React Native SDK Reference

This topic explains how to use the Harness Feature Flags SDK in your React Native application.

NOTE

This SDK is not currently supported on Harness Self-Managed Enterprise Edition (on premises).

This topic describes how to use the Harness Feature Flags SDK for your React Native application.

Before You Begin

• Getting Started with Feature Flags

• Feature Flags Overview

• Client-Side and Server-Side SDKs

• Communication Strategy Between SDKs and Harness Feature Flags

Version

Latest SDK version can be found on the GitHub Release Page

Prerequisites

To use this SDK, make sure you:

• Install React 17 or a newer version.

• Install Node.js 16 or a newer version.

To follow along with our test code sample, make sure you've done the following:

• Create a Feature Flag on the Harness Platform. If you are following along with the SDK README sample code, make sure your flag is called harnessappdemodarkmode.

• Create an SDK key and make a copy of it.

• Create a project using Expo.

Install the SDK

To set up the React Native Client SDK, you have the following options:

• You can install the React Native SDK using npm by running the following command in your terminal:

npm install @harnessio/ff-react-native-client-sdk

• You can also install with yarn by running the following command in your terminal:

yarn add @harnessio/ff-react-native-client-sdk

Initialize the SDK

To initialize the React Native SDK, you need to:

1. import the FF context provider

2. provide a client SDK key

3. provide a target

Import the FF context provider

The FF context provider is responsible for connecting to the Harness Feature Flags service and retrieving your feature flag values. It provides the various hooks and higher-order components with the data they need to operate. You should try to include this as high up in your JSX render tree as possible to ensure that it can serve all the views of your application.

import { FFContextProvider } from '@harnessio/ff-react-native-client-sdk';

Add your client SDK key

To use the provider, wrap your application code in the FFContextProvider component and pass in your client SDK key and target.

To connect to the correct Environment that you set up on the Harness Platform, you need to add the Client SDK Key from that Environment. Input the Client SDK Key into the apiKey prop, for example:

export default function App() {
  return (
    <View>
      <FFContextProvider
        apiKey="YOUR_API_KEY"
        target={target}
      >
        <MyApplicationCode/>
      </FFContextProvider>
    </View>
  )
}

Add a Target

What is a Target?

Targets are used to control which users see which Variation of a Feature Flag, for example, if you want to do internal testing, you can enable the Flag for some users and not others. When creating a Target, you give it a name and a unique identifier. Often Targets are users, but you can create a Target from anything that can be uniquely identified, such as an app or a machine.

For more information about Targets, go to Targeting Users With Flags.

To add a Target that you want to Evaluate, pass an object using the target prop.

Parameter	Description	Required?	Example
identifier	Unique ID for the Target.	Required	user1234
name	Name for the Target.	Optional	User 1234
attributes	Object of key/value pairs of meta data for the target	Optional	{ region: 'US-East' }

Regex requirements for Target names and identifiers

Identifier

Regex: ^[A-Za-z0-9.@_-]*$ Must consist of only alphabetical characters, numbers, and the following symbols: . (period) @ (at sign) -(dash) _ (underscore)

The characters can be lowercase or uppercase but cannot include accented letters, for example Cafe_789.

Name Regex: ^[\\p{L}\\d .@_-]*$

Must consist of only alphabetical characters, numbers, and the following symbols: . (period) @ (at sign) -(dash) _ (underscore) (space)

The characters can be lowercase or uppercase and can include accented letters, for example Café_123.

Configure the SDK

By default, the React Native Client SDK is configured to connect to the Harness Feature Flags service, establish a stream (where supported) and periodically report back flag metrics. Using the options prop, you can configure the following options of the SDK:

Name	Example	Description	Default Value
cache	cache: true	Cache evaluations to use during the next start up of the SDK.	false
baseUrl	baseUrl: 'https://config.ff.harness.io/api/1.0'	The URL used to fetch Feature Flag Evaluations. When using the Relay Proxy, change this to: http://localhost:7000	https://config.ff.harness.io/api/1.0
eventUrl	eventUrl: 'https://events.ff.harness.io/api/1.0'	The URL for posting metrics data to the Feature Flag service. When using the Relay Proxy, change this to: http://localhost:7000	https://events.ff.harness.io/api/1.0
streamEnabled	streamEnabled: true	Set to true to enable streaming mode. Set to false to disable streaming mode.	true
debug	debug: true	Set to true to enable debug mode. Set to false to disable debug mode.	false
eventsSyncInterval	eventsSyncInterval: 60000	The interval in milliseconds at which we send metrics.	60000 (milliseconds)
pollingEnabled	pollingEnabled: true	Set to true to enable polling mode. Set to false to disable polling mode.	false
pollingInterval	pollingInterval: 60000	The interval in milliseconds that we poll for changes when you are using polling mode.	60000 (milliseconds)
logger	logger: { debug: myDebugFn, info: myInfoFn, error: myErrorFn, warn: myWarnFn }	The logger to use when logging debug, info, error and warning messages.	console

Use the options prop to declare the configuration options you want to use, for example:

<FFContextProvider
  apiKey="YOUR_API_KEY"
  target={target}
  options={{
    cache: true,
    streamEnabled: false
  }}
>
 <MyApplicationCode/>
</FFContextProvider>

Code example using Expo

The following is a complete code example using Expo that you can use to test the harnessappdemodarkmode flag you created on the Harness Platform.

When you run the code, it will:

• Render a loading screen

• Connect to the FF service.

• Retrieve all flags.

• Access a flag using the useFeatureFlag hook.

• Access several flags using the useFeatureFlags hook.

First create your Expo project and install the dependencies.

npx create-expo-app my-demo-app
cd my-demo-app
npm install @harnessio/ff-react-native-client-sdk

The following code can be placed in the src/App.js file:

import { StyleSheet, Text, View } from 'react-native';
import { StatusBar } from 'expo-status-bar';

import { FFContextProvider, useFeatureFlag, useFeatureFlags } from '@harnessio/ff-react-native-client-sdk';

export default function App() {
  // typically your target identifier and name should be retrieved from
  // user details and pertain to the current user
  const target = {
    identifier: 'YOUR_TARGET_IDENTIFIER',
    name: 'YOUR TARGET NAME'
  }
  
  return (
    <View style={styles.container}>
      <FFContextProvider
        apiKey="YOUR_API_KEY"
        target={target}
      >
        <SingleFeatureFlag />
        <MultipleFeatureFlags />
      </FFContextProvider>

      <StatusBar style="auto" />
    </View>
  )
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: 'orange',
    alignItems: 'center',
    justifyContent: 'center',
    minHeight: '100%'
  }
})

function SingleFeatureFlag() {
  const flagValue = useFeatureFlag('harnessappdemodarkmode')

  return (
    <Text>The value of "harnessappdemodarkmode" is {JSON.stringify(flagValue)}</Text>
  )
}

function MultipleFeatureFlags() {
  const flags = useFeatureFlags()

  return (
    <>
      <Text>Here are all our flags:</Text>
      <Text>{JSON.stringify(flags, null, 2)}</Text>
    </>
  )
}

Use the React Native SDK

Async mode

By default, the React Native Client SDK will block rendering of children until the initial load of feature flags has completed.

This ensures that children have immediate access to all flags when they are rendered.

However, in some circumstances it might be beneficial to immediately render the application and handle display of loading on a component-by-component basis.

The React Native Client SDK's asynchronous mode allows this by passing the optional async prop when connecting with the FFContextProvider.

Streaming and polling

By default, the React Native Client SDK will set up a stream to keep the feature flag values up-to-date when things change in your Harness project. When a change is made in the Harness project, Harness will send an event to the SDK and the SDK will serve the changed value. This is great when your application needs to change in near-real-time when a feature flag changes (for example, your application might need to display a maintenance screen when the backend APIs are being updated). However, in some circumstances, polling might be a better option. When streaming is disabled and polling is enabled, the SDK will periodically poll for current feature flag values and keep your application up-to-date. By default, the interval for polling is 60 seconds and can be adjusted to suit your application.

Streaming

Streaming is enabled by default and can be disabled using the streamEnabled option and passing false. In the event that the stream is interrupted, the SDK will attempt to reconnect automatically. If after a number of attempts the stream cannot be re-established, the SDK will switch to polling unless specifically disabled using the pollingEnabled option.

Polling

Polling is disabled by default and can be enabled using the pollingEnabled option and passing true. When enabled, the SDK will poll for feature flag value changes every 60 seconds, this can be adjusted using the pollingInterval option and passing the number of milliseconds you want the SDK to wait between polling.

Cache evaluations

In practice flags rarely change and so it can be useful to cache the last received evaluations from the server to allow your application to get started as fast as possible. Setting the cache option as true or as an object (see interface below) will allow the SDK to store its evaluations to localStorage and retrieve at startup. This lets the SDK get started near instantly and begin serving flags, while it carries on authenticating and fetching up-to-date evaluations from the server behind the scenes.

<FFContextProvider
  apiKey="YOUR_API_KEY"
  target={target}
  options={{
    cache: true
  }}
>
  <MyApp />
</FFContextProvider>

The cache option can also be passed as an object with the following options.

interface CacheOptions {
  // maximum age of stored cache, in ms, before it is considered stale
  ttl?: number
  // storage mechanism to use, conforming to the Web Storage API standard, can be either synchronous or asynchronous
  // defaults to localStorage
  storage?: AsyncStorage | SyncStorage
}

interface SyncStorage {
  getItem: (key: string) => string | null
  setItem: (key: string, value: string) => void
  removeItem: (key: string) => void
}

interface AsyncStorage {
  getItem: (key: string) => Promise<string | null>
  setItem: (key: string, value: string) => Promise<void>
  removeItem: (key: string) => Promise<void>
}

Override the internal logger

By default, the React Client SDK will log errors and debug messages using the console object. In some cases, it can be useful to instead log to a service or silently fail without logging errors.

const myLogger = {
  debug: (...data) => {
    // do something with the logged debug message
  },
  info: (...data) => {
    // do something with the logged info message
  },
  error: (...data) => {
    // do something with the logged error message
  },
  warn: (...data) => {
    // do something with the logged warning message
  }
}

return (
  <FFContextProvider
    apiKey="YOUR_API_KEY"
    target={target}
    options={{
      logger: myLogger
    }}
  >
    <MyApp />
  </FFContextProvider>
)

Fast startup

By default, the React Native Client SDK will connect to the Harness Feature Flags service to get the current feature flag values and then render your application. Using a combination of the cache option (see Caching evaluations above) and Async mode (see Async mode above), you can instruct the SDK to instead render immediately using previously cached values (in the case of a returning user) or default values (in the case of new users). The SDK will immediately render your application and asynchronously connect to the Harness Feature Flags service to make sure the cached feature flag values are kept up-to-date.

<FFContextProvider
  async
  apiKey="YOUR_API_KEY"
  target={target}
  options={{
    cache: true
  }}
>
  <MyApp />
</FFContextProvider>

Use the API

FFContextProvider

The FFContextProvider component is used to set up the React context to allow your application to access feature flags using the useFeatureFlag and useFeatureFlags hooks and withFeatureFlags HOC. At minimum, it requires the apiKey you have set up in your Harness Feature Flags account, and the target. You can think of a target as a user.

The FFContextProvider component also accepts an options object, a fallback component, an array of initialEvaluations, an onError handler, and can be placed in Async mode using the async prop. The fallback component will be displayed while the SDK is connecting and fetching your flags. The initialEvaluations prop allows you pass an array of evaluations to use immediately as the SDK is authenticating and fetching flags. The onError prop allows you to pass an event handler which will be called whenever a network error occurs.

import { Text } from 'react-native';
import { FFContextProvider } from '@harnessio/ff-react-native-client-sdk';

// ...

function MyComponent() {
  return (
    <FFContextProvider
      async={false} // OPTIONAL: whether or not to use async mode
      apiKey="YOUR_API_KEY" // your SDK API key
      target={{
        identifier: 'YOUR_TARGET_IDENTIFIER', // replace with a unique ID for the Target 
        name: 'YOUR TARGET NAME',  // replace with the unique name of the Target
        attributes: { // OPTIONAL: key/value pairs of attributes of the Target
          customAttribute: 'this is a custom attribute',
          anotherCustomAttribute: 'this is something else'
        }
      }}
      fallback={<Text>Loading...</Text>} // OPTIONAL: component to display when the SDK is connecting
      options={{ // OPTIONAL: advanced configuration options
        cache: false,
        baseUrl: 'https://url-to-access-flags.com',
        eventUrl: 'https://url-for-events.com',
        streamEnabled: true,
        debug: false,
        eventsSyncInterval: 60000,
        pollingEnabled: false,
        pollingInterval: 60000
      }}
      initialEvaluations={evals} // OPTIONAL: array of evaluations to use while fetching
      onError={handler} // OPTIONAL: event handler to be called on network error
    >
      <CompontToDisplayAfterLoad /> 
    </FFContextProvider>
  )
}

useFeatureFlag

The useFeatureFlag hook returns a single named flag value. An optional second argument allows you to set what value will be returned if the flag does not have a value. By default useFeatureFlag will return undefined if the flag cannot be found.

N.B. when rendered in Async mode, the default value will be returned until the flags are retrieved. Consider using the useFeatureFlagsLoading hook to determine when the SDK has finished loading.

import { Text } from 'react-native';
import { useFeatureFlag } from '@harnessio/ff-react-native-client-sdk';

// ...

function MyComponent() {
  const myFlagValue = useFeatureFlag('flagIdentifier', 'default value')

  return <Text>My flag value is: {myFlagValue}</Text>
}

useFeatureFlags

The useFeatureFlags hook returns an object of flag identifier/flag value pairs. You can pass an array of flag identifiers or an object of flag identifier/default value pairs. If an array is used and a flag cannot be found, the returned value for the flag will be undefined. If no arguments are passed, all flags will be returned.

N.B. when rendered in Async mode, the default value will be returned until the flags are retrieved. Consider using the useFeatureFlagsLoading hook to determine when the SDK has finished loading.

import { Text } from 'react-native';
import { useFeatureFlag } from '@harnessio/ff-react-native-client-sdk';

// ...

function MyComponent() {
  const myFlagValues = useFeatureFlags()

  return (
    <>
      <Text>My flag values are:</Text>
      <Text>{JSON.stringify(myFlagValues, null, 2)}</Text>
    </>
  )
}

Get a subset of Flags

const myFlagValues = useFeatureFlags(['flag1', 'flag2'])

Get a subset of Flags with custom default values

const myFlagValues = useFeatureFlags({
  flag1: 'defaultForFlag1',
  flag2: 'defaultForFlag2'
})

useFeatureFlagsLoading

The useFeatureFlagsLoading hook returns a boolean value indicating whether the SDK is currently loading flags from the server.

import { Text } from 'react-native';
import { useFeatureFlagsLoading, useFeatureFlags } from '@harnessio/ff-react-native-client-sdk';

// ...

function MyComponent() {
  const isLoading = useFeatureFlagsLoading()
  const flags = useFeatureFlags()

  if (isLoading) {
    return <Text>Loading...</Text>
  }

  return (
    <>
      <Text>My flag values are:</Text>
      <Text>{JSON.stringify(flags, null, 2)}</Text>
    </>
  )
}

useFeatureFlagsClient

The React Native Client SDK internally uses the Javascript Client SDK to communicate with Harness. Sometimes it can be useful to be able to access the instance of the Javascript Client SDK rather than use the existing hooks or higher-order components (HOCs). The useFeatureFlagsClient hook returns the current Javascript Client SDK instance that the React Native Client SDK is using. This instance will be configured, initialized, and hooked up to the various events the Javascript Client SDK provides.

import { Text } from 'react-native';
import { useFeatureFlagsClient, useFeatureFlagsLoading } from '@harnessio/ff-react-native-client-sdk';

// ...

function MyComponent() {
  const client = useFeatureFlagsClient()
  const loading = useFeatureFlagsLoading()

  if (loading || !client) {
    return <Text>Loading...</Text>
  }

  return (
    <Text>
      My flag value is: {client.variation('flagIdentifier', 'default value')}
    </Text>
  )
}

ifFeatureFlag

The ifFeatureFlag higher-order component (HOC) wraps your component and conditionally renders only when the named flag is enabled or matches a specific value.

import { Text } from 'react-native';
import { ifFeatureFlag } from '@harnessio/ff-react-native-client-sdk';

// ...

function MyComponent() {
  return <Text>This should render if the flag is on</Text>
}

const MyConditionalComponent = ifFeatureFlag('flag1')(MyComponent)

You can then use MyConditionalComponent as a normal component, and only render if flag1's value is truthy.

Use conditions with a specific value

import { Text } from 'react-native';
import { ifFeatureFlag } from '@harnessio/ff-react-native-client-sdk';

// ...

function MyComponent() {
  return <Text>This should render if the flag evaluates to 'ABC123'</Text>
}

const MyConditionalComponent = ifFeatureFlag('flag1', { matchValue: 'ABC123' })(
  MyComponent
)

You can then use MyConditionalComponent as a normal component, and only render if flag1's value matches the passed condition.

Load fallback when in async mode

If Async mode is used, by default the component will wait for flags to be retrieved before showing. This behaviour can be overridden by passing an element as loadingFallback; when loading the loadingFallback will be displayed until the flags are retrieved, at which point the component will either show or hide as normal.

import { Text } from 'react-native';
import { ifFeatureFlag } from '@harnessio/ff-react-native-client-sdk';

// ...

function MyComponent() {
  return <Text>This should render if the flag is on</Text>
}

const MyConditionalComponent = ifFeatureFlag('flag1', {
  loadingFallback: <Text>Loading...</Text>
})(MyComponent)

withFeatureFlags

The withFeatureFlags higher-order component (HOC) wraps your component and adds flags and loading as additional props. flags contains the evaluations for all known flags and loading indicates whether the SDK is actively fetching flags.

import { Text } from 'react-native';
import { withFeatureFlags } from '@harnessio/ff-react-native-client-sdk';

// ...

function MyComponent({ flags }) {
  return <Text>Flag1's value is {flags.flag1}</Text>
}

const MyComponentWithFlags = withFeatureFlags(MyComponent)

Load in async mode

If Async mode is used, the loading prop will indicate whether the SDK has completed loading the flags. When loading completes, the loading prop will be false and the flags prop will contain all known flags.

import { Text } from 'react-native';
import { withFeatureFlags } from '@harnessio/ff-react-native-client-sdk';

// ...

function MyComponent({ flags, loading }) {
  if (loading) {
    return <Text>Loading...</Text>
  }

  return <Text>Flag1's value is {flags.flag1}</Text>
}

const MyComponentWithFlags = withFeatureFlags(MyComponent)

withFeatureFlagsClient

The React Native Client SDK internally uses the Javascript Client SDK to communicate with Harness. Sometimes it can be useful to be able to access the instance of the Javascript Client SDK rather than use the existing hooks or higher-order components (HOCs). The withFeatureFlagsClient HOC wraps your component and adds featureFlagsClient as additional prop. featureFlagsClient is the current Javascript Client SDK instance that the React Native Client SDK is using. This instance will be configured, initialized, and been hooked up to the various events the Javascript Client SDK provides.

import { Text } from 'react-native';
import { withFeatureFlagsClient } from '@harnessio/ff-react-native-client-sdk';

// ...

function MyComponent({ featureFlagsClient }) {
  if (featureFlagsClient) {
    return (
      <Text>
        Flag1's value is {featureFlagsClient.variation('flag1', 'no value')}
      </Text>
    )
  }

  return <Text>The Feature Flags client is not currently available</Text>
}

const MyComponentWithClient = withFeatureFlagsClient(MyComponent)