React Native Client SDK For Harness Feature Flags

Use this README to get started with our Feature Flags (FF) Client SDK for React Native. This guide outlines the basics of getting started with the SDK and provides a full code sample for you to try out.

This sample doesn't include configuration options, for in depth steps and configuring the SDK, see the React Native Client SDK Reference.

Requirements

To use this SDK, make sure you’ve:

• Installed Node.js v16 or a newer version
• Installed React.js v17 or a newer version

To follow along with our test code sample, make sure you’ve:

• Created a Feature Flag on the Harness Platform called harnessappdemodarkmode
• Created a client SDK key and made a copy of it
• Created a project using Expo

npx create-expo-app my-demo-app
cd my-demo-app
npm install

Installing the SDK

The first step is to install the FF SDK as a dependency in your application. To install using npm, use:

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

Or to install with yarn, use:

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

Code Sample

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

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() {
  return (
    <View style={styles.container}>
      <FFContextProvider
        apiKey="YOUR_API_KEY"
        target={{
          identifier: 'YOUR_TARGET_IDENTIFIER', // <- replace with an identifier unique to the user, e.g. email or UUID
          name: 'YOUR TARGET NAME' // <- replace with a name unique to the user
        }}
      >
        <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>
    </>
  )
}

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 may 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 asyncMode 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.

Caching 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={{
    identifier: 'YOUR_TARGET_IDENTIFIER',
    name: 'YOUR TARGET NAME'
  }}
  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>
}

Overriding 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={{
      identifier: 'YOUR_TARGET_IDENTIFIER',
      name: 'YOUR TARGET NAME'
    }}
    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
  asyncMode
  apiKey="YOUR_API_KEY"
  target={{
    identifier: 'YOUR_TARGET_IDENTIFIER',
    name: 'YOUR TARGET NAME'
  }}
  options={{
    cache: true
  }}
>
  <MyApp />
</FFContextProvider>

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 asyncMode 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
      asyncMode={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 /> <!-- component to display when Flags are available -->
    </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 have been 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.

Conditionally 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.

Loading 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)

Loading 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 have 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)

Additional Reading

For further examples and config options, see the React Native Client SDK Reference For more information about Feature Flags, see our Feature Flags documentation.