LibHTML: Add TreeNode::for_each_in_subtree_of_type<T>()

This allows you to iterate a subtree and get a callback for every node
where is<T>(node) == true. This makes for quite pleasant DOM traversal.

Libraries/LibHTML/DOM/Document.cpp

@@ -177,9 +177,9 @@ void Document::layout()
 
 void Document::update_style()
 {
-    for_each_in_subtree([&](Node& node) {
-        if (node.needs_style_update())
-            to<Element>(node).recompute_style();
+    for_each_in_subtree_of_type<Element>([&](auto& element) {
+        if (element.needs_style_update())
+            element.recompute_style();
         return IterationDecision::Continue;
     });
     update_layout();
@@ -252,23 +252,23 @@ void Document::set_hovered_node(Node* node)
 
 const Element* Document::get_element_by_id(const String& id) const
 {
-    const Element* element = nullptr;
-    for_each_in_subtree([&](auto& node) {
-        if (is<Element>(node) && to<Element>(node).attribute("id") == id) {
-            element = &to<Element>(node);
+    const Element* found_element = nullptr;
+    for_each_in_subtree_of_type<Element>([&](auto& element) {
+        if (element.attribute("id") == id) {
+            found_element = &element;
             return IterationDecision::Break;
         }
         return IterationDecision::Continue;
     });
-    return element;
+    return found_element;
 }
 
 Vector<const Element*> Document::get_elements_by_name(const String& name) const
 {
     Vector<const Element*> elements;
-    for_each_in_subtree([&](auto& node) {
-        if (is<Element>(node) && to<Element>(node).attribute("name") == name)
-            elements.append(&to<Element>(node));
+    for_each_in_subtree_of_type<Element>([&](auto& element) {
+        if (element.attribute("name") == name)
+            elements.append(&element);
         return IterationDecision::Continue;
     });
     return elements;

Libraries/LibHTML/DOM/HTMLFormElement.cpp

@@ -34,12 +34,10 @@ void HTMLFormElement::submit()
 
     Vector<NameAndValue> parameters;
 
-    for_each_in_subtree([&](auto& node) {
-        if (is<HTMLInputElement>(node)) {
-            auto& input = to<HTMLInputElement>(node);
-            if (!input.name().is_null())
-                parameters.append({ input.name(), input.value() });
-        }
+    for_each_in_subtree_of_type<HTMLInputElement>([&](auto& node) {
+        auto& input = to<HTMLInputElement>(node);
+        if (!input.name().is_null())
+            parameters.append({ input.name(), input.value() });
         return IterationDecision::Continue;
     });
 

Libraries/LibHTML/DOM/Node.cpp

@@ -74,9 +74,8 @@ RefPtr<LayoutNode> Node::create_layout_node(const StyleProperties*) const
 
 void Node::invalidate_style()
 {
-    for_each_in_subtree([&](auto& node) {
-        if (is<Element>(node))
-            node.set_needs_style_update(true);
+    for_each_in_subtree_of_type<Element>([&](auto& element) {
+        element.set_needs_style_update(true);
         return IterationDecision::Continue;
     });
     document().schedule_style_update();

Libraries/LibHTML/Layout/LayoutDocument.cpp

@@ -34,11 +34,8 @@ void LayoutDocument::layout()
 void LayoutDocument::did_set_viewport_rect(Badge<Frame>, const Rect& a_viewport_rect)
 {
     FloatRect viewport_rect(a_viewport_rect.x(), a_viewport_rect.y(), a_viewport_rect.width(), a_viewport_rect.height());
-    for_each_in_subtree([&](auto& layout_node) {
-        if (is<LayoutImage>(layout_node)) {
-            auto& image = to<LayoutImage>(layout_node);
-            const_cast<HTMLImageElement&>(image.node()).set_volatile({}, !viewport_rect.intersects(image.rect()));
-        }
+    for_each_in_subtree_of_type<LayoutImage>([&](auto& layout_image) {
+        const_cast<HTMLImageElement&>(layout_image.node()).set_volatile({}, !viewport_rect.intersects(layout_image.rect()));
         return IterationDecision::Continue;
     });
 }

Libraries/LibHTML/TreeNode.h

@@ -4,6 +4,15 @@
 #include <AK/NonnullRefPtr.h>
 #include <AK/Weakable.h>
 
+// FIXME: I wish I didn't have to forward declare these, but I can't seem to avoid
+//        it if I still want to have for_each_in_subtree_of_type<U> inline here.
+class Node;
+class LayoutNode;
+template<typename T>
+bool is(const Node&);
+template<typename T>
+bool is(const LayoutNode&);
+
 template<typename T>
 class TreeNode : public Weakable<T> {
 public:
@@ -122,6 +131,34 @@ public:
         return IterationDecision::Continue;
     }
 
+    template<typename U, typename Callback>
+    IterationDecision for_each_in_subtree_of_type(Callback callback)
+    {
+        if (is<U>(static_cast<const T&>(*this))) {
+            if (callback(static_cast<U&>(*this)) == IterationDecision::Break)
+                return IterationDecision::Break;
+        }
+        for (auto* child = first_child(); child; child = child->next_sibling()) {
+            if (child->template for_each_in_subtree_of_type<U>(callback) == IterationDecision::Break)
+                return IterationDecision::Break;
+        }
+        return IterationDecision::Continue;
+    }
+
+    template<typename U, typename Callback>
+    IterationDecision for_each_in_subtree_of_type(Callback callback) const
+    {
+        if (is<U>(static_cast<const T&>(*this))) {
+            if (callback(static_cast<const U&>(*this)) == IterationDecision::Break)
+                return IterationDecision::Break;
+        }
+        for (auto* child = first_child(); child; child = child->next_sibling()) {
+            if (child->template for_each_in_subtree_of_type<U>(callback) == IterationDecision::Break)
+                return IterationDecision::Break;
+        }
+        return IterationDecision::Continue;
+    }
+
 protected:
     TreeNode() {}