Async repository

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Goal

Write the BookRepository that supplies data to the ViewModel via suspend functions. After this page the ViewModel from the previous page can compile and run.

Prerequisites

• 02 UiState and ViewModel

The repository

import kotlinx.coroutines.delay

class BookRepository {
    suspend fun getBooks(): List<Book> {
        delay(600) // simulate async work — no network, no API keys
        return listOf(
            Book(1, "The Go Programming Language", "Donovan & Kernighan"),
            Book(2, "Kotlin in Action", "Jemerov & Isakova"),
            Book(3, "Effective Java", "Bloch"),
        )
    }

    suspend fun getBook(id: Int): Book? = getBooks().firstOrNull { it.id == id }
}

delay is a non-blocking suspend function from kotlinx.coroutines. It yields the coroutine for 600 ms and resumes it — identical behaviour to a real I/O wait but without any network calls or credentials.

getBook re-uses getBooks for simplicity. In production you would query by primary key instead of loading the full list each time.

Why no network?

This tier stays offline deliberately. There are two reasons:

1. No API keys. This is a public repository. Embedding credentials — even test ones — would expose them in git history forever.
2. Focused scope. Adding Retrofit, OkHttp, and a live endpoint is its own tier. The architecture here (sealed UiState, ViewModel, repository) is identical whether the data source is an in-memory list or a REST API. Swapping delay(600) and the hard-coded list for a retrofit.create(BookService::class.java).getBooks() call is a one-function change once you reach the networking tier.

Where suspend comes from

suspend marks a function that can pause without blocking a thread. The Kotlin compiler rewrites suspend functions into state machines under the hood. You do not need to manage threads manually — viewModelScope.launch picks the right dispatcher automatically.

Next → 04 Collect state in Compose