如何解决NavDestination切换页面后浏览位置无法保存问题

问题现象
在HarmonyOS中，使用NavDestination实现页面跳转至其他页面后，下次进入如何保持跳转前页面的浏览位置？

背景知识
Navigation：Navigation组件是路由导航的根视图容器，一般作为Page页面的根容器使用，其内部默认包含了标题栏、内容区和工具栏，其中内容区默认首页显示导航内容（NavDestination的子组件）或非首页显示（NavDestination的子组件），首页和非首页通过路由进行切换。
@ohos.arkui.observer(无感监听)提供UI组件行为变化的无感监听能力。可以监听Navigation的页面切换事件进行相应操作。
Scroll是一种可滚动的容器组件，当子组件的布局尺寸超过父组件的尺寸时，内容可以滚动。
onDidScroll方法在Scroll滚动时触发，可用于在滑动过程中获取Scroll组件的偏移量yOffset。
scrollTo方法可用于让Scroll组件滑动到指定位置。
AppStorage是应用全局的UI状态存储，和应用的进程绑定，只能在UI主线程中使用，无法在子线程中使用、修改。
解决方案
方案一：使用路由模式保留页面实例。
在页面跳转时不要使用pop和clear等方法，否则NavDestination页面会被回收。可以采用单例模式MOVE_TO_TOP_SINGLETON跳转至指定的页面，该方式会使用栈内已存在的页面实例（即浏览位置不变）。

由于单例模式是从栈底到栈顶依次查找，当栈内存在多个同名页面实例时，会默认跳转最底层的同名页面实例，所以需确保栈内只存在一个同名实例，否则跳转的页面保留的滚动位置与上一次显示的页面会不一致（显示的是栈底同名实例保留的滚动位置）。

import { PageOneS1Builder } from './PageOneS1';
import { PageTwoS1Builder } from './PageTwoS1';

@Entry
@Component
struct Solution1 {
private pathStack: NavPathStack = new NavPathStack();

aboutToAppear(): void {
this.pathStack.pushPath({ name: 'PageOneS1' }); // 推送第一个子页作为首页
}

@Builder
pageMap(name: string) {
if (name === 'PageOneS1') {
PageOneS1Builder();
} else if (name === 'PageTwoS1') {
PageTwoS1Builder();
}
}

build() {
Navigation(this.pathStack) {
}
.navDestination(this.pageMap)
.width('100%')
.height('100%')
.hideNavBar(true);
}
}
@Builder
export function PageOneS1Builder() {
PageOneS1();
}

@Component
struct PageOneS1 {
private pathStack: NavPathStack = new NavPathStack();
private arr: number[] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15];

build() {
NavDestination() {
Column({ space: 20 }) {
Row() {
Button('跳转页面')
.margin({ top: 20 })
.onClick(() => {
this.pathStack.pushPathByName('PageTwoS1', null, false);
});
};

    List({ space: 16 }) {  
      ForEach(this.arr, (item: number) => {  
        ListItem() {  
          Text(item.toString())  
            .width('100%')  
            .height(100)  
            .fontSize(16)  
            .textAlign(TextAlign.Center)  
            .borderRadius(16)  
            .backgroundColor('#F1F3F5');  
        }.width('100%');  
      }, (item: string) => item);  
    }.padding({ left: 12, right: 12 })  
    .expandSafeArea([SafeAreaType.SYSTEM], [SafeAreaEdge.BOTTOM])  
    .alignListItem(ListItemAlign.Center)  
    .height('90%');  
  };  
}.width('100%').height('100%')  
.onReady((context: NavDestinationContext) => {  
  this.pathStack = context.pathStack;  
});  

}
}
@Builder
export function PageTwoS1Builder() {
PageTwoS1();
}

@Component
struct PageTwoS1 {
pathStack: NavPathStack = new NavPathStack();

build() {
NavDestination() {
Column({ space: 20 }) {
Button('单例跳转')
.width('30%')
.margin('20vp')
.onClick(() => {
// 或者使用单例模式跳转回其它页面
this.pathStack.pushPath({ name: 'PageOneS1' }, { launchMode: LaunchMode.MOVE_TO_TOP_SINGLETON });
});
}.width('100%').height('100%');
}
.onReady((context: NavDestinationContext) => {
this.pathStack = context.pathStack;
});
}
}
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