Explain the roles of ViewModel, Repository, and data source in MVVM

What's really being asked

The interviewer wants to see if you can draw clean architectural boundaries in Android MVVM. Specifically, they care whether you understand that each layer owns a single responsibility and that dependencies point inward toward data, not outward toward the UI. They also want to hear how state flows up and events flow down.

The full answer

First, the ViewModel acts as a state holder and transformation layer. It receives raw data from the Repository, converts it into UI state, and survives configuration changes. It never talks to Retrofit or Room directly. Second, the Repository serves as a single source of truth and abstraction boundary. It mediates between different data sources, for example returning cached Room data immediately while triggering a Retrofit refresh in the background. Third, the data source does the actual I/O. Room handles local persistence and SQL mapping; Retrofit handles network serialization and HTTP calls. Fourth, the interaction pattern is unidirectional. The UI observes ViewModel state; the ViewModel calls Repository methods; the Repository decides which data source to query; the data source returns results back up the chain. Fifth, testing implications. ViewModels are tested with fake Repositories; Repositories are tested with fake data sources; and data sources contain the only truly slow I/O.

The mistakes people make

A major red flag is describing the ViewModel as a place for business rules or direct network calls. Another is claiming the Repository is just unnecessary boilerplate that can be skipped in small apps. Some candidates blur the lines by saying Room lives inside the ViewModel or that the Repository emits UI-specific strings. Confusing the Repository pattern with a DAO is also common.

What usually comes next

The interviewer may ask how you handle errors across these layers, where coroutines and Flow fit in, or how you inject these dependencies with Hilt. They might also ask what happens if the Repository needs to merge local and remote data, or how you prevent memory leaks when the ViewModel outlives the UI.

A concrete example

Imagine a news app. The user opens the headlines screen. The Fragment observes a UiState sealed class from the ViewModel. The ViewModel asks the NewsRepository for headlines. The Repository checks Room for cached articles and returns them immediately via Flow, then calls Retrofit to fetch updates. Retrofit returns JSON that the Repository maps to entities and writes back to Room. Room emits the new cache, the Repository passes it along, and the ViewModel maps it to UiState.Success. The Fragment simply renders what it receives.