LibWeb/HTML: Add a cache for the list of Options for HTMLSelectElement

This is still called _way_ too often, and we need to be much smarter
about when this needs to run. But we get two wins from this very
naive implementation:

  1. There is a inner text setter nested within the selectedness
     steps uses this list of elements. This saves us building
     up a list of elements again within these steps.
  2. More importantly, it caches the number of selected elements.
     This already allows us to skip a minor amount of work iterating
     over the children again. But in future commits, this will serve
     as one of the criteria for skipping running the selectedness
     algorithm altogether for certain cases, which is a very big win.

A example future idea might be to append to this list directly when
something like appendChild is run instead of iterating over all of
the children again. But that's left as future work.

Libraries/LibWeb/HTML/HTMLSelectElement.cpp

@@ -51,6 +51,7 @@ void HTMLSelectElement::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_selected_options);
     visitor.visit(m_inner_text_element);
     visitor.visit(m_chevron_icon_element);
+    visitor.visit(m_cached_list_of_options);
 
     for (auto const& item : m_select_items) {
         if (item.has<SelectItemOption>())
@@ -197,25 +198,40 @@ GC::Ref<DOM::HTMLCollection> HTMLSelectElement::selected_options()
 }
 
 // https://html.spec.whatwg.org/multipage/form-elements.html#concept-select-option-list
-Vector<GC::Root<HTMLOptionElement>> HTMLSelectElement::list_of_options() const
+void HTMLSelectElement::update_cached_list_of_options() const
 {
     // The list of options for a select element consists of all the option element children of the select element,
     // and all the option element children of all the optgroup element children of the select element, in tree order.
-    Vector<GC::Root<HTMLOptionElement>> list;
+    m_cached_list_of_options.clear();
+    m_cached_number_of_selected_options = 0;
 
     for (auto* node = first_child(); node; node = node->next_sibling()) {
         if (auto* maybe_option = as_if<HTMLOptionElement>(*node)) {
-            list.append(GC::make_root(const_cast<HTMLOptionElement&>(*maybe_option)));
+            if (maybe_option->selected())
+                ++m_cached_number_of_selected_options;
+            m_cached_list_of_options.append(const_cast<HTMLOptionElement&>(*maybe_option));
             continue;
         }
 
         if (auto* maybe_opt_group = as_if<HTMLOptGroupElement>(node)) {
             maybe_opt_group->for_each_child_of_type<HTMLOptionElement>([&](HTMLOptionElement& option_element) {
-                list.append(GC::make_root(option_element));
+                if (option_element.selected())
+                    ++m_cached_number_of_selected_options;
+                m_cached_list_of_options.append(option_element);
                 return IterationDecision::Continue;
             });
         }
     }
+}
+
+// https://html.spec.whatwg.org/multipage/form-elements.html#concept-select-option-list
+Vector<GC::Root<HTMLOptionElement>> HTMLSelectElement::list_of_options() const
+{
+    update_cached_list_of_options();
+    Vector<GC::Root<HTMLOptionElement>> list;
+    list.ensure_capacity(m_cached_list_of_options.size());
+    for (auto& item : m_cached_list_of_options)
+        list.unchecked_append(GC::make_root(item));
 
     return list;
 }
@@ -223,8 +239,10 @@ Vector<GC::Root<HTMLOptionElement>> HTMLSelectElement::list_of_options() const
 // https://html.spec.whatwg.org/multipage/form-elements.html#the-select-element:concept-form-reset-control
 void HTMLSelectElement::reset_algorithm()
 {
+    update_cached_list_of_options();
+
     // The reset algorithm for select elements is to go through all the option elements in the element's list of options,
-    for (auto const& option_element : list_of_options()) {
+    for (auto const& option_element : m_cached_list_of_options) {
         // set their selectedness to true if the option element has a selected attribute, and false otherwise,
         option_element->set_selected_internal(option_element->has_attribute(AttributeNames::selected));
         // set their dirtiness to false,
@@ -239,9 +257,10 @@ WebIDL::Long HTMLSelectElement::selected_index() const
 {
     // The selectedIndex IDL attribute, on getting, must return the index of the first option element in the list of options
     // in tree order that has its selectedness set to true, if any. If there isn't one, then it must return −1.
+    update_cached_list_of_options();
 
     WebIDL::Long index = 0;
-    for (auto const& option_element : list_of_options()) {
+    for (auto const& option_element : m_cached_list_of_options) {
         if (option_element->selected())
             return index;
         ++index;
@@ -251,17 +270,17 @@ WebIDL::Long HTMLSelectElement::selected_index() const
 
 void HTMLSelectElement::set_selected_index(WebIDL::Long index)
 {
+    update_cached_list_of_options();
     // On setting, the selectedIndex attribute must set the selectedness of all the option elements in the list of options to false,
     // and then the option element in the list of options whose index is the given new value,
     // if any, must have its selectedness set to true and its dirtiness set to true.
-    auto options = list_of_options();
-    for (auto& option : options)
+    for (auto& option : m_cached_list_of_options)
         option->set_selected_internal(false);
 
-    if (index < 0 || index >= static_cast<int>(options.size()))
+    if (index < 0 || static_cast<size_t>(index) >= m_cached_list_of_options.size())
         return;
 
-    auto& selected_option = options[index];
+    auto& selected_option = m_cached_list_of_options[index];
     selected_option->set_selected_internal(true);
     selected_option->m_dirty = true;
 }
@@ -276,6 +295,7 @@ i32 HTMLSelectElement::default_tab_index_value() const
 void HTMLSelectElement::children_changed()
 {
     Base::children_changed();
+    update_cached_list_of_options();
     update_selectedness();
 }
 
@@ -307,7 +327,8 @@ Optional<ARIA::Role> HTMLSelectElement::default_role() const
 
 String HTMLSelectElement::value() const
 {
-    for (auto const& option_element : list_of_options())
+    update_cached_list_of_options();
+    for (auto const& option_element : m_cached_list_of_options)
         if (option_element->selected())
             return option_element->value();
     return ""_string;
@@ -315,7 +336,8 @@ String HTMLSelectElement::value() const
 
 WebIDL::ExceptionOr<void> HTMLSelectElement::set_value(String const& value)
 {
-    for (auto const& option_element : list_of_options())
+    update_cached_list_of_options();
+    for (auto const& option_element : m_cached_list_of_options)
         option_element->set_selected(option_element->value() == value);
     update_inner_text_element();
     return {};
@@ -474,7 +496,8 @@ void HTMLSelectElement::did_select_item(Optional<u32> const& id)
     if (!id.has_value())
         return;
 
-    for (auto const& option_element : list_of_options())
+    update_cached_list_of_options();
+    for (auto const& option_element : m_cached_list_of_options)
         option_element->set_selected(false);
 
     for (auto const& item : m_select_items) {
@@ -559,6 +582,7 @@ void HTMLSelectElement::create_shadow_tree_if_needed()
 
 void HTMLSelectElement::update_inner_text_element(Badge<HTMLOptionElement>)
 {
+    update_cached_list_of_options();
     update_inner_text_element();
 }
 
@@ -569,7 +593,7 @@ void HTMLSelectElement::update_inner_text_element()
         return;
 
     // Update inner text element to the label of the selected option
-    for (auto const& option_element : list_of_options()) {
+    for (auto const& option_element : m_cached_list_of_options) {
         if (option_element->selected()) {
             m_inner_text_element->set_text_content(strip_newlines(option_element->label()));
             return;
@@ -583,23 +607,15 @@ void HTMLSelectElement::update_selectedness()
     if (has_attribute(AttributeNames::multiple))
         return;
 
-    auto list_of_options = this->list_of_options();
+    update_cached_list_of_options();
 
     // If element's multiple attribute is absent, and element's display size is 1,
     if (display_size() == 1) {
-        bool has_selected_elements = false;
-        for (auto const& option_element : list_of_options) {
-            if (option_element->selected()) {
-                has_selected_elements = true;
-                break;
-            }
-        }
-
         // and no option elements in the element's list of options have their selectedness set to true,
-        if (!has_selected_elements) {
+        if (m_cached_number_of_selected_options == 0) {
             // then set the selectedness of the first option element in the list of options in tree order
             // that is not disabled, if any, to true, and return.
-            for (auto const& option_element : list_of_options) {
+            for (auto const& option_element : m_cached_list_of_options) {
                 if (!option_element->disabled()) {
                     option_element->set_selected_internal(true);
                     update_inner_text_element();
@@ -614,19 +630,13 @@ void HTMLSelectElement::update_selectedness()
     // and two or more option elements in element's list of options have their selectedness set to true,
     // then set the selectedness of all but the last option element with its selectedness set to true
     // in the list of options in tree order to false.
-    int number_of_selected = 0;
-    for (auto const& option_element : list_of_options) {
-        if (option_element->selected())
-            ++number_of_selected;
-    }
-    // and two or more option elements in element's list of options have their selectedness set to true,
-    if (number_of_selected >= 2) {
+    if (m_cached_number_of_selected_options >= 2) {
         // then set the selectedness of all but the last option element with its selectedness set to true
         // in the list of options in tree order to false.
         GC::Ptr<HTML::HTMLOptionElement> last_selected_option;
         u64 last_selected_option_update_index = 0;
 
-        for (auto const& option_element : list_of_options) {
+        for (auto const& option_element : m_cached_list_of_options) {
             if (!option_element->selected())
                 continue;
             if (!last_selected_option
@@ -636,7 +646,7 @@ void HTMLSelectElement::update_selectedness()
             }
         }
 
-        for (auto const& option_element : list_of_options) {
+        for (auto const& option_element : m_cached_list_of_options) {
             if (option_element != last_selected_option)
                 option_element->set_selected_internal(false);
         }

Libraries/LibWeb/HTML/HTMLSelectElement.h

@@ -111,6 +111,7 @@ private:
 
     virtual void children_changed() override;
 
+    void update_cached_list_of_options() const;
     void show_the_picker_if_applicable();
 
     void create_shadow_tree_if_needed();
@@ -119,6 +120,9 @@ private:
 
     u32 display_size() const;
 
+    mutable Vector<GC::Ref<HTMLOptionElement>> m_cached_list_of_options;
+    mutable size_t m_cached_number_of_selected_options { 0 };
+
     GC::Ptr<HTMLOptionsCollection> m_options;
     GC::Ptr<DOM::HTMLCollection> m_selected_options;
     bool m_is_open { false };