Component如何监听应用前后台切换

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') 
} 
} 
}