How to set up feature flags in iOS

Feature flags help you conditionally roll out and release features safely. This tutorial shows you how integrate them in iOS using PostHog.

We'll create a basic iOS app, add PostHog, create a feature flag, and then implement the flag to control content in our app.

1. Create a new iOS app

Our app will have two screens. The first screen will have a button which takes you to a second screen. The second screen will either have a red or green background color depending on whether our feature flag is enabled or not.

The first step is to create a new app. Open XCode and click "Create new project". Select iOS as your platform, then "App" and press next. Give your app a name, select SwiftUI as the interface, and the defaults for everything else. Click next and then "Create".

Then, replace your code in ContentView.swift with the following:

import SwiftUI

struct ContentView: View {
    @State private var navigateToFeatureScreen = false

    var body: some View {
        NavigationStack {
            VStack {
                Image(systemName: "globe")
                    .imageScale(.large)
                    .foregroundStyle(.tint)
                Text("Hello, world!")

                Button("Go to Next Screen") {
                    navigateToFeatureScreen = true
                }
                .padding()
                .background(Color.blue)
                .foregroundColor(.white)
                .cornerRadius(10)
            }
            .padding()
            .navigationDestination(isPresented: $navigateToFeatureScreen) {
                FeatureScreenView(isFlagEnabled: false) // We'll update this later
            }
        }
    }
}

Then, create a new SwiftUI View called FeatureScreenView. Replace the default code in the new file with the following:

import SwiftUI

struct FeatureScreenView: View {
    var isFlagEnabled: Bool

    var body: some View {
        ZStack {
            Color(isFlagEnabled ? .green : .red)
                .edgesIgnoringSafeArea(.all)
            .padding()
            .cornerRadius(10)
        }
    }
}

Our basic set up is now complete. Build and run your app to see it 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.

First, add posthog-ios as a dependency to your app using Swift Package Manager (or if you prefer, you can use CocoaPods.

To add the package dependency to your Xcode project, select File > Add Package Dependency and enter the URL https://github.com/PostHog/posthog-ios.git. Select posthog-ios and click Add Package.

Note that for this tutorial we use version 3.1.3 of the SDK.

Next, configure your PostHog instance in App.swift using your project API key and instance address (you can find these in your project settings):

import SwiftUI
import PostHog

@main
struct posthog_feature_flagsApp: App {
    init() {
        let POSTHOG_API_KEY = "<ph_project_api_key>"
        // usually 'https://us.i.posthog.com' or 'https://eu.i.posthog.com'
        let POSTHOG_HOST = "https://us.i.posthog.com"
        let configuration = PostHogConfig(apiKey: POSTHOG_API_KEY, host: POSTHOG_HOST) 
        PostHogSDK.shared.setup(configuration)
    }
    
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

To check your setup, build and run your app. Click your button a few times. You should start seeing events in the activity tab.

3. Create a feature flag in PostHog

With PostHog set up, your app is ready for feature flags. To create one, go to the feature flags tab in PostHog and click New feature flag. Enter a flag key (like my-cool-flag), set the release condition to roll out to 100% of users, and press "Save."

You can customize your release conditions with rollout percentages, and user or group properties to fit your needs.

4. Implement the flag code

To implement the feature flag, we:

1. Fetch the my-cool-flag flag using PostHogSDK.shared.isFeatureEnabled().
2. Change the background color of FeatureScreenView based on the value of the flag.

To do this, update the code in ContentView.swift with the following:

import SwiftUI
import PostHog

// ...

    @State private var isFlagEnabled = false

// ...
    var body: some View {
        NavigationStack {
            VStack {
               // ...

               Button("Go to Next Screen") {
                    // Fetch feature flag here
                    isFlagEnabled = PostHogSDK.shared.isFeatureEnabled("my-cool-flag")
                    navigateToFeatureScreen = true
                }

// ...

            .navigationDestination(isPresented: $navigateToFeatureScreen) {
                FeatureScreenView(isFlagEnabled: isFlagEnabled) 
            }
        }
    }
}

That's it! When you restart your app and click the button, you should see the green background color on the second screen.

Further reading

• How to set up iOS session replay
• How to run A/B tests in iOS
• How to set up analytics in iOS

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