ladybird/Libraries/LibWeb/SVG/SVGUseElement.cpp

/*
 * Copyright (c) 2023, Preston Taylor <95388976+PrestonLTaylor@users.noreply.github.com>
 * Copyright (c) 2024-2026, Shannon Booth <shannon@serenityos.org>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <LibGC/HeapHashTable.h>
#include <LibWeb/Bindings/Intrinsics.h>
#include <LibWeb/Bindings/SVGUseElement.h>
#include <LibWeb/DOM/Document.h>
#include <LibWeb/DOM/DocumentLoadEventDelayer.h>
#include <LibWeb/DOM/ElementFactory.h>
#include <LibWeb/DOM/Event.h>
#include <LibWeb/DOM/ShadowRoot.h>
#include <LibWeb/HTML/PotentialCORSRequest.h>
#include <LibWeb/HTML/SharedResourceRequest.h>
#include <LibWeb/Layout/Box.h>
#include <LibWeb/Layout/SVGGraphicsBox.h>
#include <LibWeb/Namespace.h>
#include <LibWeb/SVG/AttributeNames.h>
#include <LibWeb/SVG/SVGDecodedImageData.h>
#include <LibWeb/SVG/SVGSVGElement.h>
#include <LibWeb/SVG/SVGSymbolElement.h>
#include <LibWeb/SVG/SVGUseElement.h>

namespace Web::SVG {

GC_DEFINE_ALLOCATOR(SVGUseElement);

SVGUseElement::SVGUseElement(DOM::Document& document, DOM::QualifiedName qualified_name)
    : SVGGraphicsElement(document, move(qualified_name))
{
}

void SVGUseElement::initialize(JS::Realm& realm)
{
    WEB_SET_PROTOTYPE_FOR_INTERFACE(SVGUseElement);
    Base::initialize(realm);

    // NOTE: The spec says "The shadow tree is open (inspectable by script), but read-only."
    //       This doesn't actually match other browsers, and there's a spec issue to change it.
    //       Spec bug: https://github.com/w3c/svgwg/issues/875
    auto shadow_root = realm.create<DOM::ShadowRoot>(document(), *this, Bindings::ShadowRootMode::Closed);
    shadow_root->set_user_agent_internal(true);

    // The user agent must create a use-element shadow tree whose host is the ‘use’ element itself
    set_shadow_root(shadow_root);

    m_document_observer = realm.create<DOM::DocumentObserver>(realm, document());
    m_document_observer->set_document_completely_loaded([this]() {
        clone_element_tree_as_our_shadow_tree(referenced_element());
    });
}

void SVGUseElement::visit_edges(Cell::Visitor& visitor)
{
    Base::visit_edges(visitor);
    SVGURIReferenceMixin::visit_edges(visitor);
    visitor.visit(m_document_observer);
    visitor.visit(m_resource_request);
}

void SVGUseElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value, Optional<FlyString> const& namespace_)
{
    Base::attribute_changed(name, old_value, value, namespace_);

    // https://svgwg.org/svg2-draft/struct.html#UseLayout
    if (name == SVG::AttributeNames::x) {
        m_x = AttributeParser::parse_coordinate(value.value_or(String {}));
    } else if (name == SVG::AttributeNames::y) {
        m_y = AttributeParser::parse_coordinate(value.value_or(String {}));
    } else if (name == SVG::AttributeNames::href || name == "xlink:href"_fly_string) {
        // When the ‘href’ attribute is set (or, in the absence of an ‘href’ attribute, an ‘xlink:href’ attribute), the user agent must process the URL.
        process_the_url(value);
    }
}

// https://www.w3.org/TR/SVG2/linking.html#processingURL
void SVGUseElement::process_the_url(Optional<String> const& href)
{
    // In all other cases, the URL is for a resource to be used in this SVG document. The user agent
    // must parse the URL to separate out the target fragment from the rest of the URL, and compare
    // it with the document base URL. If all parts other than the target fragment are equal, this is
    // a same-document URL reference, and processing the URL must continue as indicated in Identifying
    // the target element with the current document as the referenced document.
    m_href = document().url().complete_url(href.value_or(String {}));
    if (!m_href.has_value())
        return;

    if (is_referenced_element_same_document()) {
        clone_element_tree_as_our_shadow_tree(referenced_element());
    } else {
        fetch_the_document(*m_href);
    }
}

bool SVGUseElement::is_referenced_element_same_document() const
{
    return m_href->equals(document().url(), URL::ExcludeFragment::Yes);
}

Gfx::AffineTransform SVGUseElement::element_transform() const
{
    // The x and y properties define an additional transformation (translate(x,y), where x and y represent the computed value of the corresponding property)
    // to be applied to the ‘use’ element, after any transformations specified with other properties
    return Base::element_transform().translate(m_x.value_or(0), m_y.value_or(0));
}

void SVGUseElement::svg_element_changed(SVGElement& svg_element)
{
    auto to_clone = referenced_element();
    if (!to_clone) {
        return;
    }

    // NOTE: We need to check the ancestor because attribute_changed of a child doesn't call children_changed on the parent(s)
    if (to_clone == &svg_element || to_clone->is_ancestor_of(svg_element)) {
        clone_element_tree_as_our_shadow_tree(to_clone);
    }
}

void SVGUseElement::svg_element_removed(SVGElement& svg_element)
{
    if (!m_href.has_value() || !m_href->fragment().has_value() || !is_referenced_element_same_document()) {
        return;
    }

    auto id = String::from_utf8_with_replacement_character(URL::percent_decode(*m_href->fragment()), String::WithBOMHandling::No);
    if (AK::StringUtils::matches(svg_element.get_attribute_value("id"_fly_string), id)) {
        shadow_root()->remove_all_children();
    }
}

// https://svgwg.org/svg2-draft/linking.html#processingURL-target
GC::Ptr<DOM::Element> SVGUseElement::referenced_element() const
{
    if (!m_href.has_value())
        return nullptr;

    if (!m_href->fragment().has_value())
        return nullptr;

    if (is_referenced_element_same_document()) {
        auto id = String::from_utf8_with_replacement_character(URL::percent_decode(*m_href->fragment()), String::WithBOMHandling::No);
        return document().get_element_by_id(id);
    }

    if (!m_resource_request)
        return nullptr;

    auto data = m_resource_request->image_data();
    if (!data || !is<SVG::SVGDecodedImageData>(*data))
        return nullptr;

    return as<SVG::SVGDecodedImageData>(*data).svg_document().get_element_by_id(*m_href->fragment());
}

// https://svgwg.org/svg2-draft/linking.html#processingURL-fetch
void SVGUseElement::fetch_the_document(URL::URL const& url)
{
    m_load_event_delayer.emplace(document());
    m_resource_request = HTML::SharedResourceRequest::get_or_create(realm(), document().page(), url);
    m_resource_request->add_callbacks(
        [this] {
            clone_element_tree_as_our_shadow_tree(referenced_element());
            m_load_event_delayer.clear();
        },
        [this] {
            m_load_event_delayer.clear();
        });

    if (m_resource_request->needs_fetching()) {
        auto request = HTML::create_potential_CORS_request(vm(), url, Fetch::Infrastructure::Request::Destination::Image, HTML::CORSSettingAttribute::NoCORS);
        request->set_client(&document().relevant_settings_object());
        m_resource_request->fetch_resource(realm(), request);
    }
}

// https://svgwg.org/svg2-draft/struct.html#UseShadowTree
void SVGUseElement::clone_element_tree_as_our_shadow_tree(Element* to_clone)
{
    shadow_root()->remove_all_children();

    // https://svgwg.org/svg2-draft/struct.html#UseStyleInheritance
    // When the referenced element is from the same document as the ‘use’ element, the same document stylesheets will
    // apply in both the original document and the shadow tree document fragment.
    shadow_root()->set_uses_document_style_sheets(to_clone && is_referenced_element_same_document());

    if (to_clone && is_valid_reference_element(*to_clone)) {
        // The ‘use’ element references another element, a copy of which is rendered in place of the ‘use’ in the document.
        auto cloned_reference_node = MUST(to_clone->clone_node(nullptr, true));
        if (is<SVGSVGElement>(cloned_reference_node.ptr()) || is<SVGSymbolElement>(cloned_reference_node.ptr())) {
            auto& cloned_element = as<SVGElement>(*cloned_reference_node);

            // The width and height properties on the ‘use’ element override the values for the corresponding
            // properties on a referenced ‘svg’ or ‘symbol’ element when determining the used value for that property
            // on the instance root element. However, if the computed value for the property on the ‘use’ element is
            // auto, then the property is computed as normal for the element instance.
            if (has_attribute(AttributeNames::width)) {
                cloned_element.set_attribute_value(AttributeNames::width, get_attribute_value(AttributeNames::width));
            }
            if (has_attribute(AttributeNames::height)) {
                cloned_element.set_attribute_value(AttributeNames::height, get_attribute_value(AttributeNames::height));
            }
        }
        shadow_root()->append_child(cloned_reference_node).release_value_but_fixme_should_propagate_errors();
    }
}

bool SVGUseElement::is_valid_reference_element(Element const& reference_element) const
{
    // If the referenced element that results from resolving the URL is not an SVG element, then the reference is invalid and the ‘use’ element is in error.
    if (!reference_element.is_svg_element())
        return false;

    // https://svgwg.org/svg2-draft/struct.html#UseShadowTree
    // When a ‘use’ references another element which is another ‘use’ or whose content contains a ‘use’ element, then
    // the shadow DOM cloning approach described above is recursive. However, a set of references that directly or
    // indirectly reference a element to create a circular dependency is an invalid circular reference. The ‘use’
    // element or element instance whose shadow tree would create the circular reference is in error and must not be
    // rendered by the user agent.
    return !would_create_circular_reference(reference_element);
}

bool SVGUseElement::would_create_circular_reference(Element const& target) const
{
    auto visited = heap().allocate<GC::HeapHashTable<GC::Ref<Element const>>>();
    return would_create_circular_reference_impl(target, visited);
}

bool SVGUseElement::would_create_circular_reference_impl(
    Element const& target,
    GC::HeapHashTable<GC::Ref<Element const>>& visited) const
{
    // FIXME: I am certain there is a much more efficient way of keeping track of cycles here!
    if (visited.table().contains(target))
        return true;

    visited.table().set(target);

    bool found_circular_reference = false;
    target.for_each_in_inclusive_subtree_of_type<SVGUseElement>([&](auto& element) {
        auto referenced = element.referenced_element();
        if (!referenced)
            return TraversalDecision::Continue;

        if (would_create_circular_reference_impl(*referenced, visited)) {
            found_circular_reference = true;
            return TraversalDecision::Break;
        }

        return TraversalDecision::Continue;
    });

    visited.table().remove(target);

    return found_circular_reference;
}

// https://www.w3.org/TR/SVG11/shapes.html#RectElementXAttribute
GC::Ref<SVGAnimatedLength> SVGUseElement::x() const
{
    // FIXME: Populate the unit type when it is parsed (0 here is "unknown").
    // FIXME: Create a proper animated value when animations are supported.
    auto base_length = SVGLength::create(realm(), 0, m_x.value_or(0), SVGLength::ReadOnly::No);
    auto anim_length = SVGLength::create(realm(), 0, m_x.value_or(0), SVGLength::ReadOnly::Yes);
    return SVGAnimatedLength::create(realm(), base_length, anim_length);
}

// https://www.w3.org/TR/SVG11/shapes.html#RectElementYAttribute
GC::Ref<SVGAnimatedLength> SVGUseElement::y() const
{
    // FIXME: Populate the unit type when it is parsed (0 here is "unknown").
    // FIXME: Create a proper animated value when animations are supported.
    auto base_length = SVGLength::create(realm(), 0, m_y.value_or(0), SVGLength::ReadOnly::No);
    auto anim_length = SVGLength::create(realm(), 0, m_y.value_or(0), SVGLength::ReadOnly::Yes);
    return SVGAnimatedLength::create(realm(), base_length, anim_length);
}

GC::Ref<SVGAnimatedLength> SVGUseElement::width() const
{
    return fake_animated_length_fixme();
}

GC::Ref<SVGAnimatedLength> SVGUseElement::height() const
{
    return fake_animated_length_fixme();
}

// https://svgwg.org/svg2-draft/struct.html#TermInstanceRoot
GC::Ptr<SVGElement> SVGUseElement::instance_root() const
{
    return const_cast<DOM::ShadowRoot&>(*shadow_root()).first_child_of_type<SVGElement>();
}

GC::Ptr<SVGElement> SVGUseElement::animated_instance_root() const
{
    return instance_root();
}

GC::Ptr<Layout::Node> SVGUseElement::create_layout_node(GC::Ref<CSS::ComputedProperties> style)
{
    return heap().allocate<Layout::SVGGraphicsBox>(document(), *this, move(style));
}

}