feat: add JSON string support for ABAC enforcement

[Copilot]

Minimized code changes to only include the core JSON string support functionality in CasbinEvaluator.

Changes

• Enhanced CasbinEvaluator.evalMemberExpression() to handle JSON strings, Maps, and AbacClass objects
• Parses JSON strings via jsonDecode() before property access
• Falls back to .toMap() for backward compatibility
• Provides clear error messages for unsupported types
• Applied Dart formatting to pass CI checks

Files Changed

• lib/src/utils/utils.dart - Added JSON string parsing support with proper formatting

All 452 existing tests pass. Backward compatible with existing AbacClass implementations.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Implementing ABAC with Casbin in Flutter</issue_title>
<issue_description>## Issue with Casbin ABAC in Flutter – Need Help!

I'm trying to implement Attribute-Based Access Control (ABAC) using Casbin in my Flutter application. While the setup works fine for Role-Based Access Control (RBAC), ABAC enforcement isn't working as expected.

I've set up a Casbin model with a policy that should allow access if the user's age is between 18 and 60. However, the enforcement always returns false, denying access even when the conditions should be met.

Here’s my setup:

Casbin Model:

[request_definition]
r = sub, obj, act

[policy_definition]
p = sub, obj, act, condition

[policy_effect]
e = some(where (p.eft == allow))

[matchers]
m = eval(p.condition) && r.obj == p.obj && r.act == p.act

Casbin Policy:

p, {"age": 18}, /data1, read, r.sub.age >= 18 && r.sub.age < 60

Flutter Code:

import 'package:flutter/material.dart';
import 'package:casbin/casbin.dart';
import 'dart:convert';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'Casbin ABAC Demo',
      theme: ThemeData(primarySwatch: Colors.blue),
      home: ABACScreen(),
    );
  }
}

class ABACScreen extends StatefulWidget {
  @override
  _ABACScreenState createState() => _ABACScreenState();
}

class _ABACScreenState extends State<ABACScreen> {
  final TextEditingController ageController = TextEditingController();
  String resultMessage = "";

  Future<void> checkAccess() async {
    int? age = int.tryParse(ageController.text);
    if (age == null) {
      setState(() {
        resultMessage = "❌ Please enter a valid age.";
      });
      return;
    }

    final model = Model()..loadModelFromText(getModel());
    final enforcer = await Enforcer.fromModelAndAdapter(model);

    // Load policies
    getPolicy().split('\n').forEach((element) {
      if (element.trim().isNotEmpty) {
        final parts = element.split(',').map((e) => e.trim()).toList();
        if (parts.length == 5) {
          enforcer.addPolicy([parts[0], parts[1], parts[2], parts[3], parts[4]]);
        }
      }
    });

    try {
      String subJson = jsonEncode({"age": age});
      bool? result = await enforcer.enforce([subJson, '/data1', 'read']);

      setState(() {
        resultMessage = result??false ? "✅ Access Granted" : "❌ Access Denied";
      });
    } catch (e) {
      debugPrint("⚠️ Error: $e");
      setState(() {
        resultMessage = "⚠️ Error: ${e.toString()}";
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Casbin ABAC Demo")),
      body: Padding(
        padding: EdgeInsets.all(20),
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            TextField(
              controller: ageController,
              keyboardType: TextInputType.number,
              decoration: InputDecoration(labelText: "Age", border: OutlineInputBorder()),
            ),
            SizedBox(height: 20),
            ElevatedButton(
              onPressed: checkAccess,
              child: Text("Check Access"),
            ),
            SizedBox(height: 20),
            SelectableText(resultMessage, style: TextStyle(fontSize: 18, fontWeight: FontWeight.bold)),
          ],
        ),
      ),
    );
  }
}

// Casbin Model
String getModel() {
  return '''
[request_definition]
r = sub, obj, act

[policy_definition]
p = sub, obj, act, condition

[policy_effect]
e = some(where (p.eft == allow))

[matchers]
m = eval(p.condition) && r.obj == p.obj && r.act == p.act
''';
}

// Casbin Policy
String getPolicy() {
  return '''
p, {"age": 18}, /data1, read, r.sub.age >= 18 && r.sub.age < 60
''';
}

Issue:

Even when I enter an age within the valid range (e.g., 25), Casbin denies access. It seems like the ABAC condition r.sub.age >= 18 && r.sub.age < 60 isn't being evaluated correctly.

Has anyone successfully implemented ABAC with Casbin in Flutter? What am I missing? Any help would be appreciated! 🚀</issue_description>

Comments on the Issue (you are @copilot in this section)

@hsluoyz @ankit3r try Casbin editor first: https://editor.casbin.org/

• Fixes Implementing ABAC with Casbin in Flutter #58

---

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[nomeguy]

@copilot don't add README, don't add any new test code, keep the code diff minimized

[Copilot]

@copilot don't add README, don't add any new test code, keep the code diff minimized

Done. Removed README changes, all test files, and examples. Only kept the core functionality change in lib/src/utils/utils.dart (+32 lines, -1 line). All 452 existing tests still pass. (commit 305682b)

[nomeguy]

@copilot fix failed CI check:

Run dart format --output=none --set-exit-if-changed .
  dart format --output=none --set-exit-if-changed .
  shell: /usr/bin/bash -e {0}
  env:
    PUB_ENVIRONMENT: bot.github
    DART_HOME: /opt/hostedtoolcache/dart/3.10.3/x64
    PUB_CACHE: /home/runner/.pub-cache
Changed lib/src/utils/utils.dart
Formatted 50 files (1 changed) in 0.07 seconds.
Error: Process completed with exit code 1.

[Copilot]

@copilot fix failed CI check:

...

Fixed. Applied dart format to the file. CI check should now pass. (commit 0ca4322)