Define a Repository Interface
Expose data needs without leaking HTTP details.
Define a Repository Interface is a free Kotlin Multiplatform Academy lesson on CoddyKit — lesson 1 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.
What a Repository Is
A repository is the single doorway your apps walk through to get data, hiding where that data actually comes from.
Start With an Interface
Define the repository as a Kotlin interface first, so callers depend on a contract instead of a concrete class.
interface UserRepository {
suspend fun getUser(id: String): User
}Speak in Domain Terms
Name methods after what the app needs, like getUser, not after how you fetch it. The intent stays clear and stable.
Return Domain Models
The interface returns clean domain models, never raw JSON or HTTP responses, so the UI stays simple.
suspend fun getUsers(): List<User>No HTTP Words Allowed
Keep words like Ktor, headers, and status codes out of the interface. Callers should never know HTTP exists.
Suspend for Async Work
Mark data methods as suspend so they can do async work without blocking, and fit naturally into coroutines on both apps.
suspend fun refresh()It Lives in commonMain
Put the interface in commonMain so Android and iOS share the exact same data contract with zero duplication.
Many Implementations Possible
One interface can have several implementations: a real network one and a fake one for tests, swapped freely.
Easier to Test
Because callers depend on the interface, you can inject a fake repository in tests and avoid hitting the real network.
class FakeUserRepository : UserRepositoryClear, Focused Methods
Keep each method small and purposeful. A focused API is easier to read, mock, and evolve over time.
A Complete Example
Here is a tidy repository contract that exposes everything the user feature needs and nothing more.
interface UserRepository {
suspend fun getUsers(): List<User>
suspend fun getUser(id: String): User
}Quick Check
Where should the repository interface live in a KMP project?
Recap
A repository interface in commonMain defines what data the app needs using suspend methods and domain models, hiding all HTTP details.
Frequently asked questions
Is the “Define a Repository Interface” lesson free?
Yes — the full text of “Define a Repository Interface” 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 “Define a Repository Interface”?
Expose data needs without leaking HTTP details. 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 1 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Define a Repository Interface” 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
- Define a Repository Interface
- Wrap the API Behind the Repository
- Map DTOs to Domain Models
- Expose Results as Flow