feature-flags.md

Feature Flags

In WordPress for iOS, we use feature flags to allow us to merge in-progress features into trunk, while still allowing us to safely deliver builds for testing and production. It's mostly useful for features that require multiple PRs to ship, and may take some time to complete.

We currently do this through BuildConfiguration and FeatureFlag.

BuildConfiguration

BuildConfiguration is an enum which reflects the type of build that is currently running:

case localDeveloper // debug

/// Continuous integration builds for Automattic employees to test branches & PRs
case a8cBranchTest // alpha

/// Beta released internally for Automattic employees
case a8cPrereleaseTesting. // internal - AppCenter / TestFlight

/// Production build released in the app store
case appStore // release

FeatureFlag

The FeatureFlag enum contains a case for any in-progress features that are currently feature flagged. It contains a single computed var: enabled. This determines whether the feature should currently be enabled. This is typically determined based on the current BuildConfiguration. Here are a couple of examples:

enum FeatureFlag: Int {
    case exampleFeature
    case newMediaPicker
    case replaceAllImagesWithRaccoons

    /// Returns a boolean indicating if the feature is enabled
    var enabled: Bool {
        switch self {
        case .exampleFeature:
            return true
        case .newMediaPicker:
            return BuildConfiguration.current == .localDeveloper
        case .replaceAllImagesWithRaccoons:
            return BuildConfiguration.current ~= [.localDeveloper, .a8cBranchTest, .a8cPrereleaseTesting]
        }
    }
}

Here, we have three features which will be active in the following circumstances:

• exampleFeature will always be enabled
• newMediaPicker will only be enabled for local / debug builds
• replaceAllImagesWithRaccoons will be enabled for debug, alpha, and internal builds

Putting it all together

The final step is to check the current status of a feature flag to selectively enable the feature within the app. For example, this might be displaying a button in the UI if a feature is enabled, or perhaps switching to a different code path in a service:

if FeatureFlag.replaceAllImagesWithRaccoons.enabled {
    avatarImageView.image = UIImage.randomRaccoonImage()
} else {
    avatarImageView.image = account.userAvatarImage
}

Once a feature is ready for release, you can remove the feature flag and the old code path.