Modern Concurrency and Legacy code

‘We can’t touch that code — it’s all callback-based!’

Sound familiar? As a developer, you’ve probably heard this countless times, or even said it yourself. Modern Concurrency promises cleaner more maintainable code with its elegant async/await syntax. Yet many teams find themselves stuck with legacy codebase, feeling like they’re forced to choose between modern development practices and stable working systems.

But what if you didn’t have to choose?

At , we faced this exact challenge. Our network layer was deeply rooted in completion handlers — a common scenario in many iOS applications. However, through careful planning and strategic implementation, we successfully modernized our entire codebase to use Swift Concurrency without breaking existing functionality or requiring extensive QA resources.

In this article, I’ll walk you through our journey, sharing practical strategies and real-world examples of how we bridged the gap between old and new, You’ll learn how to:

• Gradually adopt modern concurrency patterns
• Safely refactor callback-based code
• Maintain backward compatibility throughout the transition
• Implement these changes with minimal risk and testing overhead

Before diving in, I recommend reading my previous article “Future of concurrent code in Swift” for helpful context on Swift’s concurrency model. Let’s explore how you can bring your…