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How to set up A/B tests in Flutter

Mar 05, 2024

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A/B tests help you improve your Flutter app by enabling you to compare the impact of changes on key metrics. To show you how to set one up, we create a basic Flutter app, add PostHog, create an A/B test, and implement the code for it.

1. Create a new Flutter app

Our app will have two screens:

  1. The first screen will have a button which takes you to a second screen.
  2. The second screen will either have a red or green background color, depending on whether the user is in the control or test variant of our A/B test. This screen will also have a button which captures an event when it's pressed. We'll use this event as our goal metric for our test.

To set this up, install the Flutter extension for VS Code. Then, create a new app by opening the Command Palette in VS Code (Ctrl/Cmd + Shift + P), typing flutter and selecting Flutter: New Project.

Select Empty Application and name your app flutter_ab_tests. Then, replace your code in lib/main.dart with the following:

lib/main.dart
import 'package:flutter/material.dart';
import 'feature_screen_view.dart';


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


class MyApp extends StatelessWidget {
  const MyApp({super.key});
  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'Flutter A/B Test App',
      home: MainScreen(),
    );
  }
}


class MainScreen extends StatelessWidget {
  const MainScreen({super.key});
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Main Screen')),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text('Hello, world!'),
            ElevatedButton(
              child: const Text('Go to Next Screen'),
              onPressed: () {
                Navigator.push(
                  context,
                  MaterialPageRoute(builder: (context) => const FeatureScreenView(isTestVariant: false)), // We update this later
                );
              },
            ),
          ],
        ),
      ),
    );
  }
}

Lastly, in the lib directory, create a new file for our second screen called feature_screen_view.dart. Add the following code to it:

Dart
import 'package:flutter/material.dart';


class FeatureScreenView extends StatelessWidget {
  final bool isTestVariant;


  const FeatureScreenView({super.key, required this.isTestVariant});


  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: isTestVariant ? Colors.green : Colors.red,
      body: Center(
        child: ElevatedButton(
          child: const Text('Click Me!'),
          onPressed: () {
            // Event capturing will go here
          },
        ),
      ),
    );
  }
}

Press F5 and run the app in any emulator (we chose Android) to see your app in action.

2. Add PostHog to your app

With our app set up, it’s time to install and set up PostHog. If you don't have a PostHog instance, you can sign up for free.

To start, install PostHog’s Flutter SDK by adding posthog_flutter to your pubspec.yaml:

pubspec.yaml
# rest of your code


dependencies:
  flutter:
    sdk: flutter
  posthog_flutter: ^4.0.1


# rest of your code

Next, we configure PostHog using our project API key and instance address. You can find these in your project settings.

Android setup

For Android, add your PostHog configuration to your AndroidManifest.xml file located in the android/app/src/main directory:

android/app/src/main/AndroidManifest.xml
<manifest xmlns:android="http://schemas.android.com/apk/res/android" package="your.package.name">
    <application>
        <!-- ... other configuration ... -->
        <meta-data android:name="com.posthog.posthog.API_KEY" android:value="<ph_project_api_key>" />
        <meta-data android:name="com.posthog.posthog.POSTHOG_HOST" android:value="<ph_instance_address>" /> <!-- usually 'https://app.posthog.com' or 'https://eu.posthog.com' -->
        <meta-data android:name="com.posthog.posthog.TRACK_APPLICATION_LIFECYCLE_EVENTS" android:value="true" />
        <meta-data android:name="com.posthog.posthog.DEBUG" android:value="true" />
    </application>
</manifest>

You'll also need to update the minimum Android SDK version to 21 in android/app/build.gradle:

android/app/build.gradle
// rest of your config


    defaultConfig {
      minSdkVersion 21
      // rest of your config
    }


// rest of your config

iOS setup

For iOS, you'll need to have Cocoapods installed. Then add your PostHog configuration with your project API key and instance address to the Info.plist file located in the ios/Runner directory:

XML
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<!-- rest of your configuration -->
  <key>com.posthog.posthog.API_KEY</key>
  <string><ph_project_api_key></string>
  <key>com.posthog.posthog.POSTHOG_HOST</key>
  <string><ph_instance_address></string>  <!--  https://app.posthog.com or https://eu.posthog.com -->
  <key>com.posthog.posthog.CAPTURE_APPLICATION_LIFECYCLE_EVENTS</key>
  <true/>
  <key>com.posthog.posthog.DEBUG</key>
  <true/>
</dict>
</plist>

Then you need to set the minimum platform version to iOS 13.0 in your Podfile:

YAML
platform :ios, '13.0'


# rest of your config

Web setup

For Web, add your Web snippet (which you can find in your project settings) in the <head> of your web/index.html file:

web/index.html
<!DOCTYPE html>
<html>


<head>
  <!-- ... other head elements ... -->


  <script>
    !function(t,e){var o,n,p,r;e.__SV||(window.posthog=e,e._i=[],e.init=function(i,s,a){function g(t,e){var o=e.split(".");2==o.length&&(t=t[o[0]],e=o[1]),t[e]=function(){t.push([e].concat(Array.prototype.slice.call(arguments,0)))}}(p=t.createElement("script")).type="text/javascript",p.async=!0,p.src=s.api_host+"/static/array.js",(r=t.getElementsByTagName("script")[0]).parentNode.insertBefore(p,r);var u=e;for(void 0!==a?u=e[a]=[]:a="posthog",u.people=u.people||[],u.toString=function(t){var e="posthog";return"posthog"!==a&&(e+="."+a),t||(e+=" (stub)"),e},u.people.toString=function(){return u.toString(1)+".people (stub)"},o="capture identify alias people.set people.set_once set_config register register_once unregister opt_out_capturing has_opted_out_capturing opt_in_capturing reset isFeatureEnabled onFeatureFlags getFeatureFlag getFeatureFlagPayload reloadFeatureFlags group updateEarlyAccessFeatureEnrollment getEarlyAccessFeatures getActiveMatchingSurveys getSurveys onSessionId".split(" "),n=0;n<o.length;n++)g(u,o[n]);e._i.push([i,s,a])},e.__SV=1)}(document,window.posthog||[]);
    posthog.init(
      '<ph_project_api_key>',
      {
        api_host:'<ph_instance_address>',
      }
    )
  </script>
</head>


<!-- ... other elements ... -->


</html>

3. Capture a custom event

The first part of setting up our A/B test in PostHog is setting up the goal metric. We'll use the number of clicks on the button on FeatureScreenView as our goal.

To measure this, we capture a custom event feature_button_clicked when the button is clicked. To do this, update the code in feature_screen_view.dart to call Posthog().capture in onPressed:

feature_screen_view.dart
import 'package:flutter/material.dart';
import 'package:posthog_flutter/posthog_flutter.dart';


class FeatureScreenView extends StatelessWidget {
  final bool isTestVariant;


  const FeatureScreenView({Key? key, required this.isTestVariant}) : super(key: key);


  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: isTestVariant ? Colors.green : Colors.red,
      body: Center(
        child: ElevatedButton(
          child: const Text('Click Me!'),
          onPressed: () async {
            await Posthog().capture(
              eventName: 'feature_button_clicked',
            );
          },
        ),
      ),
    );
  }
}

Once you’ve done this, reload your app and click the button a few times. You should see events appearing in the PostHog events explorer.

Events captured in PostHog

4. Create an A/B test in PostHog

If you haven't done so already, you'll need to upgrade your PostHog account to include A/B testing. This requires entering your credit card, but don't worry, we have a generous free tier of 1 million requests per month – so you won't be charged anything yet.

Next, go to the A/B testing tab and create an A/B test by clicking the New experiment button. Add the following details to your experiment:

  1. Name it "My cool experiment".
  2. Set "Feature flag key" to my-cool-experiment.
  3. Under the experiment goal, select the feature_button_clicked event we created in the previous step.
  4. Use the default values for all other fields.

Click "Save as draft" and then click "Launch".

Experiment setup in PostHog

5. Implement the A/B test code

The final step is to add the experiment code. We'll add code that does the following:

  1. Fetch the my-cool-experiment flag using await Posthog().getFeatureFlag('my-cool-experiment').
  2. Change the background color of FeatureScreenView based on the value of the flag (control or test).

To do this, update the code in main.dart to the following:

Dart
import 'package:flutter/material.dart';
import 'feature_screen_view.dart';
import 'package:posthog_flutter/posthog_flutter.dart';


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


class MyApp extends StatelessWidget {
  const MyApp({super.key});
  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'Flutter A/B Test App',
      home: MainScreen(),
    );
  }
}


class MainScreen extends StatelessWidget {
  const MainScreen({super.key});


  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Main Screen')),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text('Hello, world!'),
            ElevatedButton(
              child: const Text('Go to Next Screen'),
              onPressed: () async { 
                final featureFlagValue = await Posthog().getFeatureFlag('my-cool-experiment');
                bool isTestVariant = featureFlagValue == 'test';
                if (context.mounted) {
                  Navigator.push(
                    context,
                    MaterialPageRoute(builder: (context) => FeatureScreenView(isTestVariant: isTestVariant)),
                  );
                }
              }
            ),
          ],
        ),
      ),
    );
  }
}

That's it! Your A/B test is now ready. When you run your app, you see either green or red as the background color of FeatureScreenView and PostHog will capture button clicks for each variant to calculate if changing the color has a statistically significant impact.

Lastly, you can view your test results on the experiment page.

Further reading