Challenge 2: A ViewModel Backed by Room — Solution

class NoteViewModel(private val dao: NoteDao) : ViewModel() {

    val notes: StateFlow<List<Note>> = dao.getAllNotes()
        .stateIn(
            scope = viewModelScope,
            started = SharingStarted.WhileSubscribed(5000),
            initialValue = emptyList()
        )

    fun addNote(title: String, content: String) {
        viewModelScope.launch {
            dao.insert(Note(title = title, content = content))
        }
    }
}

Notes:
- dao.getAllNotes() returns a cold Flow<List<Note>> straight from Room;
  .stateIn(...) converts it into a hot StateFlow that Compose can collect
  with collectAsStateWithLifecycle(), following the exact same pattern
  as the chapter's TaskViewModel example.
- initialValue = emptyList() is what "notes" holds before the very first
  emission from the database arrives — collectAsStateWithLifecycle()
  never sees an uninitialized/null state this way.
- addNote() wraps dao.insert(...) — a suspend function — in
  viewModelScope.launch { }, since insert() can't be called directly
  from a non-suspending context like a Button's onClick lambda.
- WhileSubscribed(5000) means the underlying database query keeps
  running for 5 seconds after the last UI collector disappears (e.g.
  during a brief rotation), rather than stopping and restarting
  immediately, avoiding an unnecessary re-query.