Navigation在分栏模式下，如何给右侧空白部分设置默认页面

问题现象
Navigation在Split模式下，未点击左侧导航栏时，右侧子页显示区显示为空白。当未进行页面路由推送时，如何给右侧的空白部分设置默认展示页面？

背景知识
Navigation组件的分栏模式由mode属性控制，包括单栏（Stack）、分栏（Split）和自适应（Auto）三个属性。该属性默认为Auto模式，在该模式下会自动监听屏幕属性，当为折叠屏或平板时，默认分栏显示，在折叠状态或普通手机时可为单栏显示。
splitPlaceholder：在API20下，Navigation双栏模式支持设置右侧页面显示默认占位页，占位页仅作为UI展示页，不可获焦和响应事件。
解决方案
方案一：push方法推送默认页面。
编写默认页面，并重写onBackPressed返回，当在该页面返回时直接退出软件。
Navigation主页执行aboutToAppear，将默认页面加入页面栈进行默认显示。
import { window } from '@kit.ArkUI';

@Entry
@Component
struct NavigationExample {
@Provide('pageInfos') pageInfos: NavPathStack = new NavPathStack();
private arr: number[] = [1, 2, 3];

aboutToAppear(): void {
this.pageInfos.pushPath({ name: 'DefaultPage' });
}

@Builder
PageMap(name: string) {
if (name === 'NavDestinationTitle1') {
pageOneTmp();
} else if (name === 'NavDestinationTitle2') {
pageTwoTmp();
} else if (name === 'NavDestinationTitle3') {
pageThreeTmp();
} else if (name === 'DefaultPage') {
DefaultPage();
}
}

build() {
Column() {
Navigation(this.pageInfos) {
TextInput({ placeholder: 'search...' })
.width('90%')
.height(40)
.backgroundColor('#FFFFFF');
List({ space: 12 }) {
ForEach(this.arr, (item: number) => {
ListItem() {
Text('Page' + item)
.width('100%')
.height(72)
.backgroundColor('#FFFFFF')
.borderRadius(24)
.fontSize(16)
.fontWeight(500)
.textAlign(TextAlign.Center)
.onClick(() => {
this.pageInfos.pushPath({ name: 'NavDestinationTitle' + item });
});
};
}, (item: number) => item.toString());
}
.width('90%')
.margin({ top: 12 });
}
.title('主标题')
.mode(NavigationMode.Split)
.navDestination(this.PageMap);
}
.height('100%')
.width('100%')
.background('#F1F3F5');
}
}

// PageOne页面
@Component
export struct pageOneTmp {
@Consume('pageInfos') pageInfos: NavPathStack;

build() {
NavDestination() {
Column() {
Text('NavDestinationContent1');
}.width('100%').height('100%').justifyContent(FlexAlign.Center);
}.title('NavDestinationTitle1');
}
}

// PageTwo页面
@Component
export struct pageTwoTmp {
@Consume('pageInfos') pageInfos: NavPathStack;

build() {
NavDestination() {
Column() {
Text('NavDestinationContent2');
}.width('100%').height('100%').justifyContent(FlexAlign.Center);
}.title('NavDestinationTitle2');
}
}

// PageThree页面
@Component
export struct pageThreeTmp {
@Consume('pageInfos') pageInfos: NavPathStack;

build() {
NavDestination() {
Column() {
Text('NavDestinationContent3');
}.width('100%').height('100%').justifyContent(FlexAlign.Center);
}.title('NavDestinationTitle3');
}
}
@Component
export struct DefaultPage {
@Consume('pageInfos') pageInfos: NavPathStack;

build() {
NavDestination() {
Column() {
Text('DefaultPage');
}.width('100%').height('100%').justifyContent(FlexAlign.Center);
}.title('DefaultPage')
.onBackPressed(() => {
// 返回true表示自定义返回，能避免返回空白页面。返回false则表示系统默认返回，会返回空白页面。
try {
// 需要导入window
window.getLastWindow(this.getUIContext().getHostContext(), (err, win) => {
const errCode: number = err.code;
if (errCode) {
console.error(Failed to obtain the top window. Cause code: ${err.code}, message: ${err.message});
return;
}
win.minimize((err) => {
const errCode: number = err.code;
if (errCode) {
console.error(Failed to minimize the window. Cause code: ${err.code}, message: ${err.message});
return;
}
console.info('Succeeded in minimizing the window.');
});
});
} catch (exception) {
console.error(Failed to obtain the top window. Cause code: ${exception.code}, message: ${exception.message});
}
return true;
});
}
}
实现效果如下：

点击放大

方案二：采用API20中的splitPlaceholder属性。
详情参考：示例14。

常见FAQ
Q：API20以下，当采用Auto模式时，如何判断是否需要推送默认显示页面？

A：可以通过以下方式在aboutToAppear中判断是否需要推送默认页面：

通过设备信息@ohos.deviceInfo接口获取设备的deviceType类型。
通过屏幕属性@ohos.display的isFoldable方法返回当前设备是否可折叠的结果。
总结
实现默认展示页面的方式为，创建一个默认页面，在创建主页时自动推送该默认页面即可，其他关联实现方式参照常见FAQ实现即可。
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