基本的可保存食谱

此方案展示了一个基本示例，说明如何创建在配置更改后仍能保留的持久返回堆栈。

工作原理

为了使返回堆栈保持持久状态，我们使用了 rememberNavBackStack 函数。此函数用于创建并记住配置更改（例如屏幕旋转）后的返回堆栈。

使用 rememberNavBackStack 的前提条件是导航键（路由）必须可序列化。在此示例中，RouteA 和 RouteB 附有 @Serializable 注释，并实现了 NavKey 接口。

此示例在 entryProvider 中使用 when 语句将路线映射到其对应的可组合项，但它也可以与 entryProvider DSL 搭配使用。

/*
 * Copyright 2025 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.
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package com.example.nav3recipes.basicsaveable

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.lifecycle.compose.dropUnlessResumed
import androidx.navigation3.runtime.NavEntry
import androidx.navigation3.runtime.NavKey
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: String) : NavKey

class BasicSaveableActivity : ComponentActivity() {

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

                        else -> {
                            error("Unknown route: $key")
                        }
                    }
                }
            )
        }
    }
}
BasicSaveableActivity.kt