HarmonyOS Waterflow需要增加能力，支持某些元素强制展示在某一列上

参考示例如下：

class BasicDataSource implements IDataSource {
  private listeners: DataChangeListener[] = [];
  private originDataArray: string[] = [];

  public totalCount(): number {
    return 0;
  }

  public getData(index: number): string {
    return this.originDataArray[index];
  }

  // 该方法为框架侧调用，为LazyForEach组件向其数据源处添加listener监听
  registerDataChangeListener(listener: DataChangeListener): void {
    if (this.listeners.indexOf(listener) < 0) {
      console.info('add listener');
      this.listeners.push(listener);
    }
  }

  // 该方法为框架侧调用，为对应的LazyForEach组件在数据源处去除listener监听
  unregisterDataChangeListener(listener: DataChangeListener): void {
    const pos = this.listeners.indexOf(listener);
    if (pos >= 0) {
      console.info('remove listener');
      this.listeners.splice(pos, 1);
    }
  }

  // 通知LazyForEach组件需要重载所有子组件
  notifyDataReload(): void {
    this.listeners.forEach(listener => {
      listener.onDataReloaded();
    })
  }

  // 通知LazyForEach组件需要在index对应索引处添加子组件
  notifyDataAdd(index: number): void {
    this.listeners.forEach(listener => {
      listener.onDataAdd(index);
    })
  }

  // 通知LazyForEach组件在index对应索引处数据有变化，需要重建该子组件
  notifyDataChange(index: number): void {
    this.listeners.forEach(listener => {
      listener.onDataChange(index);
    })
  }

  // 通知LazyForEach组件需要在index对应索引处删除该子组件
  notifyDataDelete(index: number): void {
    this.listeners.forEach(listener => {
      listener.onDataDelete(index);
    })
  }

  // 通知LazyForEach组件将from索引和to索引处的子组件进行交换
  notifyDataMove(from: number, to: number): void {
    this.listeners.forEach(listener => {
      listener.onDataMove(from, to);
    })
  }
}

class MyDataSource extends BasicDataSource {
  private dataArray: string[] = [];
  private cursorOffset: number = 0;

  public totalCount(): number {
    return this.dataArray.length - this.cursorOffset;
  }

  public getData(index: number): string {
    return this.dataArray[index + this.cursorOffset];
  }

  public addData(index: number, data: string): void {
    this.dataArray.splice(index, 0, data);
    this.notifyDataAdd(index);
  }

  public pushData(data: string): void {
    this.dataArray.push(data);
    this.notifyDataAdd(this.dataArray.length - 1);
  }

  public getTopMostData(num: number) {
    this.cursorOffset = num;
    return this.dataArray.slice(0, num)
  }
}

@Entry
@Component
struct WaterFlowPage {
  private data: MyDataSource = new MyDataSource();

  aboutToAppear() {
    for (let i = 0; i <= 80; i++) {
      this.data.pushData(`Hello ${i}`)
    }
  }

  build() {
    WaterFlow() {
      FlowItem() {
        Column() {
          ForEach(this.data.getTopMostData(5), (item: string) => {
            Text(item).fontSize(20)
              .onAppear(() => {
                console.info("appear:" + item)
              })
          })
        }
      }.width("100%")
      //.height(200)
      .backgroundColor(Color.White)

      LazyForEach(this.data, (item: string, index: number) => {
        FlowItem() {
          Row() {
            Text(item).fontSize(20)
              .onAppear(() => {
                console.info("appear:" + item)
              })
          }
        }.width("100%")
        .height(30 + Math.random() * 30)
        .backgroundColor(Color.White)
      }, (item: string) => item)
    }
    .cachedCount(5)
    .columnsTemplate('1fr 1fr')
    .backgroundColor("#efefef")
    .columnsGap(10)
    .rowsGap(5)
  }
}
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