Ricetta Parcelable di base

Questa ricetta mostra un esempio di base di come creare uno stack di back persistente che sopravvive alle modifiche di configurazione, senza utilizzare kotlinx.serialization. Utilizza invece Parcelable di Android e il plug-in kotlin-parcelize per salvare e ripristinare lo stato di navigazione.

Come funziona

In questo esempio, RouteA e RouteB implementano un'interfaccia di marcatore, Route, che a sua volta estende Parcelable. Sono anche annotate con @Parcelize dal plug-in kotlin-parcelize, che genera automaticamente un'implementazione Parcelable:

sealed interface Route : Parcelable

@Parcelize
data object RouteA : Route

@Parcelize
data class RouteB(val id: String) : Route

Per rendere persistente lo stack di back, questa ricetta definisce la funzione rememberParcelableBackStack. Per garantire che NavDisplay e altri composable siano a conoscenza delle modifiche allo stack di back, quest'ultimo viene archiviato in un SnapshotStateList.

@Composable
fun <T : Parcelable> rememberParcelableBackStack(vararg elements: T): SnapshotStateList<T> {
    return rememberSaveable {
        mutableStateListOf(*elements)
    }
}

Questo funge da alternativa a rememberNavBackStack integrato da androidx.navigation3.runtime, che si basa su kotlinx.serialization. Utilizza questo approccio se la tua applicazione preferisce rigorosamente Parcelable ed evita di dipendere da kotlinx.serialization.

/*
 * Copyright 2026 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.example.nav3recipes.basicparcelable

import android.os.Bundle
import android.os.Parcelable
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.mutableStateListOf
import androidx.compose.runtime.saveable.rememberSaveable
import androidx.compose.runtime.snapshots.SnapshotStateList
import androidx.lifecycle.compose.dropUnlessResumed
import androidx.navigation3.runtime.NavEntry
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.parcelize.Parcelize

sealed interface Route : Parcelable

@Parcelize
data object RouteA : Route

@Parcelize
data class RouteB(val id: String) : Route

/**
 * Creates and remembers a [SnapshotStateList] to hold a back stack of [Parcelable] routes
 * that survives configuration changes and process death.
 *
 * @param T The route type, which must implement [Parcelable].
 * @param elements The initial routes to populate the back stack.
 * @return A reactive [SnapshotStateList] managing the navigation back stack.
 */
@Composable
fun <T : Parcelable> rememberParcelableBackStack(vararg elements: T): SnapshotStateList<T> {
    return rememberSaveable {
        mutableStateListOf(*elements)
    }
}

class BasicParcelableActivity : ComponentActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        setEdgeToEdgeConfig()
        super.onCreate(savedInstanceState)
        setContent {
            val backStack = rememberParcelableBackStack<Route>(RouteA)

            NavDisplay(
                backStack = backStack,
                onBack = { backStack.removeLastOrNull() },
                entryProvider = { key ->
                    when (key) {
                        is RouteA -> NavEntry(key) {
                            ContentGreen("Welcome to Nav3") {
                                Button(onClick = dropUnlessResumed {
                                    backStack.add(RouteB("123"))
                                }) {
                                    Text("Click to navigate")
                                }
                            }
                        }

                        is RouteB -> NavEntry(key) {
                            ContentBlue("Route id: ${key.id} ")
                        }
                    }
                }
            )
        }
    }
}

BasicParcelableActivity.kt