HarmonyOS 全屏loading

可以使用应用子窗口功能，相关文档：https://developer.huawei.com/consumer/cn/doc/harmonyos-guides-V5/application-window-stage-V5

重点需要关注WindowStage的生命周期，在WindowStage生效后进行相关操作。

参考代码：

//EntryAbility.ets:

import AbilityConstant from '@ohos.app.ability.AbilityConstant';
import hilog from '@ohos.hilog';
import UIAbility from '@ohos.app.ability.UIAbility';
import Want from '@ohos.app.ability.Want';
import window from '@ohos.window';
import { BusinessError } from '@ohos.base';

export default class EntryAbility extends UIAbility {
  onCreate(want: Want, launchParam: AbilityConstant.LaunchParam) {
    hilog.info(0x0000, 'testTag', '%{public}s', 'Ability onCreate');
  }

  onDestroy() {
    hilog.info(0x0000, 'testTag', '%{public}s', 'Ability onDestroy');
  }

  onWindowStageCreate(windowStage: window.WindowStage) {
    // Main window is created, set main page for this ability
    hilog.info(0x0000, 'testTag', '%{public}s', 'Ability onWindowStageCreate');

    windowStage.loadContent('pages/Index', (err, data) => {
      if (err.code) {
        hilog.error(0x0000, 'testTag', 'Failed to load the content. Cause: %{public}s', JSON.stringify(err) ?? '');
        return;
      }
      //子窗口创建
      let promise=windowStage.createSubWindow("subWindow");
      promise.then((data)=>{
        globalThis.subWindow=data;
        globalThis.subWindow.setUIContent("pages/Page2")
        globalThis.subWindow.resize(800,600);
        globalThis.subWindow.moveWindowTo(50,150);
      })
        .catch((err:BusinessError)=>{
          console.error('Failed to create the subwindow. Cause: ' + JSON.stringify(err));
        })
      hilog.info(0x0000, 'testTag', 'Succeeded in loading the content. Data: %{public}s', JSON.stringify(data) ?? '');
    });
  }

  onWindowStageDestroy() {
    // Main window is destroyed, release UI related resources
    //子窗口销毁
    globalThis.subWindow.destroy();
    hilog.info(0x0000, 'testTag', '%{public}s', 'Ability onWindowStageDestroy');
  }

  onForeground() {
    // Ability has brought to foreground
    hilog.info(0x0000, 'testTag', '%{public}s', 'Ability onForeground');
  }

  onBackground() {
    // Ability has back to background
    hilog.info(0x0000, 'testTag', '%{public}s', 'Ability onBackground');
  }

  //Index.ets:

  import window from '@ohos.window'
  import { BusinessError } from '@ohos.base'

@Entry
@Component
struct Index {
  @State message: string = '-----------'
  //private windowClass:window.Window|null;

  build() {
    Row() {
      Column() {
        Text(this.message)
          .fontSize(50)
          .fontWeight(FontWeight.Bold)
        Button(){
          Text("实验")
        }
        .width(80)
        .height(60)
        .onClick(()=>{
          this.message="实验已开始"
          //窗口显示
          globalThis.subWindow.showWindow();
        })
      }
      .width('100%')
    }
    .height('100%')
  }
}
//Page2.ets:

import { BusinessError } from '@ohos.base'
import window from '@ohos.window'
import router from '@ohos.router'

@Entry
@Component
struct Page2 {
  @State message: string = '子窗口'

  build() {
    Row() {
      Column() {
        Text(this.message)
          .fontSize(30)
          .fontWeight(FontWeight.Bold)
        Button(){
          Text("隐藏")
        }
        .width(80)
        .height(60)
        .onClick(()=>{
          try {
            //子窗口隐藏
            globalThis.subWindow.hide();
          } catch (exception) {
            console.error('Failed to create the window. Cause: ' + JSON.stringify(exception));
          }
        })
      }
      .width('100%')
    }
    .height('100%')
    .backgroundColor(Color.Green)
  }
}