Use Cases & Domain Boundaries
Keep business rules independent and testable.
Use Cases & Domain Boundaries is a free Kotlin Multiplatform Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Kotlin Multiplatform Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Rules Deserve a Home
Business rules scattered across screens drift and duplicate. Giving each rule its own use case creates one trusted place for important logic to live.
What a Use Case Is
A use case captures one app action, like place an order or refresh the feed. It reads as a verb and does exactly that one meaningful thing.
One Public Operator
A tidy use case exposes a single entry point. Many teams use Kotlin invoke so the object is called like a function at the call site.
class PlaceOrder(private val repo: OrderRepo) {
suspend operator fun invoke(cart: Cart): Order
}It Depends on Interfaces
A use case receives repository interfaces, never concrete clients. It orchestrates them but stays unaware of Ktor, SQL or any platform detail.
Pure and Testable
Because it has no UI and no I/O of its own, a use case is pure logic you can test in commonTest with simple fakes and zero device setup.
Drawing the Boundary
The domain boundary is the line use cases guard. Nothing technical, no JSON or HTTP types, crosses it; only clean models and clear results pass through.
Keep Frameworks Out
If a use case imports a Ktor or SQLDelight type, the boundary has leaked. Keep those framework imports in the data layer where they belong.
Composing Use Cases
Bigger flows combine smaller ones. A checkout use case might call validate cart, then place order, each independently tested and reused elsewhere.
Returning Outcomes
A use case usually returns a domain model or a sealed Result, so callers handle success and failure without catching framework exceptions.
suspend operator fun invoke(): Result<Feed>Who Calls Them
Shared ViewModels call use cases in response to user intents. The ViewModel coordinates; the use case decides. Each keeps a single, clear duty.
When to Skip Them
For a trivial passthrough, a use case can feel like ceremony. Add one when logic is real or shared; do not wrap a one-line call just for symmetry. 🙂
Quick Check
What belongs inside a well-designed use case?
Recap
You learned use cases hold one action each, depend on interfaces, and guard the domain boundary so frameworks and UI stay firmly outside. 🎉
Frequently asked questions
Is the “Use Cases & Domain Boundaries” lesson free?
Yes — the full text of “Use Cases & Domain Boundaries” is free to read here on the web, and the Kotlin Multiplatform Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Kotlin Multiplatform Academy course, upgrade to CoddyKit PRO.
What will I learn in “Use Cases & Domain Boundaries”?
Keep business rules independent and testable. You practise Kotlin Multiplatform Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Kotlin Multiplatform Academy?
No prior experience is required. Kotlin Multiplatform Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Use Cases & Domain Boundaries” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Kotlin Multiplatform Academy lesson?
Yes. Every Kotlin Multiplatform Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Layered Architecture for KMP
- Use Cases & Domain Boundaries
- Split Shared Code into Modules
- Decide What Stays Native