Android Data Layer

The data layer coordinates data from multiple sources; its public API is repository interfaces, and its internals (DAOs, API services, DTOs) never leak upward. This skill is the canonical home for the error-propagation model, plus the KMP-Room setup. Related: android-skills:android-retrofit (Retrofit service/OkHttp/Hilt wiring), android-skills:android-dev (overall app architecture).

Error propagation — the layered model

The goal: data-layer errors (IOException / HttpException / SQLiteException) must never reach the ViewModel. Map them to a domain error type at the repository boundary. If the project already has an error convention, match it; otherwise a sealed DataError hierarchy is a reasonable default:

sealed class DataError(message: String, cause: Throwable? = null) : Exception(message, cause) {
    class Network(cause: Throwable) : DataError("Network error", cause)
    class Server(val code: Int, message: String?) : DataError("Server error $code: $message")
    class Local(cause: Throwable) : DataError("Local storage error", cause)
}