Many long-running iOS apps still contain years of Objective-C. A full rewrite is risky, slow and hard to justify to the business, because users see no new features while it happens. A gradual migration is usually the better path.
Swift and Objective-C can live together
Apple designed Swift and Objective-C to work side by side in the same project. Swift code can call Objective-C, and Objective-C code can call Swift, through a bridging header and a generated Swift interface. Apple's guide to migrating Objective-C code to Swift explains the mechanics.
Pick the seams first
Start with leaf modules that have few dependencies, such as data models, formatters, networking helpers and utility classes. Convert one class at a time, run the test suite after each change, and ship regularly. Avoid starting with the core screen flow, because that is where most of the risk sits.
Add tests before you touch the code
Legacy code often has little test coverage. Write characterization tests for the behavior you depend on before converting a class. If the Swift version passes the same tests, you have evidence that nothing changed for the user.
Use a checklist for each module
- Are nullability and optional handling correct after conversion?
- Do threading and queue assumptions still hold?
- Are public headers and bridging names clear for remaining Objective-C callers?
- Is there a test that would fail if the behavior changed?
Keep a migration log
Track which modules are converted, which are in progress and which are blocked. A shared log keeps product managers informed and stops the migration from quietly stalling halfway.