Exploring Behavior-Driven Development (BDD) with flutter_gherkin in Flutter

Behavior-Driven Development (BDD) is a software development process that focuses on defining the behavior of an application using simple, human-readable language. It’s an evolution of Test-Driven Development (TDD) and promotes collaboration among developers, testers, and stakeholders. In Flutter, flutter_gherkin is a powerful package that allows you to implement BDD by writing feature files and step definitions.

What is Behavior-Driven Development (BDD)?

BDD is a development approach where the application’s behavior is defined in terms of scenarios written in a natural language format. These scenarios describe what the system should do in specific situations and serve as both documentation and automated tests. BDD helps in ensuring that everyone involved understands the system’s expected behavior and reduces ambiguity in requirements.

Why Use BDD in Flutter?

• Clear Specifications: BDD provides clear and understandable specifications of the app’s behavior.
• Collaboration: It fosters collaboration between developers, testers, and business stakeholders.
• Automated Testing: Scenarios can be automated as tests, ensuring that the application behaves as expected.
• Living Documentation: Feature files serve as living documentation, always up-to-date with the latest behavior of the system.

Getting Started with flutter_gherkin in Flutter

To implement BDD in Flutter using flutter_gherkin, follow these steps:

Step 1: Add flutter_gherkin Dependency

Add flutter_gherkin to your dev_dependencies in your pubspec.yaml file:

dev_dependencies:
  flutter_test:
    sdk: flutter
  flutter_gherkin: ^8.0.0  # Use the latest version

Run flutter pub get to install the dependency.

Step 2: Create Feature Files

Create a features directory in your Flutter project and add feature files with a .feature extension. Feature files contain scenarios written in Gherkin syntax, a simple, human-readable language.

Example: features/counter.feature

Feature: Counter Functionality
  As a user,
  I want to be able to increment a counter,
  So that I can track how many times I've performed an action.

  Scenario: Incrementing the counter
    Given the app is running
    When I tap the increment button
    Then the counter should display "1"

Step 3: Write Step Definitions

Step definitions are Dart functions that define how each step in the feature files should be executed. Create a test_driver directory in your project and add a file (e.g., steps/counter_steps.dart) containing your step definitions.

import 'package:flutter_gherkin/flutter_gherkin.dart';
import 'package:gherkin/gherkin.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:flutter/material.dart';
import 'package:integration_test/integration_test.dart';

StepDefinitionGeneric theAppIsRunning() {
  return given(RegExp(r'the app is running'), (context) async {
    final tester = context.world.rawAppDriver as FlutterWidgetTester;
    await tester.pumpWidget(const MyApp()); // Replace MyApp with your main app widget
  });
}

StepDefinitionGeneric iTapTheIncrementButton() {
  return when(RegExp(r'I tap the increment button'), (context) async {
    final tester = context.world.rawAppDriver as FlutterWidgetTester;
    final incrementButton = find.byIcon(Icons.add); // Assuming you have an increment button with an add icon
    await tester.tap(incrementButton);
    await tester.pumpAndSettle();
  });
}

StepDefinitionGeneric theCounterShouldDisplay(String expectedCount) {
  return then(RegExp(r'the counter should display "(.*)"'), (context, [String? count]) async {
    final tester = context.world.rawAppDriver as FlutterWidgetTester;
    final expectedValue = count ?? expectedCount;
    final counterTextFinder = find.text(expectedValue);
    expect(counterTextFinder, findsOneWidget);
  });
}

Explanation:

• theAppIsRunning() starts the Flutter application using tester.pumpWidget().
• iTapTheIncrementButton() simulates tapping the increment button using find.byIcon() and tester.tap().
• theCounterShouldDisplay() verifies that the counter displays the expected value using find.text() and expect().

Step 4: Create a Test Driver File

Create a file to configure and run flutter_gherkin. This file will load the feature files, step definitions, and run the tests.

Example: test_driver/run_tests.dart

import 'dart:async';
import 'package:flutter_gherkin/flutter_gherkin.dart';
import 'package:gherkin/gherkin.dart';
import 'package:glob/glob.dart';
import 'steps/counter_steps.dart';
import 'package:integration_test/integration_test_driver_extended.dart';

Future main() {
  final config = FlutterTestConfiguration()
    ..features = [Glob('test_driver/features/**.feature')]
    ..reporters = [
      ProgressReporter(),
      TestRunSummaryReporter(),
      JsonReporter(path: './report.json')
    ]
    ..stepDefinitions = [
      theAppIsRunning(),
      iTapTheIncrementButton(),
      theCounterShouldDisplay(RegExp(r'd+').pattern),
    ]
    ..restartAppBetweenScenarios = true
    ..targetAppPath = 'test_driver/app.dart' // path to your app entry point
    ..exitAfterTestRun = true; // set to false if debugging

  return GherkinRunner().execute(config);
}

Create a separate file to act as a target app for integration tests.

Example: test_driver/app.dart

import 'package:flutter/material.dart';
import 'package:your_app/main.dart' as app;  // Replace 'your_app' with the name of your app

void main() {
  app.main();
}

Step 5: Configure Flutter Integration Tests

Ensure that integration tests are properly configured in your pubspec.yaml:

dev_dependencies:
  flutter_test:
    sdk: flutter
  integration_test:
    sdk: flutter
  flutter_gherkin: ^8.0.0

Enable integration tests in your flutter_test configuration:

flutter:
  uses-material-design: true
  generate: true

Step 6: Run the Tests

Execute the tests using the following command:

flutter drive --driver=test_driver/run_tests.dart --target=test_driver/app.dart

This command runs the run_tests.dart file, which executes the feature files and their corresponding step definitions.

Example: Counter App

Let’s walk through a complete example of a simple counter app and how to test it using flutter_gherkin.

Counter App Implementation

First, let’s create a basic Flutter app with a counter. This will consist of a main.dart which is our main application code.

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Counter App',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Counter App'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);

  final String title;

  @override
  State createState() => _MyHomePageState();
}

class _MyHomePageState extends State {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            const Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headlineMedium,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ),
    );
  }
}

Step Definitions for the Counter App

Now, create or modify your step definitions to interact with your app. Here are a full set of steps for our counter application, with annotations describing their role:

import 'package:flutter_gherkin/flutter_gherkin.dart';
import 'package:gherkin/gherkin.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:flutter/material.dart';
import 'package:integration_test/integration_test.dart';

// Step to ensure the app is running
StepDefinitionGeneric theAppIsRunning() {
  return given(RegExp(r'the app is running'), (context) async {
    final tester = context.world.rawAppDriver as FlutterWidgetTester;
    await tester.pumpWidget(const MyApp());
  });
}

// Step to simulate tapping the increment button
StepDefinitionGeneric iTapTheIncrementButton() {
  return when(RegExp(r'I tap the increment button'), (context) async {
    final tester = context.world.rawAppDriver as FlutterWidgetTester;
    final incrementButton = find.byIcon(Icons.add);
    await tester.tap(incrementButton);
    await tester.pumpAndSettle(); // Wait for animations to complete
  });
}

// Step to verify the counter displays the expected value
StepDefinitionGeneric theCounterShouldDisplay(String expectedCount) {
  return then(RegExp(r'the counter should display "(.*)"'), (context, [String? count]) async {
    final tester = context.world.rawAppDriver as FlutterWidgetTester;
    final expectedValue = count ?? expectedCount;
    final counterTextFinder = find.text(expectedValue);
    expect(counterTextFinder, findsOneWidget);
  });
}

// Step to verify a certain text exists on screen (more general step)
StepDefinitionGeneric iShouldSeeText(String text) {
  return then(RegExp(r'I should see the text "(.*)"'), (context, [String? expectedText]) async {
    final tester = context.world.rawAppDriver as FlutterWidgetTester;
    final textToFind = expectedText ?? text;
    final textFinder = find.text(textToFind);
    expect(textFinder, findsOneWidget);
  });
}

Use flutter_test‘s WidgetTester from flutter_test and ensure that the Flutter application code resides in the lib folder or is accessible via imports.

This example demonstrates how to integrate and verify functionality. Let’s expand this setup.

Comprehensive Features with more steps

To improve maintainability and the value of our tests, we add additional feature files.

1. More granular tests.

2. Complex interaction, e.g. network and user interaction.

Testing Input Fields in a registration page.

Here’s the scenario.

Feature: User Registration
  As a new user,
  I want to register with my information,
  So that I can use the app.

  Scenario: Successful registration
    Given I am on the registration page
    When I fill in the "Name" field with "John Doe"
    And I fill in the "Email" field with "john.doe@example.com"
    And I fill in the "Password" field with "password123"
    And I tap the "Register" button
    Then I should see the text "Registration successful!"

Implementation

Implement associated step definitions.

StepDefinitionGeneric iAmOnTheRegistrationPage() {
  return given(RegExp(r'I am on the registration page'), (context) async {
    final tester = context.world.rawAppDriver as FlutterWidgetTester;
    await tester.pumpWidget(MaterialApp(home: RegistrationPage()));
  });
}

StepDefinitionGeneric iFillInFieldWithValue(String fieldName, String value) {
  return when(RegExp(r'I fill in the "(.*)" field with "(.*)"'), (context, [String? field, String? text]) async {
    final tester = context.world.rawAppDriver as FlutterWidgetTester;
    final fieldKey = Key(field ?? fieldName); // Ensure your TextField has a Key with this name
    await tester.enterText(find.byKey(fieldKey), text ?? value);
  });
}

StepDefinitionGeneric iTapButton(String buttonText) {
  return when(RegExp(r'I tap the "(.*)" button'), (context, [String? button]) async {
    final tester = context.world.rawAppDriver as FlutterWidgetTester;
    final buttonFinder = find.widgetWithText(ElevatedButton, button ?? buttonText);
    await tester.tap(buttonFinder);
    await tester.pumpAndSettle();
  });
}

Best Practices

1. Write Clear Scenarios: Scenarios should be easy to understand by both technical and non-technical stakeholders.
2. Keep Scenarios Concise: Each scenario should focus on a single aspect of the application’s behavior.
3. Use Descriptive Step Names: Step names should clearly describe the action being performed.
4. Automate All Scenarios: Ensure that all scenarios are automated to provide comprehensive test coverage.
5. Integrate with CI/CD: Integrate flutter_gherkin tests into your CI/CD pipeline for continuous testing and validation.

Conclusion

flutter_gherkin is a powerful tool for implementing Behavior-Driven Development in Flutter applications. It facilitates clear specifications, collaboration, and automated testing, leading to higher quality software. By following the steps outlined in this guide, you can effectively incorporate BDD into your Flutter development process and create robust, well-documented applications that meet stakeholder expectations.