Top 30 Combine Framework Interview Questions & Answers 2023

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Welcome to our blog post on the top 30 Combine framework interview questions and answers for 2023. As the iOS development community continues to evolve, so too do the tools and frameworks that developers use to build their apps. One such framework that has gained popularity in recent years is Combine. In this post, we will provide a comprehensive list of the most frequently asked questions on the Combine framework, along with their corresponding answers. Whether you’re a seasoned iOS developer or just starting out, this post is a great resource to help you prepare for your next interview and master the ins and outs of the Combine framework. So, let’s dive in!

What is the Combine framework?

The Combine framework is a reactive programming framework introduced by Apple in WWDC 2019. It enables developers to manage and respond to asynchronous events and data streams in a declarative and functional way.

What are the main components of the Combine framework?

The main components of the Combine framework are Publishers, Subscribers, and Operators. Publishers emit values, Subscribers receive and process those values, and Operators transform the values emitted by Publishers.

How does Combine differ from ReactiveCocoa or RxSwift?

Combine is a native framework for iOS, whereas ReactiveCocoa and RxSwift are third-party libraries. Combine also has a simpler API and a more Swift-like syntax, making it easier for developers to learn and use.

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How do you create a publisher in Combine?

To create a publisher in Combine, you can use the Publishers factory class or create your own custom publisher by conforming to the Publisher protocol.

How do you create a subscriber in Combine?

To create a subscriber in Combine, you can use the Subscribers factory class or create your own custom subscriber by conforming to the Subscriber protocol.

What are the two ways to cancel a subscription in Combine?

The two ways to cancel a subscription in Combine are to call the cancel() method on the subscriber or to call the subscription.cancel() method on the returned Subscription object.

What is a subject in Combine?

A subject in Combine is a special kind of publisher that can also receive values and pass them on to its subscribers.

How can you merge two publishers in Combine?

To merge two publishers in Combine, you can use the merge() operator.

How can you filter the values emitted by a publisher in Combine?

To filter the values emitted by a publisher in Combine, you can use the filter() operator.

How can you create a custom publisher in Combine?

To create a custom publisher in Combine, you need to conform to the Publisher protocol and implement the required methods. Here is an example of a custom publisher that emits the current time every second:

class TimerPublisher: Publisher {
    typealias Output = Date
    typealias Failure = Never

    func receive<S>(subscriber: S) where S : Subscriber, TimerPublisher.Failure == S.Failure, TimerPublisher.Output == S.Input {
        let subscription = TimerSubscription(subscriber: subscriber)
        subscriber.receive(subscription: subscription)
    }
}

private class TimerSubscription<S: Subscriber>: Subscription where S.Input == Date {
    private var subscriber: S?
    private var timer: Timer?

    init(subscriber: S) {
        self.subscriber = subscriber
        timer = Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { [weak self] _ in
            self?.subscriber?.receive(Date())
        }
    }

    func request(_ demand: Subscribers.Demand) {
        // no-op
    }

    func cancel() {
        subscriber = nil
        timer?.invalidate()
        timer = nil
    }
}

How can you handle errors in Combine?

To handle errors in Combine, you can use the catch() operator, which allows you to specify a closure that will be called when an error occurs. You can also use tryCatch operator to handle errors in a more elegant way.

How can you use the reduce operator in Combine?

The reduce operator allows you to accumulate the values emitted by a publisher and return the final result. Here is an example of using the reduce operator to find the sum of an array of integers:

let numbers = [1, 2, 3, 4, 5]
let publisher = numbers.publisher

publisher
    .reduce(0, +)
    .sink(receiveValue: { sum in
        print("The sum is: \(sum)")
    })
    .store(in: &cancellables)

How can you use the scan operator in Combine?

The scan operator is similar to reduce, but it emits the intermediate results as well as the final result. Here is an example of using the scan operator to find the running total of an array of integers:

let numbers = [1, 2, 3, 4, 5]
let publisher = numbers.publisher

publisher
    .scan(0, +)
    .sink(receiveValue: { total in
        print("The running total is: \(total)")
    })
    .store(in: &cancellables)

How can you use the debounce operator in Combine?

The debounce operator allows you to delay the emission of values by a certain amount of time. Here is an example of using the debounce operator to delay the emission of button presses by 0.5 seconds:

let button = UIButton()
let publisher = button.publisher(for: .touchUpInside)

publisher
    .debounce(for: .milliseconds(500), scheduler: DispatchQueue.main)
    .sink(receiveValue: { _ in
        print("Button pressed")
    })
    .store(in: &

Which frameworks do you like most apart from Combine?

iOS interview Prep 2023: Mastering Frameworks

How can you use the map operator in Combine?

The map operator allows you to transform the values emitted by a publisher. Here is an example of using the map operator to square the values emitted by an array of integers:

let numbers = [1, 2, 3, 4, 5]
let publisher = numbers.publisher

publisher
    .map { $0 * $0 }
    .sink(receiveValue: { value in
        print("The squared value is: \(value)")
    })
    .store(in: &cancellables)

How can you use the flatMap operator in Combine?

The flatMap operator allows you to transform the values emitted by a publisher and then flatten the resulting publishers into a single publisher. Here is an example of using the flatMap operator to fetch data from an API:

let publisher = URLSession.shared.dataTaskPublisher(for: url)
    .map { $0.data }
    .decode(type: User.self, decoder: JSONDecoder())
    .flatMap { user in
        user.fetchAvatar()
    }
    .sink(receiveCompletion: { completion in
        switch completion {
        case .failure(let error):
            print(error)
        case .finished:
            break
        }
    }, receiveValue: { avatar in
        // update UI with avatar
    })

How can you use the compactMap operator in Combine?

The compactMap operator allows you to transform the values emitted by a publisher and filter out nil values. Here is an example of using the compactMap operator to filter out invalid email addresses:

let publisher = emailTextField.publisher(for: .editingChanged)
    .map { $0.text }
    .compactMap { email in
        guard email.isValidEmail() else { return nil }
        return email
    }
    .sink(receiveValue: { email in
        print("Valid email: \(email)")
    })
    .store(in: &cancellables)

How can you use the zip operator in Combine?

The zip operator allows you to combine the values emitted by two or more publishers. Here is an example of using the zip operator to combine the name and age of a user:

let namePublisher = user.namePublisher
let agePublisher = user.agePublisher

Publishers.Zip(namePublisher, agePublisher)
    .sink(receiveValue: { name, age in
        print("Name: \(name), Age: \(age)")
    })
    .store(in: &cancellables)

How can you use the throttle operator in Combine?

The throttle operator allows you to limit the frequency of the values emitted by a publisher. Here is an example of using the throttle operator to limit the rate of API calls:

let publisher = searchTextField.publisher(for: .editingChanged)
    .map { $0.text }
    .throttle(for: .seconds(1), scheduler: DispatchQueue.main, latest: true)

How can you chain multiple operators together in Combine?

To chain multiple operators together in Combine, you can use the . operator to chain the operators together, like this: publisher.operator1().operator2().operator3().... This way, the output of the previous operator is passed as the input to the next operator.

What is the difference between sink and assign?

sink is used to subscribe to a publisher and receive its emitted values, while assign is used to bind the value emitted by a publisher to a property on an object. sink also allows you to handle completion and error events while assign does not.

What is a Cancellable in Combine?

A Cancellable is an object that represents an active subscription to a publisher. It allows you to cancel the subscription by calling its cancel() method.

How does Combine differ from other reactive programming frameworks?

Combine is a native framework for iOS and macOS, whereas other reactive programming frameworks such as RxSwift and ReactiveCocoa are third-party libraries. Combine has a simpler API and a more Swift-like syntax, making it easier for developers to learn and use.

How can you use the filter operator in Combine?

The filter operator allows you to filter out values emitted by a publisher based on a certain condition. Here is an example of using the filter operator to only receive even numbers:

let numbers = [1, 2, 3, 4, 5]
let publisher = numbers.publisher

publisher
    .filter { $0 % 2 == 0 }
    .sink(receiveValue: { value in
        print("Even number: \(value)")
    })
    .store(in: &cancellables)

How can you use the merge operator in Combine?

The merge operator allows you to merge multiple publishers into a single publisher. Here is an example of using the merge operator to merge two publishers that emit different types of events:

let publisher1 = NotificationCenter.default.publisher(for: .event1)
let publisher2 = NotificationCenter.default.publisher(for: .event2)

Publishers.Merge(publisher1, publisher2)
    .sink(receiveValue: { event in
        switch event {
        case .event1(let value):
            print("Event 1: \(value)")
        case .event2(let value):
            print("Event 2: \(value)")
        }
    })
    .store(in: &cancellables)

How can you use the combineLatest operator in Combine?

The combineLatest operator allows you to combine the latest values emitted by multiple publishers. Here is an example of using the combineLatest operator to combine the name and age of a user:

let namePublisher = user.namePublisher
let agePublisher = user.agePublisher

Publishers.CombineLatest(namePublisher, agePublisher)
    .sink(receiveValue: { name, age in
        print("Name: \(name), Age: \(age)")
    })
    .store(in: &cancellables)

How can you use the buffer operator in Combine?

The buffer operator allows you to buffer the values emitted by a publisher and emit them as an array. Here is an example of using the buffer operator to buffer the values emitted by a publisher and emit them every 3 seconds:

let publisher = somePublisher

publisher
    .buffer(timeSpan: 3, scheduler: DispatchQueue.main)
    .sink(receiveValue: { values in
        print("Buffered values: \(values)")
    })
    .store(in: &cancellables)

How can you use the distinctUntilChanged operator in Combine?

The distinctUntilChanged operator allows you to only receive unique values emitted by a publisher. Here is an example of using the distinctUntilChanged operator to only receive unique values emitted by a publisher:

let publisher = somePublisher

publisher
    .distinctUntilChanged()
    .sink(receiveValue: { value in
        print("Unique value: \(value)")
    })
    .store(in: &cancellables)

Fellow iOS Developers, Please Keep In Mind

  • It’s important to keep in mind a few key points as you prepare for your interview. Firstly, it’s worth noting that there are over 1000 interview questions available in the interview section for you to review and prepare for. While reading the question, take the time to carefully consider your answer and think about the information that you want to convey. The answer provided here in this blog can be explained in a different way. You should also prepare your examples.
  • It’s also important to remember that these interview questions are not meant to be difficult. The interviewer is not looking to challenge you, but rather to start a conversation that will allow your abilities and interests to come to the forefront. They want to get to know you and your experience better.
  • Finally, it’s crucial to avoid simply answering questions with a “yes” or “no.” Interviewers are looking for more in-depth responses that include basic understanding, reasoning, explanation, and examples. So, make an effort to elaborate on your answers and provide specific, relevant information to support your response. This will demonstrate your thoughtfulness and show the interviewer that you are well-prepared for the interview.

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Written by

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