📱Android: NavBackStackEntry’s Lifecycle in Jetpack Compose Navigation 👾
⚙️Introduction
Hi! 👋This is Mike and today, I wanted to share about a concept that I learned a while ago. It`s NavBackStackEntry! As you know, Jetpack Compose has been rapidly evolving, introducing unique ways to manage UI in Android apps. With the Compose Navigation component, we can switch between screens seamlessly and gain access to each screen’s lifecycle state through NavBackStackEntry. If you’ve been working with scenarios like managing video playback, animations, or any other feature that depends on the page’s lifecycle, the NavBackStackEntry lifecycle can be a powerful tool. But you might be wondering….. What is NavBackStackEntry?
🔍 So what is NavBackStackEntry?
In Compose’s Navigation component, each screen or destination is represented by a NavBackStackEntry. This object carries useful information about the current state of the screen, including arguments, saved state, and its lifecycle. You can think of it as a lightweight object that tracks each screen in the navigation stack.
đź“ŚWhy Does the Lifecycle of NavBackStackEntry Matter?
Consider a scenario where you’re playing a video on a Compose screen. You’ll want the video to pause when the screen goes out of focus or stop when the user navigates away. Before Compose, managing these lifecycle events would require careful handling of Fragment or Activity lifecycle states. With Compose’s NavBackStackEntry lifecycle, each screen’s lifecycle can be handled more intuitively, enabling you to respond to lifecycle events for each navigation destination without complex workarounds.
🛠️Setting Up NavBackStackEntry’s Lifecycle
Let’s just see a sample of how the lifecycle works and how you can use it in your Compose navigation setup. First, each NavBackStackEntry has a private LifecycleRegistry to track its lifecycle state:
private var lifecycle = LifecycleRegistry(this)
public override fun getLifecycle(): Lifecycle {
return lifecycle
}The lifecycle state is then updated based on two main properties:
1. hostLifecycleState: This represents the lifecycle state of the hosting component, typically the Activity or Fragment that manages Compose’s navigation.
2. maxLifecycle: This is the maximum lifecycle state for each destination or screen.
🕹️Controlling the Lifecycle State with updateState()
The `updateState()` function determines the current lifecycle of the `NavBackStackEntry` by comparing `hostLifecycleState` and `maxLifecycle`, using the minimum of the two. Here’s how it works:
public fun updateState() {
lifecycle.currentState = if (hostLifecycleState.ordinal < maxLifecycle.ordinal) {
hostLifecycleState
} else {
maxLifecycle
}
}This ensures that the lifecycle state of each screen aligns with the hosting component and respects the navigation logic for each screen.
đź”—Lifecycle State Management: Practical Use Cases
1. Managing Video Playback
Let’s say you’re displaying a video on one of the Compose screens. When a user leaves the screen, you’ll want to pause the video automatically. Thanks to the NavBackStackEntry lifecycle, you can monitor when the screen goes from `RESUMED` to `STOPPED` and pause the video accordingly.
2. Reducing Resource Usage with Lifecycle-Aware UI Components
Many applications include UI components that consume resources, like network connections, timers, and animations. The `NavBackStackEntry` lifecycle can help you manage these components more efficiently by starting them only when the screen is active and pausing them when the screen goes to the background.
3. Saving User Data on Navigation
Another common use case is saving unsaved data when the user navigates away. For instance, if you’re implementing a form or text input, you can use the onStop or onDestroy lifecycle events to trigger a save function when the user leaves the screen, even if they didn’t explicitly press a save button.
Understanding Lifecycle Ordinals
The State enum manages the lifecycle levels. Here’s what they look like:
public enum State {
DESTROYED,
INITIALIZED,
CREATED,
STARTED,
RESUMED;
}With DESTROYED as the minimum and RESUMED as the maximum, the lifecycle state is determined by whichever hostLifecycleState or maxLifecycle is at the lower ordinal (or lifecycle level). This essentially means that the screen lifecycle respects both the host component and its lifecycle constraints.
Example: Applying Lifecycle in Compose
Imagine you have a navigation setup where you want to manage video playback on one of the screens. In your composable, you can observe NavBackStackEntry’s lifecycle and use it to control playback. Here’s a small code snippet that demonstrates how this might work:
@Composable
fun VideoScreen(backStackEntry: NavBackStackEntry) {
val lifecycle = backStackEntry.lifecycle
val lifecycleOwner = LifecycleOwner { lifecycle }DisposableEffect(lifecycleOwner) {
val observer = LifecycleEventObserver { _, event ->
if (event == Lifecycle.Event.ON_PAUSE) {
// Pause video playback
} else if (event == Lifecycle.Event.ON_RESUME) {
// Resume video playback
}
}
lifecycle.addObserver(observer)
onDispose {
lifecycle.removeObserver(observer)
}
}
}This code leverages LifecycleEventObserver to listen for lifecycle changes, allowing you to respond to lifecycle events and manage video playback smoothly.
Conclusion
The lifecycle in NavBackStackEntry is a subtle but powerful tool within Jetpack Compose Navigation. By allowing each navigation destination to respect both its own and the host’s lifecycle states, you can more efficiently manage UI components like video playback, animations, and data handling. The ability to have these controls at the NavBackStackEntry level lets you bring in better performance, resource management, and user experience to your Compose-based Android apps.
From pausing videos when leaving a screen to optimizing resource-heavy components, understanding how to utilize NavBackStackEntry’s lifecycle is essential for developing modern, responsive applications with Jetpack Compose. Anyway, that is about it! Thank you for reading and feel free to follow me for more tips and tricks!
