Modularizing a monolithic iOS app

WHAT THIS TESTS This evaluates architectural judgment under real constraints: can you reduce coupling incrementally, choosing extraction order by dependency direction and risk rather than by what seems exciting.

A GOOD ANSWER COVERS Start by understanding the dependency graph. The best first module is a stable leaf: code that many parts depend on but that itself depends on little, so extracting it does not drag the whole app along. Common first choices are a design system or UI components, core domain models, foundational utilities, or a networking layer, each with a small, well-understood surface. Define an explicit public interface, move the implementation into a Swift Package or framework, and enforce that it builds in isolation so hidden couplings surface immediately. Then iterate inward toward feature modules, optionally hiding implementations behind protocol interfaces so features depend on abstractions. Benefits include faster incremental builds, clear ownership, enforced boundaries, and parallel team work.

COMMON WRONG ANSWERS A big-bang rewrite that pauses feature delivery and risks regressions. Extracting a tangled, high-traffic feature first, which has many upward dependencies and resists clean separation. Creating modules without enforced boundaries, so the monolith persists logically even if split physically.

LIKELY FOLLOW-UPS How do you prevent circular dependencies between modules? Static versus dynamic linking trade-offs for many modules? How do you keep build times improving as module count grows?

ONE CONCRETE EXAMPLE In a shopping app, the first extraction is a DesignSystem package holding colors, typography, and reusable buttons. It depends only on UIKit or SwiftUI, so moving it is low risk, yet nearly every screen consumes it, so the boundary immediately clarifies ownership and shrinks rebuild scope when unrelated features change. Next you extract CoreModels and a Networking module, then begin peeling feature modules like Cart and Checkout that depend on those leaves through defined interfaces.