Schema beibehalten

In diesem Rezept wird gezeigt, wie Werte für Elemente im Backstack beibehalten werden. Außerdem wird gezeigt, wie ein benutzerdefinierter NavEntryDecorator implementiert wird.

Funktionsweise

Wenn Sie „retain“ mit Nav 3 verwenden möchten, müssen Sie einen NavEntryDecorator definieren und installieren. Dieser Decorator verwaltet ein RetainedValuesStoreRegistry, das Ziele in ein eindeutiges RetainedValuesStore einbettet.

Der in diesem Beispiel implementierte Dekorator behält Werte so lange bei, wie sich das Element im Backstack befindet. Wenn ein Element entfernt und dann wieder hinzugefügt wird, erhält es neue beibehaltene Werte.

Weitere Informationen zu „retain“ finden Sie in der Dokumentation zu retain, RetainedValuesStore und RetainedValuesStoreRegistry.

Informationen zu NavEntryDecorator finden Sie in der offiziellen Dokumentation.

package com.example.nav3recipes.retain

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.material3.Button
import androidx.compose.material3.Text
import androidx.compose.runtime.Composable
import androidx.compose.runtime.remember
import androidx.compose.runtime.retain.RetainedValuesStore
import androidx.compose.runtime.retain.RetainedValuesStoreRegistry
import androidx.compose.runtime.retain.retain
import androidx.compose.runtime.retain.retainRetainedValuesStoreRegistry
import androidx.lifecycle.compose.dropUnlessResumed
import androidx.navigation3.runtime.NavEntry
import androidx.navigation3.runtime.NavEntryDecorator
import androidx.navigation3.runtime.NavKey
import androidx.navigation3.runtime.entryProvider
import androidx.navigation3.runtime.rememberNavBackStack
import androidx.navigation3.ui.NavDisplay
import com.example.nav3recipes.content.ContentBlue
import com.example.nav3recipes.content.ContentGreen
import com.example.nav3recipes.ui.setEdgeToEdgeConfig
import kotlinx.serialization.Serializable

@Serializable
private data object RouteA : NavKey

@Serializable
private data class RouteB(val id: Int) : NavKey

class RetainActivity : ComponentActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        setEdgeToEdgeConfig()
        super.onCreate(savedInstanceState)
        setContent {
            val backStack = rememberNavBackStack(RouteA)

            NavDisplay(
                backStack = backStack,
                onBack = { backStack.removeLastOrNull() },
                entryDecorators = listOf(
                    rememberRetainedValuesStoreNavEntryDecorator()
                ),
                entryProvider = entryProvider {
                    entry<RouteA> {
                        ContentGreen("Nav3 with retain support") {
                            Text("Retained value: ${retainValue()}")
                            Button(onClick = dropUnlessResumed {
                                backStack.add(RouteB(1))
                            }) {
                                Text("Click to navigate")
                            }
                        }
                    }
                    entry<RouteB> { key ->
                        ContentBlue("Route id: ${key.id}") {
                            Text("Retained value: ${retainValue()}")
                            Button(onClick = dropUnlessResumed {
                                backStack.add(RouteB(key.id + 1))
                            }) {
                                Text("Click to navigate")
                            }
                        }
                    }
                }
            )
        }
    }

    @Composable
    private fun retainValue() = retain { "Retained Value ${retainCount++}" }

    private companion object {
        // Counter to track how many values have been retained
        // Resets on process death.
        var retainCount = 0
    }
}

/**
 * Returns a [RetainedValuesStoreNavEntryDecorator] that is remembered across recompositions backed
 * by [registry].
 *
 * The underlying storage is controlled by the provided [registry]. By default, a new
 * [RetainedValuesStoreRegistry] is retained at this point in the composition hierarchy and will be
 * destroyed when the composition is permanently discarded or when the returned decorator is removed
 * from the composition hierarchy. If you need the backing storage of this decorator to have a
 * different lifespan, you can manually manage and provide a [RetainedValuesStoreRegistry] with the
 * intended lifespan.
 *
 * @param registry The underlying [RetainedValuesStoreRegistry] used to provide
 *   [RetainedValuesStore] instances to [NavEntries][NavEntry]. This instance should be retained to
 *   properly survive destruction and recreation scenarios.
 */
@Composable
fun <T : Any> rememberRetainedValuesStoreNavEntryDecorator(
    registry: RetainedValuesStoreRegistry = retainRetainedValuesStoreRegistry()
): RetainedValuesStoreNavEntryDecorator<T> {
    return remember(registry) {
        RetainedValuesStoreNavEntryDecorator(registry)
    }
}

/**
 * Provides the content of each [NavEntry] with a dedicated [RetainedValuesStore] so that each nav
 * entry may retain its own values.
 *
 * @param registry The underlying [RetainedValuesStoreRegistry] used to provide
 *   [RetainedValuesStore] instances to [NavEntries][NavEntry]. This instance should be retained to
 *   properly survive destruction and recreation scenarios.
 */
class RetainedValuesStoreNavEntryDecorator<T : Any>(
    registry: RetainedValuesStoreRegistry,
) : NavEntryDecorator<T>(
    onPop = { key ->
        registry.clearChild(key)
    },
    decorate = { entry ->
        registry.LocalRetainedValuesStoreProvider(entry.contentKey) { entry.Content() }
    },
)