LibWeb: Move get_element_by_id() to a NonElementParentNode mixin class

This matches the current version of the DOM spec. And since C++ doesn't
have mixins this is actually a CRTP class.

Libraries/LibWeb/DOM/Document.cpp

@@ -295,19 +295,6 @@ void Document::set_hovered_node(Node* node)
     invalidate_style();
 }
 
-const Element* Document::get_element_by_id(const String& id) const
-{
-    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 found_element;
-}
-
 Vector<const Element*> Document::get_elements_by_name(const String& name) const
 {
     Vector<const Element*> elements;

Libraries/LibWeb/DOM/Document.h

@@ -37,6 +37,7 @@
 #include <LibJS/Forward.h>
 #include <LibWeb/CSS/StyleResolver.h>
 #include <LibWeb/CSS/StyleSheet.h>
+#include <LibWeb/DOM/NonElementParentNode.h>
 #include <LibWeb/DOM/ParentNode.h>
 
 namespace Web {
@@ -50,7 +51,9 @@ class LayoutNode;
 class StyleResolver;
 class StyleSheet;
 
-class Document : public ParentNode {
+class Document
+    : public ParentNode
+    , public NonElementParentNode<Document> {
 public:
     using WrapperType = Bindings::DocumentWrapper;
 
@@ -119,7 +122,6 @@ public:
 
     void schedule_style_update();
 
-    const Element* get_element_by_id(const String&) const;
     Vector<const Element*> get_elements_by_name(const String&) const;
 
     const String& source() const { return m_source; }

Libraries/LibWeb/DOM/DocumentFragment.h

@@ -27,11 +27,14 @@
 #pragma once
 
 #include <AK/FlyString.h>
+#include <LibWeb/DOM/NonElementParentNode.h>
 #include <LibWeb/DOM/ParentNode.h>
 
 namespace Web {
 
-class DocumentFragment : public ParentNode {
+class DocumentFragment
+    : public ParentNode
+    , public NonElementParentNode<DocumentFragment> {
 public:
     DocumentFragment(Document& document)
         : ParentNode(document, NodeType::DOCUMENT_FRAGMENT_NODE)

Libraries/LibWeb/DOM/NonElementParentNode.h

@@ -0,0 +1,59 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <AK/Forward.h>
+#include <LibWeb/Forward.h>
+#include <LibWeb/TreeNode.h>
+
+namespace Web {
+
+template<typename NodeType>
+class NonElementParentNode {
+public:
+    const Element* get_element_by_id(const FlyString& id) const
+    {
+        const Element* found_element = nullptr;
+        static_cast<const NodeType*>(this)->template for_each_in_subtree_of_type<Element>([&](auto& element) {
+            if (element.attribute("id") == id) {
+                found_element = &element;
+                return IterationDecision::Break;
+            }
+            return IterationDecision::Continue;
+        });
+        return found_element;
+    }
+    Element* get_element_by_id(const FlyString& id)
+    {
+        return const_cast<Element*>(const_cast<const NonElementParentNode*>(this)->get_element_by_id(id));
+    }
+
+protected:
+    NonElementParentNode() {}
+};
+
+}

Libraries/LibWeb/HtmlView.cpp

@@ -382,7 +382,7 @@ void HtmlView::scroll_to_anchor(const StringView& name)
     if (!document())
         return;
 
-    auto* element = document()->get_element_by_id(name);
+    const auto* element = document()->get_element_by_id(name);
     if (!element) {
         auto candidates = document()->get_elements_by_name(name);
         for (auto* candidate : candidates) {