Feature Flags

Feature Toggles offer a way to enable or disable certain features that are present in your codebase, switch environments or configurations or toggle between multiple implementations of a protocol - even in your live system at runtime. Feature Flags are implemented using CloudKit and are therefor associated with no run costs for the developer. Existing Feature Toggles can be changed in the CloudKit Dashboard and are delivered immediately via silent push notifications to your users.

Installation

Currently only Swift Package Manager is supported. Swift Package Manager is a dependency manager built into Xcode.

If you are using Xcode 11 or higher, go to File / Swift Packages / Add Package Dependency... and enter package repository URL https://github.com/dehlen/FeatureFlags.git, then follow the instructions.

To remove the dependency, select the project and open Swift Packages (which is next to Build Settings). You can add and remove packages from this tab.

Usage

CloudKit Preparations

1. If your application does not support CloudKit yet start with adding the CloudKit and remote background notification entitlements to your application
2. Add a new custom record type 'FeatureStatus' with two fields:

For each feature toggle you want to support in your application later add a new record in your CloudKit public database.

In your project

1. In your AppDelegate, initialize a FeatureToggleApplicationService and hook its two UIApplicationDelegate methods into the AppDelegate lifecycle like so:

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        // Override point for customization after application launch.
        return featureToggleApplicationService.application(application, didFinishLaunchingWithOptions: launchOptions)
}

func application(_ application: UIApplication, didReceiveRemoteNotification userInfo: [AnyHashable : Any], fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
        featureToggleApplicationService.application(application, didReceiveRemoteNotification: userInfo, fetchCompletionHandler: completionHandler)
}

2. Anywhere in your code you can create an instance of FeatureToggleUserDefaultsRepository and call retrieve to fetch the current status of a feature toggle.

:warning: Note that retrieve returns the locally saved status of your toggle, this command does not trigger a fetch from CloudKit. Feature Toggles are fetched from CloudKit once at app start from within the FeatureToggleApplicationService UIApplicationDelegate hook. Additionally you can subscribe to updates whenever there was a change to the feature toggles in CloudKit as shown in the next section.

3. You have to call retrieve with your implementation of a FeatureToggleIdentifiable. What I think works well is creating an enum which implements FeatureToggleIdentifiable:

enum FeatureToggle: String, FeatureToggleIdentifiable {
    case feature1
    case feature2
    
    var identifier: String {
        return self.rawValue
    }
    
    var fallbackValue: Bool {
        switch self {
        case .feature1:
            return false
        case .feature2:
            return true
        }
    }
}

Notifications

You can subscribe to updates from your feature toggles in CloudKit by subscribing to the onRecordsUpdated Notification like so:

NotificationCenter.default.addObserver(self, selector: #selector(updateToggleStatusFromNotification), name: NSNotification.Name.onRecordsUpdated, object: nil)

@objc
private func updateToggleStatusFromNotification(notification NSNotification) {
    guard let updatedToggles = notification.userInfo[Notification.featureToggleUserInfoKey] as? [FeatureToggle] else {
        return
    }
    
    // do something with the updated toggle like e.g. disabling UI elements 
}

Note that the updated Feature Toggles are attached to the notifications userInfo dictionary. When this notification has been sent the updated values are also already stored in the repository.

Documentation

The documentation is generated thanks to jazzy.

You can find the latest version here: Documentation.

License

The MIT License

Copyright (c) 2020 David Ehlen

See LICENSE file