HarmonyOS 父组件如何调用子组件的方法？

@Component  
struct Child {  
  @State private text: string = '初始值'  
  private controller: ChildController = new ChildController();  
  
  aboutToAppear() {  
    if(this.controller) {  
      this.controller.changeText = this.changeText  
    }  
    console.log('aaa')  
  }  
  
  private changeText = (value: string) =>{  
    this.text = value  
    console.log('bbb')  
  }  
  
  build() {  
    Column() {  
      Text(this.text)  
    }  
  }  
}  
  
class ChildController {  
  changeText = (value: string) => {  
    console.log('11111')  
  }  
}  
  
export let ChildRef = new ChildController()  
  
@Entry  
@Component  
struct Parent {  
  // ChildRef = new ChildController()  
  @State noShow: boolean = false  
  
  build() {  
    Column() {  
      Text('获取Child的exposeMethods!').fontSize('18vp').fontColor(Color.Gray)  
      Divider()  
      Child({ controller: ChildRef })  
      Child()  
      Button('Parent调用childer的changeText').onClick(() => {  
        ChildRef.changeText('Parent调用childer的changeText')  
      })  
    }  
  }  
}