Why modular architecture matters for large iOS teams Design principles for Swift packages How to define module boundaries and publish clean interfaces Testing, CI, and versioning for modular packages A pragmatic incremental migration strategy Practical Application: checklists, scripts, and CI snippets Large iOS monoliths quietly tax velocity: slow local builds, noisy CI, fragile reviews, and features that collide in the same code paths. Modularizing around Swift Package Manager packages with strict interfaces turns that drag into leverage — smaller compile surfaces, clearer ownership, and true reuse. A legacy monolith shows itself in practical symptoms: PRs that touch unrelated files, 10–20 minute inner-loop wait times for the team, CI pipelines that rebuild most of the app on every change, and duplicated utilities because no one wants to plumb the monolith. You need modular architecture that enforces boundaries, not a diagram that lives in a slide deck.…