Swift is a powerful and versatile programming language known for its strong safety features. However, when it comes to asynchronous programming, there have been several approaches to handle completion handlers that can be error-prone. In this blog post, we will explore why completion handlers can be problematic and how modern Swift technologies like async/await offer a superior alternative. We'll use an example to demonstrate the pitfalls of completion handlers and showcase the advantages of the more modern approach.

The Problem with Completion Handlers

Completion handlers are often used in Swift to manage asynchronous operations, like network requests or file I/O. They require a callback function to be executed once the asynchronous task is completed. While they work, they can lead to some common issues:

• 1. Callback Hell: Asynchronous code written using completion handlers can quickly become nested and difficult to read, leading to a structure commonly referred to as "Callback Hell." This makes code maintenance and debugging a challenging task.

• 2. Error Handling: Error handling is often inconsistent and can be prone to error when using completion handlers. Developers must remember to call the completion handler in both success and failure cases.

• 3. Leaked Resources: If the completion handler is not executed properly, it may lead to resource leaks, as the developer might forget to release resources or close connections.

A Common Pitfall: Forgetting to Call the Completion Handler

To illustrate the problem, let's consider an example of downloading data from a web API using a completion handler. We'll create a function that downloads a JSON response and passes the result back using a completion handler:

func downloadData(completion: @escaping (Result<Data, Error>) -> Void) {
    guard let url = URL(string: "https://example.com/data.json") else {
        return
    }

    URLSession.shared.dataTask(with: url) { data, response, error in
        if let error = error {
            completion(.failure(error))
            return
        }

        guard let data = data else {
            // Oops! We forgot to call the completion handler here.
            return
        }

        completion(.success(data))
    }.resume()
}

In this example, there is a critical mistake - we forgot to call the completion handler if the data is nil. This can lead to unexpected behavior and resource leaks in a real-world scenario.

A Better Approach: Using async/await

Swift introduced native support for asynchronous programming using async/await. With this approach, the code becomes more readable, less error-prone, and easier to maintain. Let's rewrite the example using async/await:

func downloadData() async throws -> Data {
    guard let url = URL(string: "https://example.com/data.json") else {
        throw MyError.invalidURL
    }

    let (data, _) = try await URLSession.shared.data(from: url)
    return data
}

Using async/await, we can handle errors more gracefully by throwing exceptions. The code is linear, making it easier to follow.

An Alternative Approach: Using Combine Publishers

Combine is a powerful framework for reactive and asynchronous programming in Swift. While it's a valuable tool, it's essential to consider your project's specific requirements and your team's familiarity with Combine. Here's how the same task can be accomplished using Combine publishers:

import Combine

func downloadData() -> AnyPublisher<Data, Error> {
    guard let url = URL(string: "https://example.com/data.json") else {
        return Fail(error: MyError.invalidURL).eraseToAnyPublisher()
    }

    return URLSession.shared.dataTaskPublisher(for: url)
        .map(\.data)
        .eraseToAnyPublisher()
}

Combine publishers offer a clean and declarative way to handle asynchronous operations, with built-in error handling and composability.

Conclusion

While completion handlers have been a traditional way to manage asynchronous code in Swift, they come with inherent pitfalls such as callback hell and error handling challenges. Modern Swift technologies like async/await provide a superior alternative, making your code more readable, maintainable, and less error-prone. As Swift evolves, it's essential for developers to embrace these new techniques to write more efficient and robust asynchronous code.