MainApp.kt

/**
 * The main composable function for setting up the app's UI structure.
 * It defines the scaffold layout, navigation, and other top-level elements for the app.
 *
 * @param navController The navigation controller for managing app navigation.
 * @param snackbarHostState The state for displaying snackbars in the app.
 */
@OptIn(ExperimentalMaterial3Api::class)
@Composable
fun MainApp(
    navController: NavHostController,
    snackbarHostState: SnackbarHostState
) {
    // Defines a scroll behavior for the top app bar, enabling it to collapse on scroll.
    val scrollBehavior = TopAppBarDefaults.exitUntilCollapsedScrollBehavior(
        rememberTopAppBarState()
    )

    // Observes the current back stack entry to determine the navigation state.
    val currentBackStackEntry by navController.currentBackStackEntryAsState()

    // Determines if the back button should be shown, based on the navigation stack.
    val showBackButton by remember(currentBackStackEntry) {
        derivedStateOf { navController.previousBackStackEntry != null }
    }

    // Defines the scaffold structure, which includes the top app bar, snackbar host, and floating action button.
    Scaffold(
        snackbarHost = { SnackbarHost(hostState = snackbarHostState) }, // Host for displaying snackbars.
        topBar = {
            // The top app bar, which dynamically displays a back button based on the navigation state.
            MainTopAppBar(
                navController = navController,
                showBackButton = showBackButton,
                scrollBehavior = scrollBehavior
            )
        },
        modifier = Modifier.nestedScroll(scrollBehavior.nestedScrollConnection) // Ensures nested scrolling works with the top bar.
    ) { innerPadding ->
        // Sets up the navigation host, which manages the composable destinations in the app.
        NavHost(
            navController = navController,
            startDestination = ROUTE_HOME, // The starting destination of the app.
            modifier = Modifier
                .padding(innerPadding) // Ensures padding for the scaffold's content area.
                .fillMaxSize() // Ensures the navigation content fills the available space.
        ) {
            // Defines the home screen route.
            composable(route = ROUTE_HOME) {
                val noteViewModel = hiltViewModel<NoteViewModel>() // Retrieves the NoteViewModel using Hilt.
                NotesScreen(viewModel = noteViewModel) // Displays the list of notes.
            }
        }
    }
}