Minor category renderer cleanup (#2537)

* Minor category renderer cleanup

* Switch to using the const constructors

Author: parlough

lib/src/render/category_renderer.dart

@@ -4,41 +4,50 @@
 
 import 'package:dartdoc/src/model/category.dart';
 
+/// A renderer for a [Category].
 abstract class CategoryRenderer {
+  const CategoryRenderer();
+
+  /// Render the label of this [category].
   String renderCategoryLabel(Category category);
+
+  /// Render the name of this [category] with a link to its specified
+  /// [Category.href] if it is documented.
   String renderLinkedName(Category category);
 }
 
+/// A HTML renderer for a [Category].
 class CategoryRendererHtml extends CategoryRenderer {
-  static final CategoryRendererHtml _instance = CategoryRendererHtml._();
-
-  factory CategoryRendererHtml() {
-    return _instance;
-  }
-
-  CategoryRendererHtml._();
+  const CategoryRendererHtml();
 
   @override
   String renderCategoryLabel(Category category) {
-    var spanClasses = <String>[];
-    spanClasses.add('category');
-    spanClasses.add(category.name.split(' ').join('-').toLowerCase());
-    spanClasses.add('cp-${category.categoryIndex}');
+    final buffer = StringBuffer('<span class="category ');
+    final name = category.name;
+    buffer.writeAll(name.toLowerCase().split(' '), '-');
+    buffer.write(' cp-');
+    buffer.write(category.categoryIndex);
+
     if (category.isDocumented) {
-      spanClasses.add('linked');
+      buffer.write(' linked');
     }
-    var spanTitle = 'This is part of the ${category.name} ${category.kind}.';
 
-    var buf = StringBuffer();
-    buf.write('<span class="${spanClasses.join(' ')}" title="$spanTitle">');
-    buf.write(renderLinkedName(category));
-    buf.write('</span>');
-    return buf.toString();
+    buffer.write('"'); // Wrap up the class list and begin title
+    buffer.write(' title="This is part of the ');
+    buffer.write(name);
+    buffer.write(' ');
+    buffer.write(category.kind);
+    buffer.write('.">'); // Wrap up the title
+
+    buffer.write(renderLinkedName(category));
+    buffer.write('</span>');
+
+    return buffer.toString();
   }
 
   @override
   String renderLinkedName(Category category) {
-    var unbrokenName = category.name.replaceAll(' ', '&nbsp;');
+    final unbrokenName = category.name.replaceAll(' ', '&nbsp;');
     if (category.isDocumented) {
       return '<a href="${category.href}">$unbrokenName</a>';
     } else {
@@ -47,13 +56,16 @@ class CategoryRendererHtml extends CategoryRenderer {
   }
 }
 
+/// A markdown renderer for a [Category].
 class CategoryRendererMd extends CategoryRenderer {
+  const CategoryRendererMd();
+
   @override
   String renderCategoryLabel(Category category) => renderLinkedName(category);
 
   @override
   String renderLinkedName(Category category) {
-    var name = category.name;
+    final name = category.name;
     if (category.isDocumented) {
       return '[$name](${category.href})';
     }

lib/src/render/renderer_factory.dart

@@ -63,7 +63,7 @@ class HtmlRenderFactory extends RendererFactory {
   TemplateRenderer get templateRenderer => HtmlTemplateRenderer();
 
   @override
-  CategoryRenderer get categoryRenderer => CategoryRendererHtml();
+  CategoryRenderer get categoryRenderer => const CategoryRendererHtml();
 
   @override
   DocumentationRenderer get documentationRenderer =>
@@ -115,7 +115,7 @@ class MdRenderFactory extends RendererFactory {
   TemplateRenderer get templateRenderer => MdTemplateRenderer();
 
   @override
-  CategoryRenderer get categoryRenderer => CategoryRendererMd();
+  CategoryRenderer get categoryRenderer => const CategoryRendererMd();
 
   // We render documentation as HTML for now.
   // TODO(jdkoren): explore using documentation directly in the output file.