In Swift, asynchronous programming is essential for developing high-quality applications. Asynchronous programming allows us to write code that executes concurrently, thereby avoiding blocking the user interface (UI) thread. One of the most popular frameworks for handling asynchronous events and data streams in Swift is the Combine framework.

Combine is a declarative framework that provides a unified and powerful way to work with asynchronous data streams. It is available starting from iOS 13, macOS 10.15, watchOS 6, and tvOS 13. Combine introduces a functional programming paradigm to handle asynchronous events and data streams in Swift, allowing developers to write clean, composable, and easy-to-understand code.

In this article, we’ll cover the essential aspects of using Combine for asynchronous event and data stream handling in Swift. We’ll start by explaining the basic building blocks of Combine, publishers and subscribers. We’ll then move on to combining publishers, transforming and filtering publishers, controlling the flow of data, and handling errors in Combine. By the end of the article, you’ll have a solid understanding of how to leverage Combine for asynchronous event and data stream handling in your Swift applications.

Getting Started with Combine:

Let’s start with the basics. Combine is a framework that provides two fundamental building blocks to handle asynchronous events and data streams: publishers and subscribers. A publisher is an object that emits a sequence of values over time, while a subscriber is an object that receives and handles the emitted values.

To create a publisher, you need to conform to the Publisher protocol. Publishers come in many types, including Just, Future, and PassthroughSubject. Just publishers emit a single value and then complete, while Future publishers emit a single value and then complete with an error or success. PassthroughSubject is a type of publisher that allows you to manually send values to subscribers.

Subscribers, on the other hand, conform to the Subscriber protocol. Subscribers come in many types, including Sink, Assign, and AnyCancellable. The Sink subscriber receives and handles the emitted values from the publisher, while the Assign subscriber assigns the emitted values to a property. AnyCancellable is a type of subscriber that allows you to cancel the subscription to the publisher.

Now that we have a basic understanding of publishers and subscribers, let’s move on to combining publishers.

Combining Publishers: Combine provides several operators to combine multiple publishers into one. These operators include merge, zip, and combineLatest.

The merge operator combines the emitted values from multiple publishers into a single stream of values. The values are emitted in the order in which they are received.

The zip operator combines the emitted values from multiple publishers into a single stream of tuples. The tuples contain the values from each publisher in the order in which they were combined.

The combineLatest operator combines the latest values from multiple publishers into a single stream of values. When any publisher emits a new value, the operator combines the latest value from each publisher and emits the result.

Let’s see these operators in action with some examples.

Example 1: Merging Publishers

let publisher1 = PassthroughSubject<Int, Never>()
let publisher2 = PassthroughSubject<Int, Never>()

let mergedPublisher = publisher1.merge(with: publisher2)

let subscription = mergedPublisher.sink { value in
    print("Merged value: \(value)")
}

publisher1.send(1)
publisher2.send(2)
publisher1.send(3)
publisher2.send(4)

// Output:
// Merged value: 1
// Merged value: 2
// Merged value: 3
// Merged value: 4

Example 2: Zipping Publishers

let publisher1 = PassthroughSubject<Int, Never>()

let publisher2 = PassthroughSubject<String, Never>()

let zippedPublisher = publisher1.zip(publisher2)

let subscription = zippedPublisher.sink { value in
print("Zipped value: (value)")
}

publisher1.send(1)
publisher2.send("Hello")
publisher1.send(2)
publisher2.send("World")

// Output:
// Zipped value: (1, "Hello")
// Zipped value: (2, "World")

Example 3: Combining Latest Publishers

let publisher1 = PassthroughSubject<Int, Never>()
let publisher2 = PassthroughSubject<Int, Never>()

let combinedLatestPublisher = publisher1.combineLatest(publisher2)

let subscription = combinedLatestPublisher.sink { value in
print("Combined latest value: (value)")
}

publisher1.send(1)
publisher2.send(2)
publisher1.send(3)
publisher2.send(4)

// Output:
// Combined latest value: (1, 2)
// Combined latest value: (3, 2)
// Combined latest value: (3, 4)

Transforming and Filtering Publishers:

Transforming and filtering publishers are essential operations in Combine. Combine provides several operators to transform and filter data streams, including map, flatMap, and filter. The map operator transforms the emitted values from a publisher into a new value. The new value can be of a different type from the emitted value. The flatMap operator transforms the emitted values from a publisher into a new publisher. The new publisher can emit a different sequence of values from the original publisher. The filter operator filters the emitted values from a publisher based on a condition. Let’s see these operators in action with some examples.

Example 1: Transforming Publishers with Map

let publisher = PassthroughSubject<Int, Never>()

let mappedPublisher = publisher.map { value in
return value * 2
}

let subscription = mappedPublisher.sink { value in
print("Mapped value: (value)")
}

publisher.send(1)
publisher.send(2)
publisher.send(3)

// Output:
// Mapped value: 2
// Mapped value: 4
// Mapped value: 6

Example 2: Transforming Publishers with FlatMap

let publisher = PassthroughSubject<Int, Never>()

let flatMappedPublisher = publisher.flatMap { value in
return Just(value * 2)
}

let subscription = flatMappedPublisher.sink { value in
print("Flat-mapped value: (value)")
}

publisher.send(1)
publisher.send(2)
publisher.send(3)

// Output:
// Flat-mapped value: 2
// Flat-mapped value: 4
// Flat-mapped value: 6

Example 3: Filtering Publishers

let publisher = PassthroughSubject<Int, Never>()
let filteredPublisher = publisher.filter { value in return value % 2 == 0 }
let subscription = filteredPublisher.sink { value in print("Filtered value: (value)") }
publisher.send(1) publisher.send(2) publisher.send(3) publisher.send(4)
// Output: // Filtered value: 2 // Filtered value: 4

Controlling the Flow of Data:

Controlling the flow of data is essential in asynchronous programming. Combine provides several operators to control the flow of data, including debounce, throttle, and delay. The debounce operator delays the emissions of values from a publisher until a specified amount of time has passed without receiving any new values. The throttle operator emits the first value from a publisher and then ignores any subsequent values for a specified amount of time. The delay operator delays the emissions of values from a publisher for a specified amount of time.

Let’s see these operators in action with some examples.

Example 1: Debouncing Publishers

let publisher = PassthroughSubject<Int, Never>()

let debouncedPublisher = publisher.debounce(for: .seconds(1), scheduler: DispatchQueue.main)

let subscription = debouncedPublisher.sink { value in
    print("Debounced value: \(value)")
}

publisher.send(1)
publisher.send(2)

DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
    publisher.send(3)
}

DispatchQueue.main.asyncAfter(deadline: .now() + 1.5) {
    publisher.send(4)
}

// Output:
// Debounced value: 2
// Debounced value: 4

Example 2: Throttling Publishers

let publisher = PassthroughSubject<Int, Never>()

let throttledPublisher = publisher.throttle(for: .seconds(1), scheduler: DispatchQueue.main, latest: true)

let subscription = throttledPublisher.sink { value in
    print("Throttled value: \(value)")
}

publisher.send(1)

DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
    publisher.send(2)
}

DispatchQueue.main.asyncAfter(deadline: .now() + 1.5) {
    publisher.send(3)
}

// Output:
// Throttled value: 1
// Throttled value: 3

Example 3: Delaying Publishers

let publisher = PassthroughSubject<Int, Never>()

let delayedPublisher = publisher.delay(for: .seconds(1), scheduler: DispatchQueue.main)

let subscription = delayedPublisher.sink { value in
    print("Delayed value: \(value)")
}

publisher.send(1)
publisher.send(2)

// Output (after 1 second):
// Delayed value: 1
// Delayed value: 2

Handling Errors:

Handling errors is crucial in asynchronous programming. Combine provides several operators to handle errors, including catch and retry.

The catch operator allows you to handle errors from a publisher and replace them with new values or a new publisher.

The retry operator resubscribes to a publisher in case of an error a specified number of times.

Let’s see these operators in action with some examples.

Example 1: Catching Errors

enum MyError: Error {
    case failed
}

let publisher = PassthroughSubject<Int, MyError>()

let catchPublisher = publisher.catch { error in
    return Just(0)
}

let subscription = catchPublisher.sink { value in
    print("Caught value: \(value)")
}

publisher.send(1)
publisher.send(completion: .failure(.failed))
publisher.send(2)

// Output:
// Caught value: 1
// Caught value: 0
// Caught value: 2

Example 2: Retrying Publishers

enum MyError: Error {
    case failed
}

let publisher = PassthroughSubject<Int, MyError>()

let retryPublisher = publisher.retry(2)

let subscription = retryPublisher.sink { value in
    print("Retried value: \(value)")
}

publisher.send(1)
publisher.send(completion: .failure(.failed))
publisher.send(2)

// Output:
// Retried value: 1
// Retried value: 1
// Retried value: 1
// Retried value: 2

Conclusion

Combine is a powerful and declarative framework that allows you to handle asynchronous events and data streams in Swift. In this article, we covered the essential aspects of using Combine, including publishers and subscribers, combining publishers, transforming and filtering publishers, controlling the flow of data, and handling errors. We saw how to use operators like merge, zip, combineLatest, map, flatMap, filter, debounce, throttle, delay, catch, and retry to manipulate data streams.

By leveraging the power of Combine, you can write clean, composable, and easy-to-understand code. Combine allows you to avoid callback hell and write asynchronous code that is more readable, maintainable, and testable.

If you want to learn more about Combine, Apple provides excellent documentation, including a Combine framework overview, a Combine framework tutorial, and a Combine framework sample code. You can also find many online resources, including books, videos, and blogs, to deepen your knowledge of Combine.

In conclusion, Combine is an essential framework for handling asynchronous events and data streams in Swift. By mastering Combine, you can write asynchronous code that is more elegant, efficient, and reliable. Happy coding!