Is SavedStateHandle in ViewModel and currentBackStackEntry the same?

Is SavedStateHandle in ViewModel and currentBackStackEntry the same?

In Jetpack Compose, managing UI state is essential for maintaining the user experience. Two key components that play a crucial role in this process are the SavedStateHandle and the currentBackStackEntry. While both are related to state management, they differ in their scope and purpose.

SavedStateHandle

The SavedStateHandle is a class in ViewModel that provides a mechanism for saving and restoring UI state. It’s designed to persist state across configuration changes such as screen rotation or process death.

Key Features

• State Persistence: Saves and restores data when the ViewModel is recreated due to configuration changes.
• Scoped to ViewModel: The SavedStateHandle is associated with a specific ViewModel instance.
• Data Type Flexibility: Can store various data types, including primitives, objects, and collections.

currentBackStackEntry

The currentBackStackEntry is a property of the NavBackStackEntry class, which represents an entry in the navigation stack. It provides access to the current navigation state.

Key Features

• Navigation State: Stores information about the currently active destination in the navigation graph.
• Scoped to Navigation Graph: The currentBackStackEntry is associated with the current navigation graph.
• Lifecycle Events: Provides lifecycle events for handling navigation changes.

Comparison

When to Use Each

• SavedStateHandle: Use for persisting UI state across configuration changes within the context of a specific ViewModel.
• currentBackStackEntry: Use for managing navigation state, handling navigation transitions, and accessing information about the current destination.

Example

Using SavedStateHandle

import androidx.compose.runtime.Composable
import androidx.compose.ui.Modifier
import androidx.lifecycle.viewmodel.compose.viewModel
import androidx.compose.material.Text
import androidx.compose.material.TextField
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.setValue
import androidx.compose.ui.tooling.preview.Preview
import androidx.lifecycle.SavedStateHandle

@Composable
fun SavedStateHandleExample() {
    val viewModel: MainViewModel = viewModel()
    var text by remember { mutableStateOf("") }
    
    Column {
        Text("Current Text: ${viewModel.text}")
        TextField(
            value = text,
            onValueChange = { text = it },
            modifier = Modifier.fillMaxWidth()
        )
    }
}

class MainViewModel(private val savedStateHandle: SavedStateHandle) : ViewModel() {
    var text by savedStateHandle.mutableStateOf("")
}

@Preview(showBackground = true)
@Composable
fun DefaultPreview() {
    SavedStateHandleExample()
}

Using currentBackStackEntry

import androidx.compose.foundation.layout.Column
import androidx.compose.material.Button
import androidx.compose.material.Text
import androidx.compose.runtime.Composable
import androidx.compose.ui.Modifier
import androidx.compose.ui.tooling.preview.Preview
import androidx.navigation.NavController
import androidx.navigation.NavHostController
import androidx.navigation.compose.NavHost
import androidx.navigation.compose.composable
import androidx.navigation.compose.rememberNavController

@Composable
fun NavigationExample() {
    val navController = rememberNavController()
    NavHost(navController = navController, startDestination = "screen1") {
        composable("screen1") {
            Screen1(navController)
        }
        composable("screen2") {
            Screen2(navController)
        }
    }
}

@Composable
fun Screen1(navController: NavHostController) {
    Column {
        Text("Screen 1")
        Button(onClick = { navController.navigate("screen2") }) {
            Text("Navigate to Screen 2")
        }
    }
}

@Composable
fun Screen2(navController: NavHostController) {
    Column {
        Text("Screen 2")
        Button(onClick = { navController.popBackStack() }) {
            Text("Go Back")
        }
    }
}

@Preview(showBackground = true)
@Composable
fun DefaultPreview() {
    NavigationExample()
}

Conclusion

While SavedStateHandle and currentBackStackEntry are both valuable tools for managing state in Jetpack Compose, they serve distinct purposes. Choosing the right tool depends on your specific needs. SavedStateHandle is ideal for persisting UI state across configuration changes, while currentBackStackEntry is well-suited for managing navigation state and handling navigation events.