求大佬告知$$的用途是什么

@Entry 
@Component 
struct bindPopupPage { 
  @State customPopup: boolean = false; 
 
  build() { 
    Column() { 
      Button('Popup').margin(20).onClick(() => { 
        this.customPopup = !this.customPopup 
      }).bindPopup($$this.customPopup, { message: 'showPopup' }) 
    } 
  } 
}
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