feature-flags

Feature Flag Patterns

Architecture

FeatureFlagProvider Interface (Kotlin)

interface FeatureFlagProvider {
    fun getBooleanFlag(key: String, default: Boolean = false): Boolean
    fun getStringFlag(key: String, default: String = ""): String
    fun getIntFlag(key: String, default: Int = 0): Int
    fun getDoubleFlag(key: String, default: Double = 0.0): Double
    suspend fun refresh()
}

Type-Safe Flag Definitions

sealed class FeatureFlag<T>(
    val key: String,
    val defaultValue: T
) {
    // Boolean flags
    object NewOnboarding : FeatureFlag<Boolean>("new_onboarding_v2", false)
    object DarkModeEnabled : FeatureFlag<Boolean>("dark_mode_enabled", true)
    object ChatFeature : FeatureFlag<Boolean>("chat_feature", false)

    // String flags
    object CheckoutButtonText : FeatureFlag<String>("checkout_button_text", "Buy Now")
    object HomeLayoutVariant : FeatureFlag<String>("home_layout_variant", "control")

    // Numeric flags
    object MaxCartItems : FeatureFlag<Int>("max_cart_items", 50)
    object SearchDebounceMs : FeatureFlag<Int>("search_debounce_ms", 300)
}

// Type-safe evaluation
class FeatureFlagManager(private val provider: FeatureFlagProvider) {
    fun isEnabled(flag: FeatureFlag<Boolean>): Boolean {
        return provider.getBooleanFlag(flag.key, flag.defaultValue)
    }

    fun getString(flag: FeatureFlag<String>): String {
        return provider.getStringFlag(flag.key, flag.defaultValue)
    }

    fun getInt(flag: FeatureFlag<Int>): Int {
        return provider.getIntFlag(flag.key, flag.defaultValue)
    }
}

Firebase Remote Config

Setup and Initialization

Dependency: com.google.firebase:firebase-config via the Firebase BoM (34.18.0 → firebase-config 23.1.0). KTX APIs moved into the main module in BoM 32.5.0 and the -ktx artifacts were removed in BoM 34.0.0 — depend on firebase-config, never firebase-config-ktx.

class FirebaseFeatureFlagProvider : FeatureFlagProvider {
    private val remoteConfig = Firebase.remoteConfig.apply {
        val configSettings = remoteConfigSettings {
            minimumFetchIntervalInSeconds = if (BuildConfig.DEBUG) 0 else 3600
        }
        setConfigSettingsAsync(configSettings)
        setDefaultsAsync(R.xml.remote_config_defaults)
    }

    // getBoolean/getDouble have no per-call default — a key absent from the
    // XML defaults returns false/0.0. Register every flag in the defaults XML.
    override fun getBooleanFlag(key: String, default: Boolean): Boolean {
        return remoteConfig.getBoolean(key)
    }

    override fun getStringFlag(key: String, default: String): String {
        return remoteConfig.getString(key).ifEmpty { default }
    }

    override fun getIntFlag(key: String, default: Int): Int {
        return remoteConfig.getLong(key).toInt()
    }

    override fun getDoubleFlag(key: String, default: Double): Double {
        return remoteConfig.getDouble(key)
    }

    override suspend fun refresh() {
        remoteConfig.fetchAndActivate().await()
    }
}