FeatureFlagMacro

FeatureFlag is a Swift package that provides an easy and efficient way to manage feature flags in your application using macros introduced in Swift 5.9. This library allows developers to define feature flags as enums, persist their state with UserDefaults, and observe changes using Combine publishers.

Features

• Declarative feature flag management using macros.
• Automatic integration with UserDefaults for state persistence.
• Combine-based AnyPublisher to observe feature flag changes.
• Swift 5.9 macro support for clean, boilerplate-free code generation.

Requirements

• Swift: 5.9+
• iOS: 13.0+ / macOS: 10.15+ / tvOS: 13.0+ / watchOS: 6.0+
• Xcode: 15.0+

Installation

Swift Package Manager

Add the following to your Package.swift:

.package(url: "https://github.com/Keitaro0226/FeatureFlagMacro.git", from: "0.1.3")

Or add the repository URL directly to Xcode:

1. Go to File > Add Packages.
2. Enter https://github.com/Keitaro0226/FeatureFlagMacro.git.
3. Follow the prompts to add the package to your project.

Usage

1. Import the Library

import Combine
import FeatureFlagMacro

2. Define a Feature Flag Enum

Annotate your enum with @FeatureFlag to automatically generate the necessary boilerplate for fetching, setting, and observing feature flags:

@FeatureFlag
@MainActor
public enum FeatureFlag: String, Codable {
    case enabled
    case disabled

    public static var defaultValue: Self { .disabled }
}

3. Set and Fetch Feature Flags

// Set the feature flag
FeatureFlag.set(.enabled)

// Fetch the current feature flag
let currentFlag = FeatureFlag.fetch()
print(currentFlag) // Output: enabled

4. Observe Feature Flag Changes

You can observe changes to the feature flag using Combine's AnyPublisher:

import Combine

@MainActor
final class RootScreenViewModel: ObservableObject {
    @Published var featureFlag: FeatureFlag = .disabled
    private var cancellables = Set<AnyCancellable>()

    public init() {
        bindFeatureFlagPublisher()
    }

    private func bindFeatureFlagPublisher() {
        FeatureFlag.publisher
            .assign(to: &$featureFlag)
    }
}

5. Use the Feature Flag in Your UI

You can now reactively update your UI based on the current feature flag:

import SwiftUI

struct ContentView: View {
    @StateObject private var viewModel = RootScreenViewModel()

    var body: some View {
        VStack {
            Text("Feature is \(viewModel.featureFlag.rawValue)")
        }
    }
}

How It Works

When you annotate an enum with @FeatureFlagMacro, the following code is automatically generated:

• fetch() to retrieve the current feature flag value from UserDefaults.
• set(_:) to update the feature flag value and publish updates.
• A Combine publisher publisher to observe feature flag changes in real-time.

Example of generated code:

private static let subject = PassthroughSubject<FeatureFlag, Never>()

public static var publisher: AnyPublisher<FeatureFlag, Never> {
    subject.eraseToAnyPublisher()
}

public static func fetch() -> FeatureFlag {
    let value = UserDefaults.standard.string(forKey: "featureflag")
    return FeatureFlag(rawValue: value ?? FeatureFlag.defaultValue.rawValue) ?? .defaultValue
}

public static func set(_ value: FeatureFlag) {
    UserDefaults.standard.set(value.rawValue, forKey: "featureflag")
    subject.send(value)
}

Testing

FeatureFlag comes with unit tests for macro expansion and helper extensions.

Run the tests using Xcode or Swift Package Manager:

swift test

Example Test for Macro Expansion

assertMacroExpansion(
    """
    @FeatureFlag
    @MainActor
    enum FeatureFlag: String, Codable {
        case enabled
        case disabled

        public static var defaultValue: Self { .disabled }
    }
    """,
    expandedSource: """
    @MainActor
    enum FeatureFlag: String, Codable {
        case enabled
        case disabled

        public static var defaultValue: Self { .disabled }

        private static let subject = PassthroughSubject<FeatureFlag, Never>()

        public static var publisher: AnyPublisher<FeatureFlag, Never> {
            subject.eraseToAnyPublisher()
        }

        public static func fetch() -> FeatureFlag {
            let value = UserDefaults.standard.string(forKey: "featureflag")
            return FeatureFlag(rawValue: value ?? FeatureFlag.defaultValue.rawValue) ?? .defaultValue
        }

        public static func set(_ value: FeatureFlag) {
            UserDefaults.standard.set(value.rawValue, forKey: "featureflag")
            subject.send(value)
        }
    }
    """,
    macros: testMacros
)

Contributing

Contributions are welcome! If you'd like to improve FeatureFlag, please:

1. Fork the repository.
2. Create a new branch.
3. Make your changes.
4. Open a pull request.

Please include tests for any new functionality.

License

FeatureFlag is available under the MIT License. See the LICENSE file for more details.

Acknowledgments

This library leverages the power of Swift 5.9 macros to simplify feature flag management in Swift applications.