pkg/analyzer/lib/src/error/widget_preview_verifier.dart

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:analyzer/dart/ast/visitor.dart';
import 'package:analyzer/dart/element/element.dart';
import 'package:analyzer/src/dart/ast/ast.dart';
import 'package:analyzer/src/diagnostic/diagnostic.dart' as diag;
import 'package:analyzer/src/error/listener.dart';
import 'package:analyzer/src/utilities/extensions/flutter.dart';

/// Helper for verifying the validity of @Preview(...) applications.
///
/// The Flutter Widget Previewer relies on code generation to import code from
/// a developer's project into a generated 'widget preview scaffold' that lives
/// in the project's '.dart_tool' directory. The nature of this implementation
/// means that any symbols referenced in the declaration of the preview (e.g.,
/// in the invocation of the `Preview(...)` constructor or the name of the
/// preview function) must be publicly accessible from outside the library in
/// which they are defined. One of the main uses for this verifier is to flag
/// usages of private symbols within preview declarations.
///
/// This verifier also ensures that the `@Preview(...)` annotation can only be
/// applied to a functions or constructors that:
///
///  - Are statically accessible (e.g., no instance methods)
///  - Have explicit implementations (e.g., not abstract or external)
class WidgetPreviewVerifier {
  final DiagnosticReporter _diagnosticReporter;

  WidgetPreviewVerifier(this._diagnosticReporter);

  /// Check is [node] is a Widget Preview application and verify its
  /// correctness.
  void checkAnnotation(Annotation node) {
    if (node.elementAnnotation?.isWidgetPreview ?? false) {
      _checkWidgetPreview(node);
    }
  }

  void _checkWidgetPreview(Annotation node) {
    if (node.arguments == null) {
      // This is an invalid annotation application since there's no constructor
      // invocation.
      return;
    }

    var parent = node.parent;
    bool isValidApplication = switch (parent) {
      // First, check that the preview application is happening in a supported
      // context.
      _ when !_isSupportedParent(node: parent) => false,
      ConstructorDeclaration() => _isValidConstructorPreviewApplication(
        declaration: parent,
        name: parent.name?.lexeme,
        parentDeclaration: parent.parent?.parent,
        isExternal: parent.externalKeyword != null,
        isFactory: parent.factoryKeyword != null,
        parameters: parent.parameters,
      ),
      FunctionDeclaration() => _isValidFunctionPreviewApplication(
        declaration: parent,
      ),
      MethodDeclaration() => _isValidMethodPreviewApplication(
        declaration: parent,
      ),
      PrimaryConstructorBody(:var declaration?) =>
        _isValidConstructorPreviewApplication(
          declaration: parent,
          name: parent.declaration?.constructorName?.name.lexeme,
          parentDeclaration: parent.parent?.parent,
          isExternal: false,
          isFactory: false,
          parameters: declaration.formalParameters,
        ),
      _ => false,
    };

    if (!isValidApplication) {
      _diagnosticReporter.report(
        diag.invalidWidgetPreviewApplication.at(node.name),
      );
    }

    var visitor = _InvalidWidgetPreviewArgumentDetectorVisitor(
      diagnosticReporter: _diagnosticReporter,
    );
    node.arguments!.accept(visitor);
  }

  bool _hasRequiredParameters(NodeList<FormalParameter> parameters) {
    return parameters.any((e) => e.isRequired);
  }

  /// Returns whether [name] is private or `node.parent` is a [ClassDeclaration]
  /// that has a private name.
  bool _isPrivateContext({required String? name, required AstNode node}) {
    if (name != null && Identifier.isPrivateName(name)) return true;

    var parent = node.parent?.parent;
    if (parent == null) return false;

    var nameToken = switch (parent) {
      ClassDeclaration declaration => declaration.namePart.typeName,
      EnumDeclaration declaration => declaration.namePart.typeName,
      ExtensionDeclaration declaration => declaration.name,
      ExtensionTypeDeclaration declaration =>
        declaration.primaryConstructor.typeName,
      MixinDeclaration declaration => declaration.name,
      _ => null,
    };
    if (nameToken == null) return false;

    return Identifier.isPrivateName(nameToken.lexeme);
  }

  /// Returns true if `node.parent` is a supported context for defining widget
  /// previews.
  ///
  /// Currently, this only includes previews defined within classes and at the
  /// top level of a compilation unit.
  bool _isSupportedParent({required AstNode node}) {
    if (node.parent is CompilationUnit) {
      return true;
    }

    if (node.parent case BlockClassBody body) {
      if (body.parent is ClassDeclaration) {
        return true;
      }
    }

    return false;
  }

  /// Returns true if `declaration` is a valid constructor target for a widget
  /// preview application.
  ///
  /// Constructor preview applications are valid if:
  ///   - The class and constructor names are public
  ///   - The class is a subtype of Widget
  ///   - The class is not abstract or is a valid factory constructor
  ///   - The constructor is not external
  ///   - The constructor does not have any required arguments
  bool _isValidConstructorPreviewApplication({
    required AstNode declaration,
    required String? name,
    required AstNode? parentDeclaration,
    required bool isExternal,
    required bool isFactory,
    required FormalParameterList parameters,
  }) {
    if (isExternal) return false;
    if (parentDeclaration is! ClassDeclaration) return false;

    var element = parentDeclaration.declaredFragment?.element;
    if (element is! ClassElement) return false;

    return !_isPrivateContext(name: name, node: declaration) &&
        element.isWidget &&
        !(element.isAbstract && !isFactory) &&
        !_hasRequiredParameters(parameters.parameters);
  }

  /// Returns true if `declaration` is a valid top-level function target for a
  /// widget preview application.
  ///
  /// Function preview applications are valid if:
  ///   - The function name is public
  ///   - The function is not a nested function
  ///   - The function is not external
  ///   - The function returns a subtype of `Widget` or `WidgetBuilder`
  ///   - The function does not have any required arguments
  bool _isValidFunctionPreviewApplication({
    required FunctionDeclaration declaration,
  }) {
    if (declaration case FunctionDeclaration(
      :var name,
      :var externalKeyword,
      :NamedType returnType,
      functionExpression: FunctionExpression(
        parameters: FormalParameterList(:var parameters),
      ),
    )) {
      return !_isPrivateContext(name: name.lexeme, node: declaration) &&
          // Check for nested function.
          declaration.parent is! FunctionDeclarationStatement &&
          externalKeyword == null &&
          returnType.isValidWidgetPreviewReturnType &&
          !_hasRequiredParameters(parameters);
    }
    return false;
  }

  /// Returns true if `declaration` is a valid static class member target for a
  /// widget preview application.
  ///
  /// Class member preview applications are valid if:
  ///   - The function is static
  ///   - The function name is public
  ///   - The function is not external
  ///   - The function returns a subtype of `Widget` or `WidgetBuilder`
  ///   - The function does not have any required arguments
  bool _isValidMethodPreviewApplication({
    required MethodDeclaration declaration,
  }) {
    if (declaration case MethodDeclaration(
      :var isStatic,
      :var externalKeyword,
      :var name,
      :NamedType returnType,
      parameters: FormalParameterList(:var parameters),
    )) {
      return !_isPrivateContext(name: name.lexeme, node: declaration) &&
          isStatic &&
          // Check for nested function.
          declaration.parent is! FunctionDeclarationStatement &&
          externalKeyword == null &&
          returnType.isValidWidgetPreviewReturnType &&
          !_hasRequiredParameters(parameters);
    }
    return false;
  }
}

class _InvalidWidgetPreviewArgumentDetectorVisitor extends RecursiveAstVisitor {
  final DiagnosticReporter diagnosticReporter;

  NamedArgument? rootArgument;
  _InvalidWidgetPreviewArgumentDetectorVisitor({
    required this.diagnosticReporter,
  });

  @override
  void visitArgumentList(ArgumentList node) {
    for (var argument in node.arguments) {
      if (argument is NamedArgument) {
        rootArgument = argument;
        argument.argumentExpression.accept(this);
        rootArgument = null;
      }
    }
  }

  @override
  void visitSimpleIdentifier(SimpleIdentifier node) {
    if (Identifier.isPrivateName(node.name)) {
      diagnosticReporter.report(
        diag.invalidWidgetPreviewPrivateArgument
            .withArguments(
              privateSymbolName: node.name,
              suggestedName: node.name.replaceFirst(RegExp('_*'), ''),
            )
            .at(rootArgument!),
      );
    }
    super.visitSimpleIdentifier(node);
  }
}