Handling Asynchronous Tasks in iOS with Combine

Working with Combine Framework

In modern iOS development, handling asynchronous tasks efficiently is crucial. While there are several ways to handle asynchronous tasks in Swift, including closures, delegates, and DispatchQueue, one powerful approach is to use the Combine framework introduced in iOS 13.

The Combine framework allows you to work with asynchronous events as if they were sequences of values you can transform and manipulate using high-order functions like map, filter, reduce, etc.

The example code we have been looking at makes extensive use of the Combine framework for network requests. Here’s a method that sends a network request and returns a publisher:

func request<T: Encodable, U: Decodable>(endPoint: APIEndpoint, parameters: T?, authRequired: Bool = true) -> AnyPublisher<U, Error> {
    guard let request = buildRequest(from: endPoint, with: parameters, authRequired: authRequired) else {
        return Fail(error: NetworkError.invalidURL)
            .eraseToAnyPublisher()
    }

    return session.dataTaskPublisher(for: request)
        .tryMap { data, response in
            // handle response here...
            return data
        }
        .decode(type: U.self, decoder: JSONDecoder())
        .mapError { error in
            // handle error here...
            return NetworkError.unknownError(error)
        }
        .eraseToAnyPublisher()
}

In this method, a network request is initiated with session.dataTaskPublisher(for: request), which returns a publisher that emits the server’s response as a tuple of (Data, URLResponse).

This is then transformed using tryMap to only keep the Data part and check the HTTP response status. The resulting data is then decoded into the expected response type U with .decode(type: U.self, decoder: JSONDecoder()).

The mapError function maps any error that occurs during this process to our custom NetworkError type.

The use of Combine here provides several benefits:

• Composability: The publisher returned by the request method can be further composed with other publishers to handle complex asynchronous workflows.
• Error handling: Errors are propagated along the publisher chain and can be caught and handled at any point.
• Code readability: The high-level, declarative syntax of Combine makes the code more readable and easier to understand compared to nested closures or delegate methods.
• Integration with Swift UI: Combine works seamlessly with Swift UI, allowing you to easily update your UI based on the results of network requests.

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map and tryMap

The map function is used to transform the output of a publisher. For example, if you have a publisher that emits integers, you could use map to transform them into strings:

let intPublisher = Just(5)
let stringPublisher = intPublisher.map { "The number is \($0)" }

The tryMap function is similar, but it can throw errors, allowing you to handle possible failures during the transformation process.

filter

The filter function is used to emit only the values that satisfy a given predicate. For example, you could create a publisher that only emits even numbers like this:

let numbers = [1, 2, 3, 4, 5, 6].publisher
let evenNumbers = numbers.filter { $0 % 2 == 0 }

combineLatest

The combineLatest function is used when you need to combine the latest values of multiple publishers. It emits a value whenever any of its input publishers emit a value, combining the latest values from each one.

let publisher1 = PassthroughSubject<Int, Never>()
let publisher2 = PassthroughSubject<String, Never>()
let combined = publisher1.combineLatest(publisher2)

publisher1.send(1)
publisher2.send("a") // Emits (1, "a")
publisher1.send(2) // Emits (2, "a")
publisher2.send("b") // Emits (2, "b")

merge

The merge function combines the outputs from multiple publishers into a single publisher. Unlike combineLatest, it does not wait for each publisher to emit a value, but emits values as soon as they arrive from any publisher.

switchToLatest

The switchToLatest operator is used when you have a publisher of publishers and you want to transform it into a publisher that emits only the latest values from the latest publisher. This is particularly useful when working with asynchronous tasks like network requests.

zip

The zip operator combines multiple publishers by pairing their values together. Unlike combineLatest, it emits a value only when all of its input publishers have emitted a value.

These are just a few examples of the operators available in the Combine framework. By composing these operators together, you can express complex asynchronous workflows in a clear and concise way.

However, as with any tool, it’s important to understand its strengths and limitations. While Combine is extremely powerful for handling asynchronous tasks, it also has a steep learning curve and requires a good understanding of Swift and functional programming concepts. For simpler tasks, other approaches like closures or the new async/await syntax introduced in Swift 5.5 might be more suitable.

In the next part of this series, we will explore how to test Combine code and handle common pitfalls. Stay tuned!

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