Lesson 17: Introduction to ViewModel

Introduction to ViewModel

A ViewModel holds UI-related data and survives configuration changes (like screen rotation) that would otherwise destroy and recreate your Activity, along with any plain remember-based state inside it.

Add the dependency

implementation("androidx.lifecycle:lifecycle-viewmodel-compose:2.8.0")

A basic ViewModel

class TaskListViewModel : ViewModel() {
    private val _tasks = mutableStateListOf(
        Task(1, "Buy groceries"),
        Task(2, "Finish report")
    )
    val tasks: List get() = _tasks

    fun addTask(title: String) {
        _tasks.add(Task(_tasks.size + 1, title))
    }
}

Using it in a composable

@Composable
fun TaskListScreen(viewModel: TaskListViewModel = viewModel()) {
    LazyColumn {
        items(viewModel.tasks) { task ->
            Text(task.title, modifier = Modifier.padding(16.dp))
        }
    }
}

viewModel() is a special composable function that either creates a new TaskListViewModel the first time, or hands you back the same instance if one already exists for this screen — including after a rotation. This is what actually solves the "data disappears on rotation" problem from lesson 7.

Why underscore-prefixed private properties?

Notice _tasks is private, with a public tasks exposing a read-only view. This is a deliberate pattern: the ViewModel controls all mutations (through functions like addTask), while the UI can only read the current data, never modify it directly. This keeps your data flow predictable — the UI reacts to state, it doesn't reach in and change it arbitrarily.

Next: Lesson 18 — the bigger architectural picture this pattern is part of.