HarmonyOS 鸿蒙Next求助实现效果（的方法

import { common } from '[@kit](/user/kit).AbilityKit';
import { hilog } from '[@kit](/user/kit).PerformanceAnalysisKit';

[@ComponentV2](/user/ComponentV2)
[@Entry](/user/Entry)
struct TopIndex {
  private scroller: Scroller = new Scroller()
  [@Local](/user/Local) whiteOpacity: number = 0
  [@Local](/user/Local) isTop: boolean = false

  aboutToAppear(): void {
    let sta = getContext(this) as common.UIAbilityContext
    sta.windowStage.getMainWindowSync().setWindowLayoutFullScreen(true)
  }

  [@Builder](/user/Builder)
  BuilderBar() {
    Row() {
      Text("1")
      Text("2")
      Text("3")
      Text("4")
    }.height('44vp')
    .width('100%')
    .backgroundColor(Color.Gray)
    .justifyContent(FlexAlign.SpaceBetween)
  }

  build() {
    Stack() {
      Scroll(this.scroller) {
        Column() {
          Column() {
            Text("我是标题")
          }
          .height('200vp')
          .justifyContent(FlexAlign.End)
          .backgroundColor(Color.Green)
          .width('100%')

          Column() {
            this.BuilderBar()
          }
          .width('100%')
          .height('1440vp')
          .backgroundColor(Color.Blue)
        }.justifyContent(FlexAlign.Start)
      }.height('100%')
      .width('100%')
      .onDidScroll((xOffset: number, yOffset: number) => {
        hilog.debug(0x000000, 'rainrain', "offset " + yOffset)
        hilog.debug(0x000000, 'rainrain', "s " + this.scroller.currentOffset().yOffset)
        let o = this.scroller.currentOffset().yOffset / 140
        if (o > 1) {
          o = 1
        }
        this.isTop = this.scroller.currentOffset().yOffset >= 140
        if (this.whiteOpacity != o) {
          this.whiteOpacity = o
        }
      })

      Column() {
        Text("我是标题").visibility(this.isTop ? Visibility.Visible : Visibility.None)
      }
      .height('64vp')
      .alignItems(HorizontalAlign.Start)
      .justifyContent(FlexAlign.End)
      .backgroundColor(Color.White)
      .width('100%')
      .opacity(this.whiteOpacity)

      Column() {
        this.BuilderBar()
      }.margin({ top: '64vp' })
      .visibility(this.isTop ? Visibility.Visible : Visibility.None)
    }.alignContent(Alignment.Top)
  }
}