Harness Feature Flag Flutter SDK

Table of Contents

Further Reading


Harness Feature Flags (FF) is a feature management solution that enables users to change the software’s functionality, without deploying new code. FF uses feature flags to hide code or behaviours without having to ship new versions of the software. A feature flag is like a powerful if statement.



Flutter SDK >= 2.10.4

For iOS Apps

For Android Apps
Android Studio or the Android SDK for CLI only

You can use Flutter doctor to verify you have the neccessary prerequisites

flutter doctor


The Feature Flag SDK provides a client that connects to the feature flag service, and fetches the value of feature flags. The following section provides an example of how to install the SDK and initalize it from an application. This quickstart assumes you have followed the instructions to setup a Feature Flag project and have created a flag called harnessappdemodarkmode and created a client API Key.

Install the SDK

To add the SDK to your own project run

ff_flutter_client_sdk: ^1.0.4

Then, you may import package to your project

import 'package:ff_flutter_client_sdk/CfClient.dart';

A Simple Example

Here is a complete example that will connect to the feature flag service and report the flag value. An event listener is registered to receive flag change events. Any time a flag is toggled from the feature flag service you will receive the updated value.

// @dart=2.9
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:ff_flutter_client_sdk/CfClient.dart';

// The SDK API Key to use for authentication.  Configure it when installing the app by setting FF_API_KEY
// e.g..
const apiKey = String.fromEnvironment('FF_API_KEY', defaultValue: '');

// The flag name
const flagName = String.fromEnvironment('FF_FLAG_NAME', defaultValue: 'harnessappdemodarkmode');

void main() => runApp(MyApp());

// A simple StatelessWidget that fetches the latest value from the FF Service.
// Everytime it receives an update the value of the flag is updated.
class MyApp extends StatelessWidget {
  Widget build(BuildContext context) {
    return MaterialApp(title: 'Harness Flutter SDK Getting Started', home: FlagState());

class FlagState extends StatefulWidget {
  _FlagState createState() => _FlagState();

class _FlagState extends State<FlagState> {
  bool _flagValue = false;

  void initState() {

    // Create Default Configuration for the SDK.  We can use this to disable streaming,
    // change the URL the client connects to etc
    var conf = CfConfigurationBuilder().build();

    // Create a target (different targets can get different results based on rules.  This include a custom attribute 'location')
    var target = CfTargetBuilder().setIdentifier("fluttersdk").setName("FlutterSDK").build();

    // Init the default instance of the Feature Flag Client
    CfClient.getInstance().initialize(apiKey, conf, target)
      if (value.success) {
        print("Successfully initialized client");

        // Evaluate flag and set initial state
        CfClient.getInstance().boolVariation(flagName, false).then((value) {
          print("$flagName: $value");
          setState(() {
            _flagValue = value;

        // Setup Event Handler
        CfClient.getInstance().registerEventsListener((data, eventType) {
          print("received event: ${eventType.toString()} with Data: ${data.toString()}");
          switch (eventType) {
                case EventType.EVALUATION_CHANGE:
                    String flag = (data as EvaluationResponse).flag;
                    dynamic value = (data as EvaluationResponse).value;
                    // If the change concerns the flag we care about, then update the state
                    if ( flag == flagName ) {
                        setState(() {
                           _flagValue = value.toLowerCase() == 'true';

  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Harness Flutter SDK Getting Started'),
      body: Container(
        child: Center(
          child: new Text("${flagName} : ${_flagValue}", style: TextStyle(fontSize: 25)),

Running the example

If you want to run the getting started example, then you can use flutter to on the cli. You just need to have either an Android or iOS emulator running.

To start an android emulator run (replace @Pixel_4.4_API_32 with your own device id)

$ANDROID_SDK/emulator/emulator @Pixel_4.4_API_32

or for iOS run

open -a simulator 

Confirm you have an ioS or android device with

flutter devices
2 connected devices:

sdk gphone64 x86 64 (mobile) • emulator-5554                        • android-x64    • Android 12 (API 32) (emulator)
iPhone 13 (mobile)           • 425E99F8-702F-4E15-8BBE-B792BF15ED88 • ios            • com.apple.CoreSimulator.SimRuntime.iOS-15-5 (simulator)

Build the project

Using the SDK API key, and a device ID from above you can build and install your app on a emulator

cd examples/getting_started
export FF_API_KEY=<your api key>

flutter pub get
flutter run --dart-define=FF_API_KEY=$FF_API_KEY -d <device id>

The app should show the configured flags current value. As you toggle the flag in the Harrness UI you will see the value update.

Alt Text

Additional Reading

Further examples and config options are in the further reading section:

Further Reading
Getting Started Example
Advanced Example

Harness is a feature management platform that helps teams to build better software and to test features quicker.