Lesson 21: Displaying Network Data in a List

Displaying Network Data in a List

Let's finish the networking flow properly, with the three states every network-backed screen actually needs: loading, error, and success.

A sealed class for UI state

sealed class TaskListUiState {
    object Loading : TaskListUiState()
    data class Success(val tasks: List) : TaskListUiState()
    data class Error(val message: String) : TaskListUiState()
}

A sealed class represents a fixed set of possible states — Kotlin's compiler knows these are the only three possibilities, which lets it warn you if you forget to handle one.

Updating the ViewModel

class TaskListViewModel : ViewModel() {
    var uiState by mutableStateOf(TaskListUiState.Loading)
        private set

    init {
        viewModelScope.launch {
            uiState = try {
                val tasks = taskApi.getTasks().map { it.toTask() }
                TaskListUiState.Success(tasks)
            } catch (e: Exception) {
                TaskListUiState.Error("Couldn't load tasks. Check your connection.")
            }
        }
    }
}

Rendering all three states

@Composable
fun TaskListScreen(viewModel: TaskListViewModel = viewModel()) {
    when (val state = viewModel.uiState) {
        is TaskListUiState.Loading -> {
            Box(modifier = Modifier.fillMaxSize(), contentAlignment = Alignment.Center) {
                CircularProgressIndicator()
            }
        }
        is TaskListUiState.Error -> {
            Text(state.message, modifier = Modifier.padding(24.dp))
        }
        is TaskListUiState.Success -> {
            LazyColumn {
                items(state.tasks) { task ->
                    Text(task.title, modifier = Modifier.padding(16.dp))
                }
            }
        }
    }
}

The when here is exhaustive — Kotlin verifies every branch of the sealed class is handled. This pattern (sealed class UI state + exhaustive when) is one of the most valuable habits in real Android apps: it makes "forgot to handle the error case" a compile-time warning instead of a production bug.

Next: Lesson 22 — saving data locally with a Room database.