Conditional Features

Flint forces your code to verify that a feature is enabled before you can perform its actions, with one consistent mechanism. If it isn't enabled, you can take the user through the steps required to enable it, by making purchases or enabling the necessary permissions.

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Flint allows you to define features that may not always be available to users. Apps usually include some features that are not enabled by default, as well as those that be manually enabled by the user. On top of this, system permission authorisations, in-app purchases and feature-flagging are classic cases of conditional features, where the user first has to pay or meet some other criteria to see or use the feature.

In Flint we use the concept of constraints on conditional features to limit when they are available. There are many constraints you can apply, including operating system versions, system permissions like Location or Camera access, purchases, and build-specific or user-toggled feature “flagging” (turning features on or off explicitly).

Imagine a podcast player app with a feature hierarchy like this:

  • All App Features
    • “Podcast playback”
      • “Dynamic Speed” — requires purchase “Premium subscription”, and user must turn it on
      • “Sound enhancement” — requires purchase “Premium subscription”, and user must turn it on
    • “Podcast downloading”
      • “Turbo downloads” — experimental feature currently for internal use only
    • “Podcast favourites”
      • “Smart playlists” — requires location access and purchase “Premium subscription”

That’s three parent features with a total of four sub-features that all have some kind of condition that determines whether or not they are currently available.

What makes conditional features in your code special is that you cannot perform their actions directly. You must first know that whether the feature is available. If the feature is not available you should take appropriate action — such as prompting the user for any required permissions. Flint helps with this part too.

This conditional feature approach makes it much easier to write cross-platform code as you do not have to handle the case where the feature’s definition is not even compiled in to the binary for a platform.

Defining a conditional feature with constraints

The first step is to define your feature as conforming to the protocol ConditionalFeature instead of Feature. You then declare the constraints on the feature by implementing the constraints function, which will tell Flint when this feature can be available at runtime. This function is called only once at startup, in order to define the constraints themselves which are evaluated every time you need to use the feature.

Here’s an example from Flint’s own conditional feature for deep linking support:

/// Flint's deep linking feature for URL Routes is conditional so that you can disable
/// it if you don't want it.
public class DeepLinkingFeature: ConditionalFeature {
    public static var description: String = "Deep Linking and app-URL handling"
    // Declare the constraints that must be met for
    // the feature to be available
    public static func constraints(requirements: FeatureConstraintsBuilder) {

    // It's on by default. 
    public static var isEnabled: Bool? = true

    /// The action to use to perform the URL
    public static let performIncomingURL = action(PerformIncomingURLAction.self)
    public static func prepare(actions: FeatureActionsBuilder) {

The constraints(requirements:) function definition uses a Domain-Specific Language (DSL) provided by the “builder” passed to the function. The above constraint .runtimeEnabled means that the isEnabled property of the feature must return true for the feature to be available.. The default implementation of this provided by Flint always returns true. You can override the property in your own features to allow assignments to it at compile or runtime, or make it call into other code to find out if it should be enabled.

Some constraints do not change their value over time — such as the minimum operating system version — while others are evaluated at runtime when required, as their state can change while the app is running. For example if a user makes an in-app purchase, once it has been verified that the purchase is valid, the features reliant on that purchase will now be available.

Here are the kinds of constraints the DSL currently supports:

  • Preconditions: This includes the user toggling a feature on or off, in-app purchases and runtime enabling.
  • System Permissions: The permissions your app needs for the feature to work, such as access to Photos, Contacts or Location.
  • Platform restrictions: You can set the minimum required version for each platform operating system, or indicate “any” or “unsupported”.

Flint allows you to combine all of these as appropriate to declare a simple set of requirements that handle all of the complexities of these disparate factors affecting feature availability.

Defining preconditions

At the time of writing there are three kinds of precondition function supported:

  • purchase(requirement: PurchaseRequirement) — The feature requires one or more purchases before it is available.
  • runtimeEnabled() — Whenever a request is made to use the feature, the value of YourFeature.isEanbled will be checked.
  • userToggled(defaultValue: Bool) — The Feature will check the user’s current settings to see if this feature is enabled.

So if we wanted to have a somewhat contrived conditional feature that required a purchase but also had to be enabled by the user, say a level editor mode in a game, and the user could only access the level editor they paid for once they had completed in-game “training”, you would set it up like this:

let premiumSubscription = Product(name: "💎 Premium Subscription",
                                  description: "Unlock the level builder",
                                  productID: "SUB0001")

public class LevelBuilderFeature: ConditionalFeature {
    public static var description: String = "Premium Level Builder"

    public static func constraints(requirements: FeatureConstraintsBuilder) {
    	requirements.userToggled(defaultValue: true)

    static var isEnabled: Bool? = MyPlayerProgressTracker.shared.tutorialCompleted


Note that there are both purchase() and the plural purchases(...) forms of the function for declaring purchase requirements. You can call these functions as many times as makes sense for your requirements, but .purchase is the only precondition that can have multiple declarations with different parameters.

For purchase preconditions, purchase requirements (see PurchaseRequirement) allow you to define rules based on one or more products, so that requirements can be fulfilled by several different purchases (say “Premium subscription” or “Generous supporter”), or require specific combinations.

The user toggling precondition will read from the user’s defaults, and if there is no current value will use the default value provided in code.

The .runtimeEnabled test will always check the static isEnabled property on the feature to verify it is true. You can use this for any kind of app-supplied runtime determination of the feature availability. You can also declare it as a writable property and just assign it true/false in the source file or at runtime to flag internal features that are perhaps not yet ready to ship.

Only if all the preconditions are true and all the other constraints are also met will the Feature be available.

See In-App Purchases for more details on setting up a PurchaseTracker to determine what has been purchased, and to use the debug purchase tracker to allow you to easily test different purchasing combinations at runtime.

Defining required permissions

Permissions can be fiddly to deal with in apps. Your goal is to get people to authorise permissions so that they can use what you’ve implemented, but to improve the probability of this happening you have to explain what is happening and why. You shouldn’t spam users at startup with authorisation requests for every permission your app might ever need. Apple provide a bunch of documentation about the various permissions.

Associating permissions with the features that require them instantly solves the “don’t spam the user with requests” part. If you only prompt for permissions when the feature is used, and only the permissions that feature needs, they already have some idea of what might be required, e.g. a mapping feature requires location access. They are primed to allow the permissions.

To declare a permission constraint, you use the permission() or permissions(...) functions:

public class SelfieFeature: ConditionalFeature {
    public static var description: String = "Selfie Posting"

    public static func constraints(requirements: FeatureConstraintsBuilder) {
                                 .location(usage: .whenInUse))


The permissions you pass to this function are any values from the SystemPermission enum. The kinds of permissions currently supported are:

  • .photos — Photos access
  • .camera — Camera access
  • .location(usage:) — Location tracking
  • .contacts(entity:) — Access to Contacts
  • .calendarEvents — EventKit access to calendar events
  • .reminders — EventKit access to reminders
  • .motion — Access to CoreMotion motion and fitness data
  • .speechRecognition — Speech Recognition
  • .siriKit — Permission to integrate SiriKit extensions
  • .blueteooth — Access to Core Bluetooth peripherals
  • .mediaLibrary — Access to the user’s media library and Apple Music

Once you add multiple permissions into the mix, and some features require different combinations of those permissions, things get complicated quickly. Using conditional features with permission constraints solves this problem in a way that makes it easy for you to provide the best experience for the user, using Flint’s tools for requesting the permissions. It also makes it very clear what your app’s permission behaviours should be for testing and QA of the various features in all permutations with and without the permissions granted.

Permissions support hides all the details of verifying the different permissions as well as how you request authorisation, and aids with decoupling as your call sites do not need to import the frameworks that provide permission authorisations.

We’ll see how to request permission authorisations shortly, and after that we’ll see how to handle the case where there are missing permissions.

Defining platform restrictions

The constraints DSL also provides convenient ways to specify per-platform minimum version requirements.

Using the .iOS, .watchOS, .tvOS, macOS properties on the builder you can set a minimum version required for this feature to be available. You can alternatively set it to .any, or .unsupported if you want to prevent a feature on a certain platform where it doesn’t make sense. The default for all platforms is .any.

There is also support for limiting a feature to a single platform by assigning a value to .iOSOnly, .watchOSOnly, .macOSOnly or .tvOSOnly. Assigning a version to any of these will set all the other platforms to .unsupported automatically.

Note that you can set version constraints either as an Int like “11”, or a String like "10.13.4":

public class ExampleFeature: ConditionalFeature {
    public static func constraints(requirements: FeatureConstraintsBuilder) {
    	requirements.iOS = 9
    	requirements.macOS = "10.12.1"
    	requirements.tvOS = .any
    	requirements.watchOS = .unsupported


public class ExampleiOSOnlyFeature: ConditionalFeature {
    public static func constraints(requirements: FeatureConstraintsBuilder) {
    	// Availble on any iOS version, and nothing else
    	requirements.iOSOnly = .any


This mechanism makes it easier to have reusable code cross-platform, while not allowing your code to run actions that should not be available.

Performing actions of a conditional feature

To actually perform an action of a conditional feature you must be sure the feature is available. Flint uses Swift’s type system to enforce this: there is no way to perform an action of a conditional feature without first checking availability, unlike normal features where you just call perform.

Instead you must first obtain a VerifiedActionBinding by calling request() on the action binding, and if you get a result back you can then call perform on that:

if let request = DeepLinkingFeature.performIncomingURL.request() {
    request.perform(withInput: url, presenter: presenter)
} else {
    // This often means there was a programmer error - 
    // your UI probably should not allow people to invoke actions from
    // features that are not enabled. A common exception is the case of permissions
    // and purchases where you want to let the user choose the action but then show
    // them UI explaining what they have to do to be able to use it.
    log.error("Attempt to use feature that is not enabled")

This type safe mechanism deliberately makes it painful to ignore the situations where a feature may not be available, and prevents confusing an always-available feature with one that isn’t. Your code that performs actions of conditional features always needs to be prepared to do something reasonable in case the feature is not available.

This is a far better than testing boolean feature flags through your code, where these can be easily missed, leading to undefined behaviours where some interactions are not gated on the feature flag by accident. Because the “glue code” for your actions lives in actions that can only be executed through the feature mechanism, you are protected from this.

This is a fundamental tenet of Flint: all your feature-related code should be triggered by action invocations, and the action invocations are protected by type safety so you cannot make these kinds of mistakes.

What to do when your conditional feature indicates that it is not available

If you do not get a request instance back when you want to perform an action of a conditional feature, you need to find out why and help the user do something about it if this is possible.

There are many reasons why a feature may not be available, and Flint provides the information you need to deal with this. Some of the conditions you can help the user deal with — permissions and purchases for example — whereas others are programmer error such as a feature being disabled programmatically but the UI elements not being hidden or disabled.

One of the most common challenges is prompting the user to authorise permissions that are required. The various APIs of iOS, tvOS and watchOS that require permissions have subtle differences in their authorisation requirements, but Flint wraps all these up into a common interface. It also provides a controller and coordinator mechanism so that you don’t have to do anything about authorisation except provide any onboarding UI that you want to include to help the user understand what is happening.

Prompting for required permissions

When the feature request() call comes back with a nil, you’ll need to see if there are any permissions that the user has not yet been asked to authorise. You do this using the permissions.notDetermined property on your feature type (see FeaturePermissionRequirements. Then you can use Flint’s controller mechanism to request all the required authorisations.

if PhotoAttachmentsFeature.permissions.notDetermined.count > 0 {
    permissionController = PhotoAttachmentsFeature.permissionAuthorisationController(using: self)
    permissionController?.begin(retryHandler: { self.doAttachPhoto() })

This example does the following:

  1. Checks whether there are any permissions that the feature requires that have the .notDetermined status. This status indicates that the system has not yet prompted the user for a given permission
  2. If the number of permissions not determined is non-zero, it calls Flint’s permissionAuthorisationController(using:) function to get a controller instance that you can use to request all the outstanding authorisations, one by one
  3. It starts the controller and passes a retryHandler that will get called at the end of the authorisation flow if the user approves all the required permissions, so that the user does not have to select the UI option again and your app can automatically resume what the user wanted to do originally

The permission controller takes a single optional argument of type PermissionAuthorisationCoordinator, which you can use to affect the authorisation process. You can pass nil for the coordinator and the user will immediately see the system permission requests one by one without any hints about what is happening, other than the usage description you provide in your Info.plist.

That’s fine during development but we don’t recommend you do this in real apps. Use the coordinator to add some onboarding cards along the lines of:

“Hey, we’re going to need permission to access the camera because, well, you’re going to be taking a photo!”

You would probably want to include buttons for “OK”, “Ask me later” and “Cancel” so they can get out of the flow.

The coordinator receives callbacks at every stage in the cycle of authorisations and can do the following:

  • Show UI at the start and end of the entire batch of required authorisations, including information about all the permissions required so you can adapt your content to this.
  • Show some UI before and after the controller shows the system permission alert, so that the alert is not shown until the user has seen your UI and indicated they are ready to proceed.
  • Set the order in which permissions will be requested — perhaps your onboarding wants to get the most important permissions requested first to avoid “prompt fatigue”, or at least you want the ordering to be deterministic for automated testing.
  • Skip authorising a specific permission - e.g. if your onboarding UI has an “Ask me later” button you can instruct the controller to skip a single permission.
  • Cancel all the authorisations.

These are all very important for avoiding problems where users, through fatigue or confusion, deny one or more of the permissions when the system alert is displayed.

Remember that once permissions are denied you cannot prompt for them again in-app and at best you are forced to show UI that tells the user to open the “Settings” app and find the switch to enable the permissions for your app. In many cases if you end up in that situation you have “lost” the user. Flint also helps deal with these situations.

For more details see the protocol definition for PermissionAuthorisationCoordinator.

Dealing with the wider range of permission and purchases issues

Whether your features require permissions, in-app purchases, or both, things get more complicated than you may at first think when you want to ensure a good user experience.

In terms of permissions, there are several authorisation statuses but only .authorized grants access:

  • .notDetermined
  • .denied
  • .restricted
  • .unsupported
  • .authorized

The .notDetermined status we have already mentioned indicates that the system has not yet asked the user whether or not your app can be granted the requested permissions. For permissions with this status you can request authorisation.

The status .denied indicates that the user was previously asked for permission but denied it, and the app cannot recover from this or request permission again. So you need to show UI when you find some required permissions are denied, to tell them to go to the Settings app to enable them. Flint provides access to the denied permissions that your feature requires in the permissions.denied property on the feature. You can check the number of permissions in that set to work out if you need to tell the user about the denied permissions.

Another status that requires special handling is .restricted. This indicates that for some reason the permission is simply not available and cannot be granted. Usually this means there are parental restrictions or a custom mobile device management profile installed that has denied access to the requested resource. For example some corporate managed devices may not permit camera or location access.

Your app can’t do anything about this situation, and often the user cannot solve it either. However you need to show UI to explain to them why they can’t use the feature that requires the permission. Flint also provides access to these restricted permissions that your feature requires in the permissions.restricted property on the feature.

Beyond permissions, with in-app purchases you can easily ask Flint whether there are purchases required to unlock the feature, using the purchases.requiredToUnlock property on the feature:

if feature.purchases.requiredToUnlock.count > 0 {
    // Show your in-app store onboarding UI

This is straightforward enough. However when this is combined with the possibility of missing permissions, care should be taken to handle the user interactions in a sensible way.

One example is that the user may not have granted permission to access the camera as well as not having purchased the in-app purchase required for the feature. In this case, you probably don’t want to prompt for camera permission unless they have actually purchased the feature first.

What’s more, if any of the permissions required are returning “restricted” you should not let them purchase the feature, but you need to tell them why.

Whatever the situation, you should usually only ask for one thing at a time to avoid confusing the user. Here’s an example of how you might approach this, taken from the “photo attachments” feature of the FlintDemo-iOS project:

func selectPhoto() {
    if let request = PhotoAttachmentsFeature.request(PhotoAttachmentsFeature.showPhotoSelection) {
        request.perform(withPresenter: self)
    } else {
        handleUnsatisfiedConstraints(for: PhotoAttachmentsFeature.self, retry: { [weak self] in self?.selectPhoto() })

func handleUnsatisfiedConstraints<T>(for feature: T.Type, retry retryHandler: (() -> Void)?) where T: ConditionalFeature {
    // Check for required permissions that are restricted on this device through parental controls or a profile.
    // We must ask for these first in case the user purchases a feature they cannot use
    if feature.permissions.restricted.count > 0 {
        let permissions ={ String(describing: $0) }).joined(separator: ", ")
        showRestrictedPermissionsWarning(for: permissions)
    // Check for required purchases next, only if there are no permissions that are restricted
    if feature.purchases.requiredToUnlock.count > 0 {
        showInAppPurchaseUpsell(for: feature.purchases.requiredToUnlock)

    // Check for required permissions that are already denied
    if feature.permissions.denied.count > 0 {
        let permissions ={ String(describing: $0) }).joined(separator: ", ")
        showDeniedPermissionsInstructions(for: feature.permissions.denied)

    // Start the flow of requesting authorisation for any permissions not determined
    if feature.permissions.notDetermined.count > 0 {
        permissionController = feature.permissionAuthorisationController(using: self)
        permissionController?.begin(retryHandler: retryHandler)

This example calls functions you supply to tell the user what is wrong with the permissions or upsell them on purchases. In a real app you would usually handle the permissions part with non-modal UI provided by the coordinator passed to feature.permissionAuthorisationController.

Next steps

  • Add Routes to Actions
  • Add Activities support to some Actions
  • Add Analytics tracking
  • Use the Timeline to see what is going on in your app when things go wrong
  • Start using Focus to pare down your logging