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Learn how to effectively handle errors in `async calls` using Swift Combine while fetching data from the server with perfect clarity and structure.
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Handling Errors in Async Calls Using Swift Combine

As developers, we often face the challenge of dealing with asynchronous operations and the errors that arise from them. In this guide, we’ll dive into handling errors in async calls using Swift’s Combine framework.

This post will address a common scenario: you have two asynchronous methods (m1() and m2()), each capable of throwing different types of errors, and you want to manage them in a unified manner.

The Problem

You need to fetch data from the server using two async calls and ensure that you can handle errors for each one effectively. In the provided example, the methods m1() and m2() return Future types that can complete with different error types. If one of the requests fails, you want to show an error message based on the specific error, but if both succeed, you continue with the next steps of your flow.

Here’s what your current implementation looks like:

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The Challenge

The challenge arises from the fact that a Publisher in Combine can only emit one type of error. Therefore, when combining the two methods (m1() and m2()), you encounter a type conflict due to their differing error types (Error1 and Error2).

The Solution

To resolve this, we can use a Result type to encapsulate the success or failure of each operation while standardizing the error type in the combine pipeline. Here’s how you can implement this solution step by step.

Step 1: Define Your Future Methods

You need to modify your m1() and m2() methods to return a Result type that handles the success or failure and uses Never for the error type in the Future.

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Step 2: Combine the Results

Now, you can use zip to wait for both asynchronous calls to complete. This involves subscribing to the results of both m1() and m2():

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Key Takeaways

By using a Result type, you can effectively manage multiple error types in Combine.

Using zip, you can coordinate multiple asynchronous calls while maintaining control over their individual success and failure states.

Implementing the shouldFail parameter helps test various scenarios, ensuring robust error handling for your async operations.

Conclusion

Handling errors in async calls using Swift Combine doesn’t have to be daunting. By leveraging the Result type and combining your calls with zip, you can create a seamless error handling process that keeps your application flow intact.

Overall, this approach allows you to gather the necessary results while keeping your error management clear and concise. Happy coding!