Swift Talking is a blog updated every two weeks about Swift, tooling, Continuous Integration and other stuff written by Alex Salom.

A/B Test: Randomly pick an item from a weighted list of items

Sometimes we are considering two approaches to solve one problem and we are unsure which one to choose. We could run an experiment using prototypes and test users but sometimes we want to do this in a production App and with real users so we can later mesure the impact of one option over the others.

Imagine we have an app with three different onboardings. All onboardings are visually different but they all lead to a screen where the user is prompted to buy a subscription. We would like to know which one of those onboardings will lead to more purchases so we can discard the others.

Instead of user testing prototypes, we could know which onboarding leads to more purchases by using A/B test with different onboardings. A/B testing is a way to compare two or more versions of a feature, typically by measuring a user’s response to feature A against feature B, or C, etc. and determining which of the of them is more effective. In other words, we could design 3 different onboarding experiences and show each one of them randomly to different users. We can choose to randomize the selection equaly so each variant has the same chances of showing up or we could use a weighted list where one onboarding will have more chances to appear than the others.

For all this to have any value we would need to have some short of tracking in place that later we can refer to and see how each variant performed. We won’t cover in this post the tracking part as this is always dependent on other factors, such as product or marketing tooling decicions. However the idea is to show one variant, measure its performance and then track the result.

In our example we want to show different onboardings to different users, measure how many users buy a subscription after a given onboarding vs how many users were presented this onboarding and finally track this results somewhere.

Sometime ago we had to implement exactly this, where we had to track the performance of different onboardings and later choose the best one. We had three options and we weren’t sure if one of the them would be too risky for our users and since this could potentially affect our sales, we decided to introduce the risky onboarding to a limited amount of users. For this we implemented the following function.

class ABTester<T> {
  typealias WeightedItem = (item: () -> T, weight: Int)

  /** Randomly chooses a weighted item from a list of items
   - item: a weighted item
   - more: more weighted items as a variadic parameter
   - randomNumber: random number generator. This is injected so we can write tests in the function.
   - returns: the winner weigthed item
   - discussion: We force the first item to be provided so we ensure that we don't end up with an empty list of items
  func ABTest(_ item: WeightedItem, _ more: WeightedItem ..., randomNumber: RandomNumber = RandomNumberImpl()) -> T {
    return [[item], more].flatMap { $0 }.sorted { (lhs: WeightedItem, rhs: WeightedItem) in
      randomNumber.generateBetween(min: 1, max: lhs.weight) >
      randomNumber.generateBetween(min: 1, max: rhs.weight)

Reading through the inline comments in the code snippet above will help you understand what each parameter represents. A WeightedItem takes an item closure which represents a variant and a weight which represents its chances to win. The heigher the weight with respect to the other items, the bigger the chances it will be chosen. The reason a WightedItem takes a closure as an item is so we only initialize the winner since we might have items which initialization is expensive. We could hide the closure from the users of this class by applying an @autoclosure annotation to the item closure but unfortunately we can’t use them yet on tuples or typealiases.

Finally we inject a RandomNumber object in order to allow unit testing this function. RandomNumber’s implementation is pretty trivial.

protocol RandomNumber {
  func generateBetween(min minValue: Int, max maxValue: Int) -> Int

class RandomNumberImpl: RandomNumber {
  func generateBetween(min minValue: Int, max maxValue: Int) -> Int {
    return minValue + Int(arc4random_uniform(UInt32(maxValue - minValue + 1)))

With all this in place, how to use it? To continue with our onboarding example, we need to initialize an ABTester instance of generic type Onboarding and call the ABTest function with a weighted list of onboardings adjusting their weight depending which onboarding we want to show more often over the others.

let abTester = ABTester<Onboarding>()
let onboarding = abTester.ABTest(
  (item: { OnboardingA() }, weight: 45),
  (item: { OnboardingB() }, weight: 45),
  (item: { OnboardingC() }, weight: 10)


In this example there will be a 45% change to show either A or B variants while only a 10% chance to show the C variant which can be a little bit risky for us.

Alex Salom

Hi! My name is Alex Salom and I’m an iOS Engineer. In this site I’ll share with you tricks & tips related to iOS and also to everything that happens around it: tooling, Continuous Integration and Continuous Delivery. You can find me as well in Twitter, GitHub and LinkedIn.