HarmonyOS 滑动切换（Tabs ）如何获取偏移量

//示例代码 第一部分  
@Entry  
@Component  
struct tabTest {  
  @State tabArray: Array<number> = [0, 1, 2, 3,4]  
  @State focusIndex: number = 0  
  @State pre: number = 0  
  @State index: number = 0  
  private controller: TabsController = new TabsController()  
  @State test: boolean = false  
  @State animationDuration: number = 300  
  @State indicatorLeftMargin: number = 0  
  @State indicatorWidth: number = 0  
  private tabsWidth: number = 0  
  private tabWidth: number = 0;  
  private scrollerForScroll: Scroller = new Scroller()  
  
  // 单独的页签  
  @Builder  
  Tab(tabName: string, tabItem: number, tabIndex: number) {  
    Row({ space: 20 }) {  
      Text(tabName)  
        .fontSize(18)  
        .fontColor(tabIndex === this.focusIndex ? Color.Blue :Color.Black)  
        .id(tabIndex.toString())  
        .onAreaChange((oldValue: Area,newValue: Area) => {  
          if (this.focusIndex === tabIndex && (this.indicatorLeftMargin === 0 || this.indicatorWidth === 0)){  
            if (newValue.position.x != undefined) {  
              let positionX = Number.parseFloat(newValue.position.x.toString())  
              this.indicatorLeftMargin = Number.isNaN(positionX) ? 0 : positionX  
            }  
            let width = Number.parseFloat(newValue.width.toString())  
            this.tabWidth = Number.isNaN(width) ? 0 : width  
            this.indicatorWidth = this.tabWidth  
          }  
        })  
    }  
    .justifyContent(FlexAlign.Center)  
    .constraintSize({ minWidth: 35 })  
    .width(100)  
    .height(30)  
    .onClick(() => {  
      this.controller.changeIndex(tabIndex)  
      this.focusIndex = tabIndex  
    })  
    .backgroundColor("#ffb7b7b7")  
  }  
  //示例代码 第2部分  
  @Builder  
  textTest(textName:string){  
    Row({ space: 20 }) {  
      Text(textName).fontSize(18)  
    }  
    .justifyContent(FlexAlign.Center)  
    .constraintSize({ minWidth: 35 })  
    .height(30)  
    .backgroundColor("#ffb7b7b7")  
  }  
  
  build() {  
    Column() {  
      Stack({ alignContent: Alignment.TopStart }) {  
        Column() {  
          // 页签  
          Row({ space: 8 }) {  
            List({ space: 20, initialIndex: 0, scroller: this.scrollerForScroll }) {  
              ForEach(this.tabArray, (item: number, index: number) => {  
                ListItem() {  
                  this.Tab("页签 " + item, item, index)  
                }  
              }, (item: string) => item)  
            }  
            .listDirection(Axis.Horizontal)  
            .height(30)  
            .width('80%')  
            .friction(0.6)  
            .alignListItem(ListItemAlign.Start)  
            .scrollBar(BarState.Off)  
            .width('80%')  
            .backgroundColor("#ffb7b7b7")  
            .onScroll((xOffset: number, yOffset: number) => {  
              this.indicatorLeftMargin -= xOffset  
            })  
            this.textTest('更多')  
  
          }  
          .alignItems(VerticalAlign.Bottom)  
          .width('100%')  
          .backgroundColor("#ffb7b7b7")  
        }  
        //示例代码 第3部分  
        .alignItems(HorizontalAlign.Start)  
        .width('100%')  
  
        Column()  
          .height(2)  
          .width(this.indicatorWidth)  
          .margin({ left: this.indicatorLeftMargin, top:30})  
          .backgroundColor(Color.Blue)  
        Column()  
          .height(10)  
          .width("20%")  
          .margin({ left: '80%', top:28})  
          .backgroundColor("#ffb7b7b7")  
      }  
      .height(40)  
      .width('100%')  
      .backgroundColor("#ffb7b7b7")  
      //tabs  
      Tabs({ barPosition: BarPosition.Start, controller: this.controller }) {  
        ForEach(this.tabArray, (item: number, index: number) => {  
          TabContent() {  
            Text('我是页面 ' + item + " 的内容")  
              .height(300)  
              .width('100%')  
              .fontSize(30)  
          }  
          .backgroundColor(Color.White)  
        }, (item: string) => item)  
      }  
      .onAreaChange((oldValue: Area,newValue: Area)=> {  
        let width = Number.parseFloat(newValue.width.toString())  
        this.tabsWidth = Number.isNaN(width) ? 0 : width  
      })  
      .width('100%')  
      .barHeight(0)  
      .animationDuration(100)  
      .onChange((index: number) => {  
        console.log('foo change')  
        this.focusIndex = index  
        this.scrollerForScroll.scrollToIndex(index-1,true)  
      })  
      //示例代码 第4部分  
      .onAnimationStart((index: number, targetIndex: number, event: TabsAnimationEvent) => {  
        // 切换动画开始时触发该回调。下划线跟着页面一起滑动  
        this.focusIndex = targetIndex  
        let targetIndexInfo = this.getTextInfo(targetIndex)  
        this.startAnimateTo(this.animationDuration, targetIndexInfo.left, targetIndexInfo.width)  
      })  
      .onAnimationEnd((index: number,event: TabsAnimationEvent) => {  
        // 切换动画结束时触发该回调。下划线动画停止。  
        let currentIndicatorInfo = this.getCurrentIndicatorInfo(index,event)  
        this.startAnimateTo(0,currentIndicatorInfo.left,currentIndicatorInfo.width)  
      })  
      .onGestureSwipe((index: number,event: TabsAnimationEvent) => {  
        // 在页面跟手滑动过程中，逐帧触发该回调。  
        let currentIndicatorInfo = this.getCurrentIndicatorInfo(index,event)  
        this.focusIndex = currentIndicatorInfo.index  
        this.indicatorLeftMargin = currentIndicatorInfo.left  
        this.tabWidth = currentIndicatorInfo.width  
        this.indicatorWidth = currentIndicatorInfo.width  
      })  
    }  
    .height('100%')  
  }  
  
  private getTextInfo(index: number): Record<string, number> {  
    let strJson = getInspectorByKey(index.toString())  
    try {  
      let obj: Record<string, string> = JSON.parse(strJson)  
      let rectInfo: number[][] = JSON.parse('[' + obj.$rect + ']')  
      return { 'left': px2vp(rectInfo[0][0]), 'width': px2vp(rectInfo[1][0] - rectInfo[0][0]) }  
    } catch (error) {  
      return { 'left': 0, 'width': 0 }  
    }  
  }  
  //示例代码 第5部分  
  private getCurrentIndicatorInfo(index: number, event: TabsAnimationEvent): Record<string, number> {  
    let nextIndex = index  
    if (index > 0 && event.currentOffset > 0) {  
      nextIndex--  
    } else if (index < 4 && event.currentOffset < 0) {  
      nextIndex++  
    }  
    let indexInfo = this.getTextInfo(index)  
    let nextIndexInfo = this.getTextInfo(nextIndex)  
    let swipeRatio = Math.abs(event.currentOffset / this.tabsWidth)  
    let currentIndex = swipeRatio > 0.5 ? nextIndex : index // 页面滑动超过一半，tabBar切换到下一页。  
    let currentLeft = indexInfo.left + (nextIndexInfo.left - indexInfo.left) * swipeRatio  
    let currentWidth = indexInfo.width + (nextIndexInfo.width - indexInfo.width) * swipeRatio  
    return { 'index': currentIndex, 'left': currentLeft, 'width': currentWidth }  
  }  
  
  private startAnimateTo(duration: number, leftMargin: number, width: number) {  
    animateTo({  
      duration: duration, // 动画时长  
      curve: Curve.Linear, // 动画曲线  
      iterations: 1, // 播放次数  
      playMode: PlayMode.Normal, // 动画模式  
      onFinish: () => {  
        console.info('play end')  
      }  
    }, () => {  
      this.indicatorLeftMargin = leftMargin  
      this.tabWidth = width  
      this.indicatorWidth = width  
    })  
  }  
}