Dynamically Show or Hide TopBar in Jetpack Compose Scaffold

Jetpack Compose has revolutionized modern Android app development with its declarative UI paradigm. Among its core components is the Scaffold, a versatile layout structure that provides a consistent design skeleton for many apps. The TopBar (often a TopAppBar) is a critical part of the Scaffold, offering navigation and branding opportunities. However, dynamically showing or hiding the TopBar based on user interaction or app state can sometimes be a challenge.

In this blog post, we’ll explore how to dynamically show or hide the TopBar in a Jetpack Compose Scaffold, leveraging best practices and advanced Compose concepts. Whether you're building a complex app or seeking a cleaner approach to managing your UI states, this guide is tailored for intermediate and advanced Android developers.

Understanding Jetpack Compose Scaffold and TopBar

The Scaffold in Jetpack Compose provides a flexible and declarative way to organize app screens. Its primary slots include:

• TopBar: For the top app bar or toolbar-like components.

• BottomBar: For the bottom navigation or actions.

• FloatingActionButton: For quick access to key actions.

• DrawerContent: For the navigation drawer.

• Content: For the main body of the screen.

Why Dynamically Toggle the TopBar?

Dynamically showing or hiding the TopBar can improve UX by adapting to user actions or app states. For example:

• Hiding the TopBar during immersive experiences like video playback.

• Displaying the TopBar only when scrolling up, similar to popular social media apps.

• Adjusting the UI dynamically based on authentication or screen state.

Building a Dynamic TopBar

1. Using MutableState to Control Visibility

In Compose, UI is a function of state. By leveraging MutableState, we can seamlessly control the visibility of the TopBar. Here's a basic implementation:

@Composable
fun DynamicTopBarScaffold() {
    // State to control visibility
    val showTopBar = remember { mutableStateOf(true) }

    Scaffold(
        topBar = {
            if (showTopBar.value) {
                TopAppBar(
                    title = { Text("Dynamic TopBar") },
                    navigationIcon = {
                        IconButton(onClick = { /* Handle navigation */ }) {
                            Icon(Icons.Default.Menu, contentDescription = null)
                        }
                    }
                )
            }
        }
    ) { innerPadding ->
        // Content goes here
        Box(modifier = Modifier.padding(innerPadding)) {
            Column {
                Button(onClick = { showTopBar.value = !showTopBar.value }) {
                    Text(if (showTopBar.value) "Hide TopBar" else "Show TopBar")
                }

                Text("Scroll or interact with the UI to trigger dynamic changes.")
            }
        }
    }
}

Key Points:

• remember ensures the state persists across recompositions.

• MutableState drives UI updates in a declarative manner.

2. Advanced Use Case: Scroll-Based Visibility

A common pattern is to hide the TopBar when scrolling down and show it when scrolling up. Here’s how to achieve this with Compose:

@Composable
fun ScrollAwareTopBarScaffold() {
    val scrollState = rememberLazyListState()
    val showTopBar = remember { derivedStateOf { scrollState.firstVisibleItemIndex == 0 } }

    Scaffold(
        topBar = {
            if (showTopBar.value) {
                TopAppBar(
                    title = { Text("Scroll Aware TopBar") }
                )
            }
        }
    ) { innerPadding ->
        LazyColumn(
            state = scrollState,
            modifier = Modifier.padding(innerPadding)
        ) {
            items(100) { index ->
                Text("Item $index", modifier = Modifier.padding(16.dp))
            }
        }
    }
}

Key Points:

• LazyListState tracks the scroll position.

• derivedStateOf optimizes recompositions by recalculating state only when dependencies change.

• This approach mimics behaviors found in apps like Instagram or Google Photos.

Best Practices for Managing TopBar State

1. Avoid Excessive Recomposition

• Use derivedStateOf for computed states to minimize recompositions.

• Structure your Composable functions to limit the scope of recompositions.

2. Leverage ViewModel for State Management

For complex apps, managing UI state in a ViewModel ensures better separation of concerns. Here’s an example:

class MainViewModel : ViewModel() {
    private val _showTopBar = MutableStateFlow(true)
    val showTopBar: StateFlow<Boolean> = _showTopBar

    fun toggleTopBar() {
        _showTopBar.value = !_showTopBar.value
    }
}

@Composable
fun ViewModelDrivenTopBar(viewModel: MainViewModel = viewModel()) {
    val showTopBar by viewModel.showTopBar.collectAsState()

    Scaffold(
        topBar = {
            if (showTopBar) {
                TopAppBar(
                    title = { Text("ViewModel TopBar") },
                    actions = {
                        IconButton(onClick = { viewModel.toggleTopBar() }) {
                            Icon(Icons.Default.MoreVert, contentDescription = null)
                        }
                    }
                )
            }
        }
    ) { innerPadding ->
        // Content
    }
}

3. Ensure Smooth Animations

Compose’s animate*AsState APIs can help provide smooth transitions:

val topBarAlpha by animateFloatAsState(if (showTopBar.value) 1f else 0f)

TopAppBar(
    modifier = Modifier.alpha(topBarAlpha),
    title = { Text("Animated TopBar") }
)

Conclusion

Dynamically showing or hiding the TopBar in Jetpack Compose opens up new possibilities for building adaptive and user-friendly UIs. By understanding the interplay between state, scrolling behaviors, and animations, you can craft polished experiences that rival the best apps on the market.

Jetpack Compose’s declarative approach ensures that managing dynamic UI elements like the TopBar is not only intuitive but also highly efficient. As you implement these techniques, remember to profile and test your app’s performance to ensure a seamless user experience.

Happy coding!