FeatureToggles

A novel approach to feature toggles for Swift programmers.

Feature toggles, also known as “feature flags,” are a tool to keep a team of developers contributing to the mainline without revealing half-implemented features on their releases.

The FeatureToggles package provides a mechanism for declaring toggles in a decentralized manner. Those toggles can stay turned off in RELEASE builds, keeping the main branch deployable. However, you can turn them on in other environments so developers and testers can access them.

Compared to other solutions for managing toggles, FeatureToggles presents several advantages.

• It’s distributed. Different modules can provide new toggles to their clients. That’s great for modular projects.
• It’s ergonomic. Inspired by SwiftUI’s environment, it lets you access any toggle with a simple @FeatureToggle-annotated property.
• It’s versatile. Turn features on or off depending on whether it’s a DEBUG or RELEASE build, or override the default via a specially-formatted URL.
• It’s strongly typed. Leveraging the power of Swift’s type system, it reduces errors and enhances code readability and maintainability.
• It’s lightweight. Just a few lines of code, zero dependencies.

First, declare a FeatureToggleKey. You can extend an existing data structure:

extension TeleportView: FeatureToggleKey, ReleaseDisabled, DebugEnabled {}

Or create a brand new entity:

enum TeleportFeatureToggle: FeatureToggleKey, ReleaseDisabled, DebugEnabled {}

Then, add a getter for your new key.

extension FeatureToggleValues {
  var teleport: Bool { self[TeleportView.self] }
}

Finally, use the @FeatureToggle property wrapper whenever you wish to access the current state of the toggle.

struct ContentView: View {
  @FeatureToggle(\.teleport) var isTeleportButtonEnabled

  var body: some View {
    VStack {
      Text("Welcome to our app")
        .font(.title)

      if isTeleportButtonEnabled {
        TeleportView()
      }
    }
  }
}

Your app can override compile-time defaults with a specially-formatted URL.

First, add a featureToggleIdentifier to your key:

extension TeleportView: FeatureToggleKey, ReleaseDisabled, DebugEnabled {
  static let featureToggleIdentifier = "teleport"
}

Then, call FeatureToggleValues.override(withValuesFrom:) when the app delegate opens a URL. In an iOS application, it would look something like this:

public func application(
  _: UIApplication,
  open url: URL,
  options _: [UIApplication.OpenURLOptionsKey: Any] = [:]
) -> Bool {
  FeatureToggleValues.override(withValuesFrom: url)
}

If your app’s custom URL is myapp, this link would enable the feature even in RELEASE builds with a disabled compile-time toggle:

myapp://toggle?id=teleport&value=true

This library is released under the MIT license. See LICENSE for details.