JustTweak 7.0.0

JustTweak 7.0.0

LangLanguage SwiftSwift
License Apache-2.0
ReleasedLast Release Jul 2021
SPMSupports SPM

Maintained by Alberto De Bortoli, Dimitar Chakarov, Mark Cousins, Alberto De Bortoli.

JustTweak 7.0.0

  • By
  • Gianluca Tranchedone, Alberto De Bortoli, Andrew Steven Grant and Dimitar Chakarov

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JustTweak is a framework for feature flagging and A/B testing for iOS apps. It provides a simple facade interface interacting with multiple 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 local configuration providing default values for experimentation
  • use a number of remote configuration 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"



  • define a JSON configuration file including your features (you can use the included ExampleConfiguration.json as a template)
  • define your features and A/B tests in your services such as Firebase and Optmizely (optional)
  • configure the JustTweak stack as following
static let tweakManager: TweakManager = {
    // mutable configuration (to override tweaks from other configurations)
    let userDefaultsConfiguration = UserDefaultsConfiguration(userDefaults: UserDefaults.standard)

    // remote configurations (optional)
    let optimizelyConfiguration = OptimizelyConfiguration()
    optimizelyConfiguration.userId = <#user_id#>
    let firebaseConfiguration = FirebaseConfiguration()

    // local JSON configuration (default tweaks)
    let jsonFileURL = Bundle.main.url(forResource: "ExampleConfiguration", withExtension: "json")!
    let localConfiguration = LocalConfiguration(jsonURL: jsonFileURL)

    // priority is defined by the order in the configurations array (from highest to lowest)
    let configurations: [Configuration] = [userDefaultsConfiguration,
    return TweakManager(configurations: configurations)

The order of the objects in the configurations array defines the priority of the configurations.

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


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 configuration with the highest priority and automatically fallback to the others if no set value is found.

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

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

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

@TweakProperty(fallbackValue: <#fallback_value#>,
               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?
  1. Run an A/B test
// check for a tweak value
let variation = tweakManager.activeVariation(for: "some_experiment")
if let variation = variation {
    // act according to the kind of variation (e.g. "control", "variation_1")
} else {
    // default behaviour


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 TweakConfigurationDidChangeNotification notification

Update a configuration at runtime

JustTweak comes with a ViewController that allows the user to edit the MutableConfiguration 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 MutableConfiguration, 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: TweakConfigurationDidChangeNotification,
                                                   object: nil)

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


JustTweak comes with three configurations out-of-the-box:

  • UserDefaultsConfiguration which is mutable and uses UserDefaults as a key/value store
  • LocalConfiguration which is read-only and uses a JSON configuration file that is meant to be the default configuration
  • EphemeralConfiguration which is simply an instance of NSMutableDictionary

In addition, JustTweak defines Configuration and MutableConfiguration protocols you can implement to create your own configurations to fit your needs. In the example project you can find a few example configurations which you can use as a starting point.


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

  • Just Eat iOS team