YMFF: Feature management made easy

YMFF is a nice little library that makes managing features with feature flags—and managing feature flags themselves—a bliss, thanks to Swift’s property wrappers and (in the future) macros.

Why & How

Every company I worked for needed a way to manage availability of features in the apps already shipped to the users. Surprisingly, feature flags (a.k.a. feature toggles a.k.a. feature switches) tend to cause a lot of struggle.

I aspire to change that.

YMFF ships completely ready-to-use, right out of the box: you get everything you need to get started in just a few lines of code.

Installation

I’m sure you know how to install dependencies. YMFF supports both SPM and CocoaPods.

Need Help?

Swift Package Manager (SPM)

To add YMFF to your project, use Xcode’s built-in support for Swift packages. Click File → Add Package Dependencies, and paste the following URL into the search field:

https://github.com/yakovmanshin/YMFF

You’re then prompted to select the version to install and indicate the desired update policy. I recommend starting with the latest version (it’s selected automatically), and choosing “up to next major” as the update rule. Then select the target you want to link YMFF to. Click Finish—and you’re ready to go!

If you need to use YMFF in another Swift package, add it to the Package.swift file as a dependency:

.package(url: "https://github.com/yakovmanshin/YMFF", from: "4.0.0")

CocoaPods

YMFF supports installation via CocoaPods, but please keep in mind this support is provided on the best-effort basis.

Add the following to your Podfile:

pod 'YMFF', '~> 4.0'

Setup

YMFF relies on the concept of feature-flag stores—“sources of truth” for feature-flag values.

Firebase Remote Config

Firebase Remote Config is one of the most popular tools to control feature flags remotely. YMFF integrates with Remote Config seamlessly, although with some manual action.

Typical Setup

import FirebaseRemoteConfig
import YMFFProtocols

extension RemoteConfig: SynchronousFeatureFlagStore {
    
    public func valueSync<Value>(for key: FeatureFlagKey) -> Result<Value, FeatureFlagStoreError> {
        // Remote Config returns a default value if the requested key doesn’t exist,
        // so you need to check the key for existence explicitly.
        guard allKeys(from: .remote).contains(key) else {
            return .failure(.valueNotFound)
        }
        
        let remoteConfigValue = self[key]
        
        let value: Value?
        // You need to use different `RemoteConfigValue` methods, depending on the return type.
        // I know, it doesn’t look fancy.
        switch Value.self {
        case is Bool.Type:
            value = remoteConfigValue.boolValue as? Value
        case is Data.Type:
            value = remoteConfigValue.dataValue as? Value
        case is Double.Type:
            value = remoteConfigValue.numberValue.doubleValue as? Value
        case is Int.Type:
            value = remoteConfigValue.numberValue.intValue as? Value
        case is String.Type:
            value = remoteConfigValue.stringValue as? Value
        default:
            value = nil
        }
        
        if let value {
            return .success(value)
        } else {
            return .failure(.typeMismatch)
        }
    }
    
}

RemoteConfig is now a valid feature-flag store.

Alternatively, you can create a custom wrapper object. That’s what I do in my projects to avoid tight coupling.

Usage

Declaring Feature Flags

Here’s how you declare feature flags with YMFF:

import YMFF

// For convenience, organize feature flags in a separate namespace using an enum.
enum FeatureFlags {
    
    // `resolver` references one or more feature-flag stores.
    private static let resolver = FeatureFlagResolver(stores: [MyFeatureFlagStore.shared])
    
    // Feature flags are initialized with three pieces of data:
    // a key string, the default (fallback) value, and the resolver.
    @FeatureFlag("ads_enabled", default: false, resolver: Self.resolver)
    static var adsEnabled
    
    // Feature flags aren’t limited to booleans. You can use any type of value!
    @FeatureFlag("number_of_banners", default: 3, resolver: Self.resolver)
    static var numberOfBanners
    
    // Advanced: Sometimes you want to map raw values from the store
    // to native values used in your app. `MyFeatureFlagStore` below
    // stores values as strings, while the app uses an enum.
    // To switch between them, you use a `FeatureFlagValueTransformer`.
    @FeatureFlag(
        "ad_unit_kind",
        transformer: FeatureFlagValueTransformer { rawValue in
            AdUnitKind(rawValue: rawValue)
        } rawValueFromValue: { value in
            value.rawValue
        },
        default: .image,
        resolver: Self.resolver
    )
    static var adUnitKind
    
}

// You can use custom types for feature-flag values.
enum AdUnitKind: String {
    case text
    case image
    case video
}

Reading Values

To the code that makes use of a feature flag, the flag acts just like the type of its value:

if FeatureFlags.adsEnabled {
    switch FeatureFlags.adUnitKind {
    case .text:
        displayAdText()
    case .image:
        displayAdBanners(count: FeatureFlags.numberOfBanners)
    case .video:
        playAdVideo()
    }
}

Writing Values

YMFF lets you write feature-flag values to mutable stores. It’s as simple as assigning a new value to the flag:

FeatureFlags.adsEnabled = true

To remove the value, you call removeValueFromMutableStores() on FeatureFlag’s projected value (i.e. the FeatureFlag instance itself, as opposed to its wrapped value):

// Here `FeatureFlags.$adsEnabled` has the type `FeatureFlag<Bool>`, 
// while `FeatureFlags.adsEnabled` is of type `Bool`.
FeatureFlags.$adsEnabled.removeValueFromMutableStore()

UserDefaults

You can use UserDefaults to read and write feature-flag values. Your changes will persist when the app restarts.

import YMFF

private static let resolver = FeatureFlagResolver(stores: [UserDefaultsStore()])

That’s it!

More

You can browse the source files to learn more about the available options.

What’s in Store

Next-Version Roadmap

• [#124] Swift macros for easier setup
• [#113] Thread-safety improvements
• [#150] Support for optional values in UserDefaultsStore
• [#144] Minimum compiler version: Swift 5.9 (Xcode 15)

This version is expected in late 2024, after Swift 6 is released.

Ideas & Bug Reports

Feel free to open a new issue if something’s not working—or if you have a suggestion.