LibWeb: Add initial implementation of IntersectionObserver

The main missing features are rootMargin, proper nested browsing
context support and content clip/clip-path support.

This makes images appear on some sites, such as YouTube and
howstuffworks.com.

Userland/Libraries/LibWeb/IntersectionObserver/IntersectionObserver.cpp

@@ -4,28 +4,61 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/QuickSort.h>
 #include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/DOM/Document.h>
 #include <LibWeb/DOM/Element.h>
 #include <LibWeb/IntersectionObserver/IntersectionObserver.h>
 
 namespace Web::IntersectionObserver {
 
 // https://w3c.github.io/IntersectionObserver/#dom-intersectionobserver-intersectionobserver
-WebIDL::ExceptionOr<JS::NonnullGCPtr<IntersectionObserver>> IntersectionObserver::construct_impl(JS::Realm& realm, WebIDL::CallbackType* callback, IntersectionObserverInit const& options)
+WebIDL::ExceptionOr<JS::NonnullGCPtr<IntersectionObserver>> IntersectionObserver::construct_impl(JS::Realm& realm, JS::GCPtr<WebIDL::CallbackType> callback, IntersectionObserverInit const& options)
 {
-    // FIXME: Implement
-    (void)callback;
-    (void)options;
+    // 4. Let thresholds be a list equal to options.threshold.
+    Vector<double> thresholds;
+    if (options.threshold.has<double>()) {
+        thresholds.append(options.threshold.get<double>());
+    } else {
+        VERIFY(options.threshold.has<Vector<double>>());
+        thresholds = options.threshold.get<Vector<double>>();
+    }
 
-    return MUST_OR_THROW_OOM(realm.heap().allocate<IntersectionObserver>(realm, realm));
+    // 5. If any value in thresholds is less than 0.0 or greater than 1.0, throw a RangeError exception.
+    for (auto value : thresholds) {
+        if (value < 0.0 || value > 1.0)
+            return WebIDL::SimpleException { WebIDL::SimpleExceptionType::RangeError, "Threshold values must be between 0.0 and 1.0 inclusive"sv };
+    }
+
+    // 6. Sort thresholds in ascending order.
+    quick_sort(thresholds, [](double left, double right) {
+        return left < right;
+    });
+
+    // 1. Let this be a new IntersectionObserver object
+    // 2. Set this’s internal [[callback]] slot to callback.
+    // 8. The thresholds attribute getter will return this sorted thresholds list.
+    // 9. Return this.
+    return MUST_OR_THROW_OOM(realm.heap().allocate<IntersectionObserver>(realm, realm, callback, options.root, move(thresholds)));
 }
 
-IntersectionObserver::IntersectionObserver(JS::Realm& realm)
+IntersectionObserver::IntersectionObserver(JS::Realm& realm, JS::GCPtr<WebIDL::CallbackType> callback, Optional<Variant<JS::Handle<DOM::Element>, JS::Handle<DOM::Document>>> const& root, Vector<double>&& thresholds)
     : PlatformObject(realm)
+    , m_callback(callback)
+    , m_root(root)
+    , m_thresholds(move(thresholds))
 {
+    intersection_root().visit([this](auto& node) {
+        node->document().register_intersection_observer({}, *this);
+    });
 }
 
-IntersectionObserver::~IntersectionObserver() = default;
+IntersectionObserver::~IntersectionObserver()
+{
+    intersection_root().visit([this](auto& node) {
+        node->document().unregister_intersection_observer({}, *this);
+    });
+}
 
 JS::ThrowCompletionOr<void> IntersectionObserver::initialize(JS::Realm& realm)
 {
@@ -35,24 +68,139 @@ JS::ThrowCompletionOr<void> IntersectionObserver::initialize(JS::Realm& realm)
     return {};
 }
 
+void IntersectionObserver::visit_edges(JS::Cell::Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(m_callback);
+    for (auto& entry : m_queued_entries)
+        visitor.visit(entry);
+    for (auto& target : m_observation_targets)
+        visitor.visit(target);
+}
+
 // https://w3c.github.io/IntersectionObserver/#dom-intersectionobserver-observe
 void IntersectionObserver::observe(DOM::Element& target)
 {
-    // FIXME: Implement
-    (void)target;
+    // Run the observe a target Element algorithm, providing this and target.
+    // https://www.w3.org/TR/intersection-observer/#observe-a-target-element
+    // 1. If target is in observer’s internal [[ObservationTargets]] slot, return.
+    if (m_observation_targets.contains_slow(JS::NonnullGCPtr { target }))
+        return;
+
+    // 2. Let intersectionObserverRegistration be an IntersectionObserverRegistration record with an observer
+    //    property set to observer, a previousThresholdIndex property set to -1, and a previousIsIntersecting
+    //    property set to false.
+    auto intersection_observer_registration = IntersectionObserverRegistration {
+        .observer = JS::make_handle(*this),
+        .previous_threshold_index = OptionalNone {},
+        .previous_is_intersecting = false,
+    };
+
+    // 3. Append intersectionObserverRegistration to target’s internal [[RegisteredIntersectionObservers]] slot.
+    target.register_intersection_observer({}, move(intersection_observer_registration));
+
+    // 4. Add target to observer’s internal [[ObservationTargets]] slot.
+    m_observation_targets.append(target);
 }
 
 // https://w3c.github.io/IntersectionObserver/#dom-intersectionobserver-unobserve
 void IntersectionObserver::unobserve(DOM::Element& target)
 {
-    // FIXME: Implement
-    (void)target;
+    // Run the unobserve a target Element algorithm, providing this and target.
+    // https://www.w3.org/TR/intersection-observer/#unobserve-a-target-element
+    // 1. Remove the IntersectionObserverRegistration record whose observer property is equal to this from target’s internal [[RegisteredIntersectionObservers]] slot, if present.
+    target.unregister_intersection_observer({}, *this);
+
+    // 2. Remove target from this’s internal [[ObservationTargets]] slot, if present
+    m_observation_targets.remove_first_matching([&target](JS::NonnullGCPtr<DOM::Element> const& entry) {
+        return entry.ptr() == &target;
+    });
 }
 
 // https://w3c.github.io/IntersectionObserver/#dom-intersectionobserver-disconnect
 void IntersectionObserver::disconnect()
 {
-    // FIXME: Implement
+    // For each target in this’s internal [[ObservationTargets]] slot:
+    // 1. Remove the IntersectionObserverRegistration record whose observer property is equal to this from target’s internal
+    //    [[RegisteredIntersectionObservers]] slot.
+    // 2. Remove target from this’s internal [[ObservationTargets]] slot.
+    for (auto& target : m_observation_targets) {
+        target->unregister_intersection_observer({}, *this);
+    }
+    m_observation_targets.clear();
+}
+
+// https://www.w3.org/TR/intersection-observer/#dom-intersectionobserver-takerecords
+Vector<JS::Handle<IntersectionObserverEntry>> IntersectionObserver::take_records()
+{
+    // 1. Let queue be a copy of this’s internal [[QueuedEntries]] slot.
+    Vector<JS::Handle<IntersectionObserverEntry>> queue;
+    for (auto& entry : m_queued_entries)
+        queue.append(*entry);
+
+    // 2. Clear this’s internal [[QueuedEntries]] slot.
+    m_queued_entries.clear();
+
+    // 3. Return queue.
+    return queue;
+}
+
+Variant<JS::Handle<DOM::Element>, JS::Handle<DOM::Document>, Empty> IntersectionObserver::root() const
+{
+    if (!m_root.has_value())
+        return Empty {};
+    return m_root.value();
+}
+
+Variant<JS::Handle<DOM::Element>, JS::Handle<DOM::Document>> IntersectionObserver::intersection_root() const
+{
+    if (!m_root.has_value())
+        return JS::make_handle(global_object().browsing_context()->top_level_browsing_context().active_document());
+    return m_root.value();
+}
+
+// https://www.w3.org/TR/intersection-observer/#intersectionobserver-root-intersection-rectangle
+CSSPixelRect IntersectionObserver::root_intersection_rectangle() const
+{
+    // If the IntersectionObserver is an implicit root observer,
+    //    it’s treated as if the root were the top-level browsing context’s document, according to the following rule for document.
+    auto intersection_root = this->intersection_root();
+
+    CSSPixelRect rect;
+
+    // If the intersection root is a document,
+    //    it’s the size of the document's viewport (note that this processing step can only be reached if the document is fully active).
+    if (intersection_root.has<JS::Handle<DOM::Document>>()) {
+        auto document = intersection_root.get<JS::Handle<DOM::Document>>();
+
+        // Since the spec says that this is only reach if the document is fully active, that means it must have a browsing context.
+        VERIFY(document->browsing_context());
+        rect = document->browsing_context()->viewport_rect();
+    } else {
+        VERIFY(intersection_root.has<JS::Handle<DOM::Element>>());
+        auto element = intersection_root.get<JS::Handle<DOM::Element>>();
+
+        // FIXME: Otherwise, if the intersection root has a content clip,
+        //          it’s the element’s content area.
+
+        // Otherwise,
+        //    it’s the result of getting the bounding box for the intersection root.
+        auto bounding_client_rect = element->get_bounding_client_rect();
+        rect = CSSPixelRect(bounding_client_rect->x(), bounding_client_rect->y(), bounding_client_rect->width(), bounding_client_rect->height());
+    }
+
+    // FIXME: When calculating the root intersection rectangle for a same-origin-domain target, the rectangle is then
+    //        expanded according to the offsets in the IntersectionObserver’s [[rootMargin]] slot in a manner similar
+    //        to CSS’s margin property, with the four values indicating the amount the top, right, bottom, and left
+    //        edges, respectively, are offset by, with positive lengths indicating an outward offset. Percentages
+    //        are resolved relative to the width of the undilated rectangle.
+
+    return rect;
+}
+
+void IntersectionObserver::queue_entry(Badge<DOM::Document>, JS::NonnullGCPtr<IntersectionObserverEntry> entry)
+{
+    m_queued_entries.append(entry);
 }
 
 }