lib/src/rules/avoid_returning_null.dart

// Copyright (c) 2017, 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.

library linter.src.rules.avoid_returning_null_if_return_type_is_bool_double_int_or_num;

import 'package:analyzer/dart/ast/ast.dart';
import 'package:analyzer/dart/ast/visitor.dart';
import 'package:analyzer/dart/element/type.dart';
import 'package:linter/src/analyzer.dart';
import 'package:linter/src/util/dart_type_utilities.dart';

const _desc =
    r'Avoid returning null from members whose return type is bool, double, int, or num.';

const _details = r'''

**AVOID** returning null from members whose return type is bool, double, int, or num.

**BAD:**
```
bool getBool() => null;
num getNum() => null;
int getInt() => null;
double getDouble() => null;
```

**GOOD:**
```
bool getBool() => false;
num getNum() => -1;
int getInt() => -1;
double getDouble() => -1.0;
```

''';

bool _isFunctionExpression(AstNode node) => node is FunctionExpression;

bool _isPrimitiveType(DartType type) =>
    DartTypeUtilities.isClass(type, 'bool', 'dart.core') ||
    DartTypeUtilities.isClass(type, 'num', 'dart.core') ||
    DartTypeUtilities.isClass(type, 'int', 'dart.core') ||
    DartTypeUtilities.isClass(type, 'double', 'dart.core');

bool _isReturnNull(AstNode node) =>
    node is ReturnStatement && DartTypeUtilities.isNullLiteral(node.expression);

class AvoidReturningNull extends LintRule {
  _Visitor _visitor;
  AvoidReturningNull()
      : super(
            name: 'avoid_returning_null',
            description: _desc,
            details: _details,
            group: Group.style) {
    _visitor = new _Visitor(this);
  }

  @override
  AstVisitor getVisitor() => _visitor;
}

class _Visitor extends SimpleAstVisitor {
  final LintRule rule;
  _Visitor(this.rule);

  @override
  visitFunctionExpression(FunctionExpression node) {
    if (_isPrimitiveType(node.element.returnType)) {
      _visitFunctionBody(node.body);
    }
  }

  @override
  visitMethodDeclaration(MethodDeclaration node) {
    if (_isPrimitiveType(node.element.returnType)) {
      _visitFunctionBody(node.body);
    }
  }

  _visitFunctionBody(FunctionBody node) {
    if (node is ExpressionFunctionBody &&
        DartTypeUtilities.isNullLiteral(node.expression)) {
      rule.reportLint(node);
      return;
    }
    DartTypeUtilities
        .traverseNodesInDFS(node, excludeCriteria: _isFunctionExpression)
        .where(_isReturnNull)
        .forEach(rule.reportLint);
  }
}