JustTweak 10.1.1

JustTweak 10.1.1

LangLanguage SwiftSwift
License Apache-2.0
ReleasedLast Release Dec 2022
SPMSupports SPM

Maintained by Alberto De Bortoli, Dimi Chakarov, Mark Cousins, Just Eat Takeaway.com iOS Team.

JustTweak 10.1.1

  • By
  • Just Eat Takeaway iOS Team

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JustTweak is a feature flagging framework for iOS apps. It provides a simple facade interface interacting with multiple tweak providers that are queried respecting a given priority. Tweaks represent flags used to drive decisions in the client code.

With JustTweak you can achieve the following:

  • use a JSON file to provide the default values for feature flagging
  • use a number of remote tweak providers such as Firebase and Optmizely to run A/B tests and feature flagging
  • enable, disable, and customize features locally at runtime
  • provide a dedicated UI for customization (this comes particularly handy for feature that are under development to showcase it to stakeholders)



JustTweak is available through CocoaPods. To install it, simply add the following line to your Podfile:

pod "JustTweak"

Swift Package Manager

JustTweak is also available through SPM. Copy the URL for this repo, and add the package in your project settings.



  • Define a LocalTweakProvider JSON file including your features. Refer to LocalTweaks_example.json for a starting point.
  • Configure the stack

To configure the stack, you have two options:

  • implement the stack manually
  • leverage the code generator tool

Manual integration

  • Configure the JustTweak stack as following:
static let tweakManager: TweakManager = {
    var tweakProviders: [TweakProvider] = []

    // Mutable TweakProvider (to override tweaks from other TweakProviders)
    let userDefaultsTweakProvider = UserDefaultsTweakProvider(userDefaults: UserDefaults.standard)
    // Optimizely (remote TweakProvider)
    let optimizelyTweakProvider = OptimizelyTweakProvider()
    optimizelyTweakProvider.userId = UUID().uuidString

    // Firebase Remote Config (remote TweakProvider)
    let firebaseTweakProvider = FirebaseTweakProvider()

    // Local JSON-based TweakProvider (default TweakProvider)
    let jsonFileURL = Bundle.main.url(forResource: "LocalTweaks_example", withExtension: "json")!
    let localTweakProvider = LocalTweakProvider(jsonURL: jsonFileURL)
    return TweakManager(tweakProviders: tweakProviders)
  • Implement the properties and constants for your features, backed by the LocalTweakProvider. Refer to TweakAccessor.swift for a starting point.

Using the code generator tool

  • Define the stack configuration in a config.json file in the following format:
    "accessorName": "GeneratedTweakAccessor"

the only currently supported value is accessorName that defines the name of the generated class.

  • Add the following to your Podfile
script_phase :name => 'TweakAccessorGenerator',
             :script => '$SRCROOT/../TweakAccessorGenerator \
             -l $SRCROOT/<path_to_the_local_tweaks_json_file> \
             -o $SRCROOT/<path_to_the_output_folder_for_the_generated_code> \
             -c $SRCROOT/<path_to_the_folder_containing_config.json>',
             :execution_position => :before_compile

Every time the target is built, the code generator tool will regenerate the code for the stack. It will include all the properties backing the features defined in the LocalTweakProvider.

  • Add the generated files to you project and start using the stack.



If you have used the code generator tool, the generated stack includes all the feature flags. Simply allocate the accessor object (which name you have defined in the .json configuration and use it to access the feature flags.

let accessor = GeneratedTweakAccessor(with: <#tweak_manager_instance#>)
if accessor.meaningOfLife == 42 {

See GeneratedTweakAccessor.swift and GeneratedTweakAccessor+Constants.swift for an example of generated code.


If you decided to implement the stack code yourself, you'll have to implemented code for accessing the features via the TweakManager.

The three main features of JustTweak can be accessed from the TweakManager instance to drive code path decisions.

  1. Checking if a feature is enabled
// check for a feature to be enabled
let enabled = tweakManager.isFeatureEnabled("some_feature")
if enabled {
    // enable the feature
} else {
    // default behaviour
  1. Get the value of a flag for a given feature. TweakManager will return the value from the tweak provider with the highest priority and automatically fallback to the others if no set value is found. It throws exeception when a nil Tweak is found which can be catched and handled as needed.

Use either tweakWith(feature:variable:) or the provided property wrappers.

// check for a tweak value
let tweak = try? tweakManager.tweakWith(feature: "some_feature", variable: "some_flag")
if let tweak = tweak {
    // tweak was found in some tweak provider, use tweak.value
} else {
    // tweak was not found in any tweak provider

Or with do-catch

// check for a tweak value
do {
    let tweak = try tweakManager.tweakWith(feature: "some_feature", variable: "some_flag")
    // tweak was found in some tweak provider, use tweak.value
    return tweak    
} catch let error as TweakError {
    switch error {
    case .notFound: () // "Feature or variable is not found"
    case .notSupported: () // "Variable type is not supported"
    case .decryptionClosureNotProvided: () // "Value is encrypted but there's no decryption closure provided"
} catch let error { // add a default catch to satisfy the compiler

@TweakProperty, @OptionalTweakProperty and @FallbackTweakProperty property wrappers are available to mark properties representing feature flags. Mind that in order to use these property wrappers, a static instance of TweakManager is needed.

@TweakProperty(feature: <#feature_key#>,
               variable: <#variable_key#>,
               tweakManager: <#TweakManager#>)
var labelText: String
@OptionalTweakProperty(fallbackValue: <#nillable_fallback_value#>,
                       feature: <#feature_key#>,
                       variable: <#variable_key#>,
                       tweakManager: <#TweakManager#>)
var meaningOfLife: Int?
@FallbackTweakProperty(fallbackValue: <#nillable_fallback_value#>,
                       feature: <#feature_key#>,
                       variable: <#variable_key#>,
                       tweakManager: <#TweakManager#>)
var shouldShowFeatureX: Bool

Tweak Providers priority

The order of the objects in the tweakProviders array defines the priority of the tweak providers.

The MutableTweakProvider with the highest priority, such as UserDefaultsTweakProvider in the example above, will be used to reflect the changes made in the UI (TweakViewController). The LocalTweakProvider should have the lowest priority as it provides the default values from a local tweak provider and it's the one used by the TweakViewController to populate the UI.

Migration notes

In order to migrate from manual to the code generated implementation, it is necessary to update to the new .json format. To aid with this process we have added the GeneratedPropertyName property to the tweak object. Set this value to align with your current property names in code, so that the generated accessor properties match your existing implementation.

Caching notes

The TweakManager provides the option to cache the tweak values in order to improve performance. Caching is disabled by default but can be enabled via the useCache property. When enabled, there are two ways to reset the cache:

  • call the resetCache method on the TweakManager
  • post a TweakProviderDidChangeNotification notification

Update a mutable tweak provider at runtime

JustTweak comes with a ViewController that allows the user to edit the MutableTweakProvider with the highest priority.

func presentTweakViewController() {
    let tweakViewController = TweakViewController(style: .grouped, tweakManager: <#TweakManager#>)

    // either present it modally
    let tweaksNavigationController = UINavigationController(rootViewController:tweakViewController)
    tweaksNavigationController.navigationBar.prefersLargeTitles = true
    present(tweaksNavigationController, animated: true, completion: nil)

    // or push it on an existing UINavigationController
    navigationController?.pushViewController(tweakViewController, animated: true)

When a value is modified in any MutableTweakProvider, a notification is fired to give the clients the opportunity to react and reflect changes in the UI.

override func viewDidLoad() {
                                                   selector: #selector(updateUI),
                                                   name: TweakProviderDidChangeNotification,
                                                   object: nil)

@objc func updateUI() {
    // update the UI accordingly


JustTweak comes with three tweak providers out-of-the-box:

  • UserDefaultsTweakProvider which is mutable and uses UserDefaults as a key/value store
  • LocalTweakProvider which is read-only and uses a JSON file that is meant to hold the default feature flagging setup
  • EphemeralTweakProvider which is simply an instance of NSMutableDictionary

In addition, JustTweak defines TweakProvider and MutableTweakProvider protocols you can implement to create your own tweak provider to fit your needs. In the example project you can find some examples which you can use as a starting point.

Encryption or pre-processing (Advanced)

JustTweak offers the ability to add a decryptionClosure to a TweakProvider. This closure takes the Tweak as input and returns a TweakValue as output. The closure allows you to do some preprocessing on your tweak which can e.g. be used to decrypt values. This can be used if you have an encrypted value in your tweaks JSON file as can be seen below:

"encrypted_answer_to_the_universe": {
  "Title": "Encrypted definitive answer",
  "Description": "Encrypted answer to the Ultimate Question of Life, the Universe, and Everything",
  "Group": "General",
  "Value": "24 ton yletinifeD",
  "GeneratedPropertyName": "definitiveAnswerEncrypted",
  "Encrypted": true

Note that you have to specify if the value is encrypted in your JSON file (with the Encrypted property) for the decryption closure to process the value. The decryption closure for the JSON above can be specified as follows:

tweakProvider.decryptionClosure = { tweak in
    // decrypt `tweak.value` with your cypher of choice and return the decrypted value

In this way, the tweak fetched from the tweak provider will have the decrypted value.


JustTweak is available under the Apache 2.0 license. See the LICENSE file for more info.

  • Just Eat iOS team