feat: introduce new code diagnostics prefer-single-widget-per-file (#406)

Author: dkrutskikh

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## Unresolved
+
+* Add static code diagnostic `prefer-single-widget-per-file`.
+
 ## 4.1.0
 
 * Add better monorepos support for CLI

README.md

@@ -358,6 +358,7 @@ Rules configuration is [described here](#configuring-a-rules-entry).
 - [avoid-unnecessary-setstate](https://github.com/dart-code-checker/dart-code-metrics/blob/master/doc/rules/avoid-unnecessary-setstate.md)
 - [avoid-wrapping-in-padding](https://github.com/dart-code-checker/dart-code-metrics/blob/master/doc/rules/avoid-wrapping-in-padding.md)
 - [prefer-extracting-callbacks](https://github.com/dart-code-checker/dart-code-metrics/blob/master/doc/rules/prefer-extracting-callbacks.md)   [![Configurable](https://img.shields.io/badge/-configurable-informational)](https://github.com/dart-code-checker/dart-code-metrics/blob/master/doc/rules/prefer-extracting-callbacks.md#config-example)
+- [prefer-single-widget-per-file](https://github.com/dart-code-checker/dart-code-metrics/blob/master/doc/rules/prefer-single-widget-per-file.md)   [![Configurable](https://img.shields.io/badge/-configurable-informational)](https://github.com/dart-code-checker/dart-code-metrics/blob/master/doc/rules/prefer-single-widget-per-file.md#config-example)
 
 ### Intl specific
 

doc/rules/prefer-single-widget-per-file.md

@@ -0,0 +1,92 @@
+# Prefer single widget per file
+
+![Configurable](https://img.shields.io/badge/-configurable-informational)
+
+## Rule id
+
+prefer-single-widget-per-file
+
+## Description
+
+Warns when a file contains more than a single widget.
+
+Ensures that files have a single responsibility so that each widget exists in its own file.
+
+### Config example
+
+```yaml
+dart_code_metrics:
+  ...
+  rules:
+    ...
+    - prefer-single-widget-per-file:
+        ignore-private-widgets: true
+```
+
+### Example
+
+Bad:
+
+some_widgets.dart
+
+```dart
+class SomeWidget extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    ...
+  }
+}
+
+// LINT
+class SomeOtherWidget extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    ...
+  }
+}
+
+// LINT
+class _SomeOtherWidget extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    ...
+  }
+}
+
+// LINT
+class SomeStatefulWidget extends StatefulWidget {
+  @override
+  _SomeStatefulWidgetState createState() => _someStatefulWidgetState();
+}
+
+class _SomeStatefulWidgetState extends State<InspirationCard> {
+  @override
+  Widget build(BuildContext context) {
+    ...
+  }
+}
+```
+
+Good:
+
+some_widget.dart
+
+```dart
+class SomeWidget extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    ...
+  }
+}
+```
+
+some_other_widget.dart
+
+```dart
+class SomeOtherWidget extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    ...
+  }
+}
+```

lib/src/analyzers/lint_analyzer/rules/rules_factory.dart

@@ -23,6 +23,7 @@ import 'rules_list/prefer_conditional_expressions/prefer_conditional_expressions
 import 'rules_list/prefer_extracting_callbacks/prefer_extracting_callbacks.dart';
 import 'rules_list/prefer_intl_name/prefer_intl_name.dart';
 import 'rules_list/prefer_on_push_cd_strategy/prefer_on_push_cd_strategy.dart';
+import 'rules_list/prefer_single_widget_per_file/prefer_single_widget_per_file.dart';
 import 'rules_list/prefer_trailing_comma/prefer_trailing_comma.dart';
 import 'rules_list/provide_correct_intl_args/provide_correct_intl_args.dart';
 
@@ -64,6 +65,8 @@ final _implementedRules = <String, Rule Function(Map<String, Object>)>{
   PreferIntlNameRule.ruleId: (config) => PreferIntlNameRule(config),
   PreferOnPushCdStrategyRule.ruleId: (config) =>
       PreferOnPushCdStrategyRule(config),
+  PreferSingleWidgetPerFileRule.ruleId: (config) =>
+      PreferSingleWidgetPerFileRule(config),
   PreferTrailingCommaRule.ruleId: (config) => PreferTrailingCommaRule(config),
   ProvideCorrectIntlArgsRule.ruleId: (config) =>
       ProvideCorrectIntlArgsRule(config),

lib/src/analyzers/lint_analyzer/rules/rules_list/prefer_single_widget_per_file/config_parser.dart

@@ -0,0 +1,8 @@
+part of 'prefer_single_widget_per_file.dart';
+
+class _ConfigParser {
+  static const _ignorePrivateWidgetsName = 'ignore-private-widgets';
+
+  static bool parseIgnorePrivateWidgets(Map<String, Object> config) =>
+      config[_ignorePrivateWidgetsName] == true;
+}

lib/src/analyzers/lint_analyzer/rules/rules_list/prefer_single_widget_per_file/prefer_single_widget_per_file.dart

@@ -0,0 +1,54 @@
+import 'package:analyzer/dart/ast/ast.dart';
+import 'package:analyzer/dart/ast/visitor.dart';
+
+import '../../../../../utils/node_utils.dart';
+import '../../../models/internal_resolved_unit_result.dart';
+import '../../../models/issue.dart';
+import '../../../models/severity.dart';
+import '../../flutter_rule_utils.dart';
+import '../../models/rule.dart';
+import '../../models/rule_documentation.dart';
+import '../../rule_utils.dart';
+
+part 'config_parser.dart';
+part 'visitor.dart';
+
+class PreferSingleWidgetPerFileRule extends Rule {
+  static const String ruleId = 'prefer-single-widget-per-file';
+
+  static const _warningMessage = 'Only a single widget per file is allowed.';
+
+  final bool _ignorePrivateWidgets;
+
+  PreferSingleWidgetPerFileRule([Map<String, Object> config = const {}])
+      : _ignorePrivateWidgets = _ConfigParser.parseIgnorePrivateWidgets(config),
+        super(
+          id: ruleId,
+          documentation: const RuleDocumentation(
+            name: 'Prefer a single widget per file',
+            brief: 'Warns when a file contains more than a single widget.',
+          ),
+          severity: readSeverity(config, Severity.style),
+          excludes: readExcludes(config),
+        );
+
+  @override
+  Iterable<Issue> check(InternalResolvedUnitResult source) {
+    final visitor = _Visitor(ignorePrivateWidgets: _ignorePrivateWidgets);
+
+    source.unit.visitChildren(visitor);
+
+    return visitor.nodes
+        .map(
+          (node) => createIssue(
+            rule: this,
+            location: nodeLocation(
+              node: node,
+              source: source,
+            ),
+            message: _warningMessage,
+          ),
+        )
+        .toList(growable: false);
+  }
+}

lib/src/analyzers/lint_analyzer/rules/rules_list/prefer_single_widget_per_file/visitor.dart

@@ -0,0 +1,24 @@
+part of 'prefer_single_widget_per_file.dart';
+
+class _Visitor extends SimpleAstVisitor<void> {
+  final bool _ignorePrivateWidgets;
+
+  final _nodes = <ClassDeclaration>[];
+
+  _Visitor({required bool ignorePrivateWidgets})
+      : _ignorePrivateWidgets = ignorePrivateWidgets;
+
+  Iterable<ClassDeclaration> get nodes =>
+      _nodes.length > 1 ? _nodes.skip(1) : [];
+
+  @override
+  void visitClassDeclaration(ClassDeclaration node) {
+    super.visitClassDeclaration(node);
+
+    final classType = node.extendsClause?.superclass.type;
+    if (isWidgetOrSubclass(classType) &&
+        (!_ignorePrivateWidgets || !Identifier.isPrivateName(node.name.name))) {
+      _nodes.add(node);
+    }
+  }
+}

test/analyzers/lint_analyzer/rules/rules_list/prefer_single_widget_per_file/examples/correct_statefull_widget_example.dart

@@ -0,0 +1,13 @@
+import 'flutter_defines.dart';
+
+class SomeStatefulWidget extends StatefulWidget {
+  @override
+  _someStatefulWidgetState createState() => _someStatefulWidgetState();
+}
+
+class _SomeStatefulWidgetState extends State<InspirationCard> {
+  @override
+  Widget build(BuildContext context) {
+//    ...
+  }
+}

test/analyzers/lint_analyzer/rules/rules_list/prefer_single_widget_per_file/examples/correct_stateless_widget_example.dart

@@ -0,0 +1,8 @@
+import 'flutter_defines.dart';
+
+class SomeWidget extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+//    ...
+  }
+}

test/analyzers/lint_analyzer/rules/rules_list/prefer_single_widget_per_file/examples/flutter_defines.dart

@@ -0,0 +1,5 @@
+class Widget {}
+
+class StatefulWidget extends Widget {}
+
+class StatelessWidget extends Widget {}