StyleResolver::cascade_declarations(StyleProperties& style, DOM::Element& element, Vector<MatchingRule> const& matching_rules, CascadeOrigin cascade_origin, bool important) const
{
auto style = StyleProperties::create();
auto* parent_style = element.parent_element() ? element.parent_element()->specified_css_values() : nullptr;
if (parent_style) {
parent_style->for_each_property([&](auto property_id, auto& value) {
if (is_inherited_property(property_id))
set_property_expanding_shorthands(style, property_id, value, m_document);
});
}
element.apply_presentational_hints(*style);
auto matching_rules = collect_matching_rules(element);
sort_matching_rules(matching_rules);
for (auto& match : matching_rules) {
for (auto& property : verify_cast<PropertyOwningCSSStyleDeclaration>(match.rule->declaration()).properties()) {
if (important != property.important)
continue;
auto property_value = property.value;
if (property.value->is_custom_property()) {
auto prop = reinterpret_cast<CSS::CustomStyleValue const*>(property.value.ptr());
auto custom_prop_name = prop->custom_property_name();
auto resolved = resolve_custom_property(element, custom_prop_name);
auto custom_property_name = static_cast<CSS::CustomStyleValue const&>(*property.value).custom_property_name();
auto resolved = resolve_custom_property(element, custom_property_name);
if (resolved.has_value()) {
property_value = resolved.value().value;
}
}
// FIXME: This also captures shorthands of which we ideally want to resolve the long names separately.
if (property_value->is_inherit()) {
// HACK: Trying to resolve the font property here lead to copious amounts of debug-spam
if (property.property_id == CSS::PropertyID::Font)
continue;
if (parent_style) {
auto maybe_parent_property_value = parent_style->property(property.property_id);
if (maybe_parent_property_value.has_value())
property_value = maybe_parent_property_value.release_value();
}
}
set_property_expanding_shorthands(style, property.property_id, property_value, m_document);
}
}
if (auto* inline_style = verify_cast<ElementInlineCSSStyleDeclaration>(element.inline_style())) {
for (auto& property : inline_style->properties()) {
set_property_expanding_shorthands(style, property.property_id, property.value, m_document);
if (cascade_origin == CascadeOrigin::Author) {
if (auto* inline_style = verify_cast<ElementInlineCSSStyleDeclaration>(element.inline_style())) {
for (auto& property : inline_style->properties()) {
if (important != property.important)
continue;
set_property_expanding_shorthands(style, property.property_id, property.value, m_document);
}
}
}
}
// https://drafts.csswg.org/css-cascade/#cascading
void StyleResolver::compute_cascaded_values(StyleProperties& style, DOM::Element& element) const
{
// First, we collect all the CSS rules whose selectors match `element`:
MatchingRuleSet matching_rule_set;
matching_rule_set.user_agent_rules = collect_matching_rules(element, CascadeOrigin::UserAgent);
sort_matching_rules(matching_rule_set.user_agent_rules);
matching_rule_set.author_rules = collect_matching_rules(element, CascadeOrigin::Author);
sort_matching_rules(matching_rule_set.author_rules);
// Then we apply the declarations from the matched rules in cascade order:
// Normal user agent declarations
cascade_declarations(style, element, matching_rule_set.user_agent_rules, CascadeOrigin::UserAgent, false);
// FIXME: Normal user declarations
// Normal author declarations
cascade_declarations(style, element, matching_rule_set.author_rules, CascadeOrigin::Author, false);
// Author presentational hints (NOTE: The spec doesn't say exactly how to prioritize these.)
element.apply_presentational_hints(style);
// FIXME: Animation declarations [css-animations-1]
// Important author declarations
cascade_declarations(style, element, matching_rule_set.author_rules, CascadeOrigin::Author, true);
// FIXME: Important user declarations
// Important user agent declarations
cascade_declarations(style, element, matching_rule_set.user_agent_rules, CascadeOrigin::UserAgent, true);
// FIXME: Transition declarations [css-transitions-1]
}
// https://drafts.csswg.org/css-cascade/#defaulting
void StyleResolver::compute_defaulted_values(StyleProperties& style, DOM::Element const& element) const
{
// FIXME: If we don't know the correct initial value for a property, we fall back to InitialStyleValue.
auto get_initial_value = [&](PropertyID property_id) -> NonnullRefPtr<StyleValue> {
auto value = property_initial_value(property_id);
if (!value)
return InitialStyleValue::the();
return value.release_nonnull();
};
auto get_inherit_value = [&](PropertyID property_id) -> NonnullRefPtr<StyleValue> {
if (!element.parent_element() || !element.parent_element()->specified_css_values())
return get_initial_value(property_id);
auto& map = element.parent_element()->specified_css_values()->m_property_values;
auto it = map.find(property_id);
VERIFY(it != map.end());
return const_cast<StyleValue&>(*it->value);
};
// Walk the list of all known CSS properties and:
// - Add them to `style` if they are missing.
// - Resolve `inherit` and `initial` as needed.
for (auto i = to_underlying(CSS::first_property_id); i <= to_underlying(CSS::last_property_id); ++i) {
auto property_id = (CSS::PropertyID)i;
auto it = style.m_property_values.find(property_id);
if (it == style.m_property_values.end()) {
if (is_inherited_property(property_id))
style.m_property_values.set(property_id, get_inherit_value(property_id));
else
style.m_property_values.set(property_id, get_initial_value(property_id));
continue;
}
if (it->value->is_initial()) {
it->value = get_initial_value(property_id);
continue;
}
if (it->value->is_inherit()) {
it->value = get_inherit_value(property_id);
continue;
}
}
}
NonnullRefPtr<StyleProperties> StyleResolver::resolve_style(DOM::Element& element) const
{
auto style = StyleProperties::create();
compute_cascaded_values(style, element);
compute_defaulted_values(style, element);
return style;
}