HarmonyOS 组件如何感知app前后台切换

//UIAbility中  
...  
onWindowStageCreate(windowStage: window.WindowStage) {...  
windowStage.loadContent('pages/Page1', (err, data) => {  
AppStorage.setOrCreate<boolean>('video',true)}  
...  
  
//Page页面中  
@Entry  
@Component  
struct Page1 {  
@State message: string = 'Hello World'  
@StorageLink('video') isOnForeground:boolean = true  
build() {  
Row() {  
Column() {  
Text(this.message)  
.fontSize(50)  
.fontWeight(FontWeight.Bold)  
Vid({isOnForeground:this.isOnForeground})  
}  
.width('100%')  
}  
.height('100%')  
}  
}  
  
@Component  
struct Vid {  
  @Watch('change')@Link isOnForeground:boolean  
  @State message: string = 'video'  
  build() {  
    Text("message")  
      .fontSize(50)  
      .fontWeight(FontWeight.Bold)  
      .onClick(()=>{  
        this.message += this .isOnForeground  
        console.log(""+this.isOnForeground)  
      })  
  }  
  change() {  
    if (this.isOnForeground) {  
      console.log('组件在前台')  
    } else {  
      console.log('组件在后台')  
    }  
  }  
}