Feature Flags

What is a feature flag?

Is the Firefox iOS codebase, we define a feature flag as a variable, inside a feature, that controls the status of the feature in the application.

Feature flags should logically be part of their own features, even if that's the only variable in that feature. - ie. Should not be part of a generalAppFeature, or a featureFlagFeature.

How feature flags work in Firefox iOS

Feature flags are all controlled by the FeatureFlagManager singleton. To access the singleton, you must make a class conform to the FeatureFlaggable protocol, which will give access to the featureFlags variable.

class BibimbapViewModel: FeatureFlaggable {
    var isNewMenuAvailable: Bool {
        return featureFlags.isFeatureEnabled(.newBibimbapMenu, checking: .buildOnly)
    }
}

Types of features flags

Name	Description	User Togglable
Core	Core features are features that are used for developer purposes and are not directly user impacting.	No
Nimbus	A nimbus feature is a feature whose configuration comes from Nimbus.	Possibly

The vast majority of feature flags should be Nimbus flags, rather than Core flags.

The FeatureFlagManager interface

Interface	Purpose
isCoreFeatureEnabled(...)	Checking where a Core feature is enabled.
isFeatureEnabled(...)	Checking whether a boolean based Nimbus feature is enabled.
getCustomState<T>(...)	Checking the status of a non-boolean based Nimbus feature.
set(...)	Saving a boolean based Nimbus feature user preference to UserDefaults.
set<T: FlaggableFeatureOptions>(...)	Saving a non-boolean based Nimbus feature user preference to UserDefaults.

The checking parameter & how feature status is checked.

One of the complexities of feature flags is that while Nimbus may have a default, a user may turn something off. Regardless of whether or not the user is in an experiment, their preferences should be respected. To accomplish this, the previously listed interfaces that check a feature status include a specific checking parameter. This has three options which should cover 100% of use cases for needing to check the status of a feature.

• buildOnly - this will only check Nimbus configuration for status
• userOnly - this will check UserDefaults to see if the user has a preference. If they do, that is what will be returned. If they do not, then the Nimbus configuration is queried for status
• buildAndUser - this will a mix of both.

How to set up a Feature Flag

Adding a feature flag to Nimbus

To add a feature to Nimbus, please read Nimbus Feature. Once this is done, add a variable to that feature named something indicative of a status. Here is an example of what that might look like

...
  isEnabled:
    description: >
      Whether or not the feature is enabled.
    type: Boolean
    default: false

Adding a feature flag in the app

To add the flag in the app (for example, for the newBibimbapMenu flag), follow these steps:

1. Add case newBibimbapMenu to the NimbusFeatureFlagID enum.
2. Add this new case to the NimbusFlaggableFeature struct. a. If the user will have a setting to interact with for the feature, you should add this such that it returns a PrefsKeys.FeatureFlags key, which you will have to also add. b. If the user doesn't have a setting for the feature, you should add it to the return nil part of the switch statement.
3. In the NimbusFeatureFlagLayer class, you should add a case for your new feature, as well as the function it will call

...
    switch featureID {
    case .newBibimbapMenu:
        return checkBibimbapFeature(for: featureID, from: nimbus)
...
private func checkBibimbapFeature(
    for featureID: NimbusFeatureFlagID,
    from nimbus: FxNimbus
) -> Bool {
    let config = nimbus.features.bibimbapFeature.value()

    switch featureID {
    case .newBibimbapMenu: return config.newBibimbapMenu
    default: return false
    }
}