页面和列表嵌套滚动，实现列表吸顶

enum ScrollPosition { 
  start, 
  center, 
  end 
} 
 
@Entry 
@Component 
struct NestedScroll { 
  @State listPosition: number = ScrollPosition.start; // 0代表滚动到List顶部，1代表中间值，2代表滚动到List底部。 
  @State scrollPosition: number = ScrollPosition.start; // 0代表滚动到页面顶部，1代表中间值，2代表滚动到页面底部。 
  @State showTitle: boolean = false; 
  @State currentYOffset: number = 0; 
  private arr: number[] = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]; 
  private scrollerForScroll: Scroller = new Scroller(); 
  private scrollerForList: Scroller = new Scroller(); 
 
  build() { 
    Stack({ alignContent: Alignment.Top }) { 
      Scroll(this.scrollerForScroll) { 
        Column() { 
          Image($r('app.media.cat')) 
            .width("100%") 
            .height("40%") 
 
          Tabs({ barPosition: BarPosition.Start }) { 
            TabContent() { 
              List({ space: 10, scroller: this.scrollerForList }) { 
                ForEach(this.arr, (item) => { 
                  ListItem() { 
                    Text("ListItem" + item) 
                      .width("100%") 
                      .height("100%") 
                      .borderRadius(15) 
                      .fontSize(24) 
                      .textAlign(TextAlign.Center) 
                      .backgroundColor(Color.White) 
                  }.width("100%").height(100) 
                }, item => item) 
              } 
              .padding({ left: 10, right: 10 }) 
              .width("100%") 
              .edgeEffect(EdgeEffect.None) 
              .scrollBar(BarState.Off) 
              .onReachStart(() => { 
                this.listPosition = ScrollPosition.start 
              }) 
              .onReachEnd(() => { 
                this.listPosition = ScrollPosition.end 
              }) 
              .onScrollFrameBegin((offset: number, state: ScrollState) => { 
                // 滑动到列表中间时 
                if (!((this.listPosition == ScrollPosition.start && offset < 0) 
                  || (this.listPosition == ScrollPosition.end && offset > 0))) { 
                  this.listPosition = ScrollPosition.center 
                } 
 
                // 如果页面已滚动到底部，列表不在顶部或列表有正向偏移量 
                if (this.scrollPosition == ScrollPosition.end 
                  && (this.listPosition != ScrollPosition.start || offset > 0)) { 
                  return { offsetRemain: offset }; 
                } else { 
                  this.scrollerForScroll.scrollBy(0, offset) 
                  return { offsetRemain: 0 }; 
                } 
              }) 
            }.tabBar('促销活动') 
 
            TabContent() { 
              Column().width('100%').height('100%').backgroundColor('#007DFF') 
            }.tabBar('行程服务') 
 
          } 
          .vertical(false) 
          .barMode(BarMode.Fixed) 
          .barWidth(360) 
          .barHeight(56) 
          .width("100%") 
          .height("92%") 
          .backgroundColor('#F1F3F5') 
        } 
      } 
      .scrollBar(BarState.Off) 
      .width("100%") 
      .height("100%") 
      .onScroll((xOffset: number, yOffset: number) => { 
        this.currentYOffset = this.scrollerForScroll.currentOffset().yOffset; 
 
        // 非（页面在顶部或页面在底部），则页面在中间 
        if (!((this.scrollPosition == ScrollPosition.start && yOffset < 0) 
          || (this.scrollPosition == ScrollPosition.end && yOffset > 0))) { 
          this.scrollPosition = ScrollPosition.center 
        } 
      }) 
      .onScrollEdge((side: Edge) => { 
        if (side == Edge.Top) { 
          // 页面在顶部 
          this.scrollPosition = ScrollPosition.start 
        } else if (side == Edge.Bottom) { 
          // 页面在底部 
          this.scrollPosition = ScrollPosition.end 
        } 
      }) 
      .onScrollFrameBegin(offset => { 
        if (this.scrollPosition == ScrollPosition.end) { 
          return { offsetRemain: 0 }; 
        } else { 
          return { offsetRemain: offset }; 
        } 
      }) 
 
      if (this.currentYOffset >= 0) { 
        Row() { 
          Text('发现').fontSize(24) 
        } 
        .justifyContent(FlexAlign.Center) 
        .backgroundColor(Color.White) 
        .width('100%') 
        .height('8%') 
        .opacity(this.currentYOffset / 50 > 1 ? 1 : this.currentYOffset / 50) 
      } 
    } 
    .width('100%') 
    .height('100%') 
    .backgroundColor(0xDCDCDC) 
  } 
}