Convert visitor classes to mixins (#1784)

Co-authored-by: Jennifer Thakar <jathak@google.com>

lib/src/ast/css/node.dart

@@ -62,7 +62,7 @@ abstract class CssParentNode extends CssNode {
 }
 
 /// The visitor used to implement [CssNode.isInvisible]
-class _IsInvisibleVisitor extends EveryCssVisitor {
+class _IsInvisibleVisitor with EveryCssVisitor {
   /// Whether to consider selectors with bogus combinators invisible.
   final bool includeBogus;
 

lib/src/ast/sass/statement/mixin_rule.dart

@@ -50,7 +50,7 @@ class MixinRule extends CallableDeclaration implements SassDeclaration {
 
 /// A visitor for determining whether a [MixinRule] recursively contains a
 /// [ContentRule].
-class _HasContentVisitor extends StatementSearchVisitor<bool> {
+class _HasContentVisitor with StatementSearchVisitor<bool> {
   const _HasContentVisitor();
 
   bool visitContentRule(_) => true;

lib/src/ast/selector.dart

@@ -98,7 +98,7 @@ abstract class Selector {
 }
 
 /// The visitor used to implement [Selector.isInvisible].
-class _IsInvisibleVisitor extends AnySelectorVisitor {
+class _IsInvisibleVisitor with AnySelectorVisitor {
   /// Whether to consider selectors with bogus combinators invisible.
   final bool includeBogus;
 
@@ -128,7 +128,7 @@ class _IsInvisibleVisitor extends AnySelectorVisitor {
 }
 
 /// The visitor used to implement [Selector.isBogus].
-class _IsBogusVisitor extends AnySelectorVisitor {
+class _IsBogusVisitor with AnySelectorVisitor {
   /// Whether to consider selectors with leading combinators as bogus.
   final bool includeLeadingCombinator;
 
@@ -159,7 +159,7 @@ class _IsBogusVisitor extends AnySelectorVisitor {
 }
 
 /// The visitor used to implement [Selector.isUseless]
-class _IsUselessVisitor extends AnySelectorVisitor {
+class _IsUselessVisitor with AnySelectorVisitor {
   const _IsUselessVisitor();
 
   bool visitComplexSelector(ComplexSelector complex) =>

lib/src/visitor/any_selector.dart

@@ -12,9 +12,7 @@ import 'interface/selector.dart';
 ///
 /// Each method returns `false` by default.
 @internal
-abstract class AnySelectorVisitor implements SelectorVisitor<bool> {
-  const AnySelectorVisitor();
-
+mixin AnySelectorVisitor implements SelectorVisitor<bool> {
   bool visitComplexSelector(ComplexSelector complex) => complex.components
       .any((component) => visitCompoundSelector(component.selector));
 

lib/src/visitor/every_css.dart

@@ -12,9 +12,7 @@ import 'interface/css.dart';
 ///
 /// Each method returns `false` by default.
 @internal
-abstract class EveryCssVisitor implements CssVisitor<bool> {
-  const EveryCssVisitor();
-
+mixin EveryCssVisitor implements CssVisitor<bool> {
   bool visitCssAtRule(CssAtRule node) =>
       node.children.every((child) => child.accept(this));
   bool visitCssComment(CssComment node) => false;

lib/src/visitor/find_dependencies.dart

@@ -19,7 +19,7 @@ Tuple2<List<Uri>, List<Uri>> findDependencies(Stylesheet stylesheet) =>
 
 /// A visitor that traverses a stylesheet and records, all `@import`, `@use`,
 /// and `@forward` rules (excluding built-in modules) it contains.
-class _FindDependenciesVisitor extends RecursiveStatementVisitor {
+class _FindDependenciesVisitor with RecursiveStatementVisitor {
   final _usesAndForwards = <Uri>[];
   final _imports = <Uri>[];
 

lib/src/visitor/recursive_ast.dart

@@ -20,10 +20,8 @@ import 'recursive_statement.dart';
 /// * [visitInterpolation]
 ///
 /// {@category Visitor}
-abstract class RecursiveAstVisitor extends RecursiveStatementVisitor
+mixin RecursiveAstVisitor on RecursiveStatementVisitor
     implements ExpressionVisitor<void> {
-  const RecursiveAstVisitor();
-
   void visitAtRootRule(AtRootRule node) {
     node.query.andThen(visitInterpolation);
     super.visitAtRootRule(node);

lib/src/visitor/recursive_selector.dart

@@ -9,9 +9,7 @@ import 'interface/selector.dart';
 /// A visitor that recursively traverses each component of a Selector AST.
 ///
 /// {@category Visitor}
-abstract class RecursiveSelectorVisitor implements SelectorVisitor<void> {
-  const RecursiveSelectorVisitor();
-
+mixin RecursiveSelectorVisitor implements SelectorVisitor<void> {
   void visitAttributeSelector(AttributeSelector attribute) {}
   void visitClassSelector(ClassSelector klass) {}
   void visitIDSelector(IDSelector id) {}

lib/src/visitor/recursive_statement.dart

@@ -18,9 +18,7 @@ import 'interface/statement.dart';
 /// * [visitChildren]
 ///
 /// {@category Visitor}
-abstract class RecursiveStatementVisitor implements StatementVisitor<void> {
-  const RecursiveStatementVisitor();
-
+mixin RecursiveStatementVisitor implements StatementVisitor<void> {
   void visitAtRootRule(AtRootRule node) {
     visitChildren(node.children);
   }

lib/src/visitor/statement_search.dart

@@ -18,9 +18,7 @@ import 'recursive_statement.dart';
 /// This supports the same additional methods as [RecursiveStatementVisitor].
 ///
 /// {@category Visitor}
-abstract class StatementSearchVisitor<T> implements StatementVisitor<T?> {
-  const StatementSearchVisitor();
-
+mixin StatementSearchVisitor<T> implements StatementVisitor<T?> {
   T? visitAtRootRule(AtRootRule node) => visitChildren(node.children);
 
   T? visitAtRule(AtRule node) => node.children.andThen(visitChildren);

pkg/sass_api/CHANGELOG.md

@@ -1,7 +1,23 @@
 ## 3.0.0
 
-* Replace the `minSpecificity` and `maxSpecificity` fields on `ComplexSelector`,
-  `CompoundSelector`, and `SimpleSelector` with a single `specificity` field.
+* **Breaking change:** Convert all visitor superclasses into mixins. This
+  includes `RecursiveAstVisitor`, `RecursiveSelectorVisitor`,
+  `RecursiveStatementVisitor`, and `StatementSearchVisitor`. This has several
+  effects;
+
+  * You must use `with` to mix in visitors rather than `extends`.
+
+  * It's now possible to mix multiple visitors into the same class, which wasn't
+    possible with `extends`.
+
+  * Because [mixins can't be composed], when mixing in `RecursiveAstVisitor` you
+    must explicitly mix in `RecursiveStatementVisitor` as well.
+
+    [mixins can't be composed]: https://github.com/dart-lang/language/issues/540
+
+* **Breaking change:** Replace the `minSpecificity` and `maxSpecificity` fields
+  on `ComplexSelector`, `CompoundSelector`, and `SimpleSelector` with a single
+  `specificity` field.
 
 ## 2.0.4