UiState and ViewModel

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Goal

Define the sealed UiState interface and a BookViewModel that drives it with a StateFlow. After this page you have the state container the remaining pages wire up to a real screen.

Prerequisites

• 01 Why state
• Classes and data classes

The data model

The beginner Book had two fields. The intermediate version adds an id so Navigation Compose can route to a detail screen:

data class Book(val id: Int, val title: String, val author: String)

UiState — three possible worlds

A sealed interface lets the compiler guarantee you handle every case. Three states cover the full lifecycle of an async operation:

sealed interface UiState {
    data object Loading : UiState
    data class Content(val books: List<Book>) : UiState
    data class Error(val message: String) : UiState
}

• Loading — a fetch is in flight; show a spinner.
• Content — the fetch succeeded; show the list.
• Error — the fetch failed; show a message and a Retry button.

data object (Kotlin 1.9+) gives Loading a sensible toString() and structural equality at no cost.

BookViewModel

import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import kotlinx.coroutines.flow.MutableStateFlow
import kotlinx.coroutines.flow.StateFlow
import kotlinx.coroutines.flow.asStateFlow
import kotlinx.coroutines.launch

class BookViewModel(
    private val repo: BookRepository = BookRepository()
) : ViewModel() {
    private val _state = MutableStateFlow<UiState>(UiState.Loading)
    val state: StateFlow<UiState> = _state.asStateFlow()

    init { load() }

    fun load() {
        viewModelScope.launch {
            _state.value = UiState.Loading
            _state.value = try {
                UiState.Content(repo.getBooks())
            } catch (e: Exception) {
                UiState.Error(e.message ?: "failed to load")
            }
        }
    }
}

Key points:

• MutableStateFlow is the private write-end; StateFlow (read-only) is what the composable sees. This keeps mutation inside the ViewModel.
• asStateFlow() wraps without copying — zero overhead.
• viewModelScope is a coroutine scope tied to the ViewModel lifecycle. It cancels automatically when the ViewModel is cleared (e.g. the screen leaves the back stack).
• init { load() } fetches on first construction so the screen never starts in a permanent Loading state.
• The try/catch wraps the repository call: any exception from the suspend function becomes an Error state rather than an app crash.

BookRepository is defined on the next page.

Next → 03 Async repository