application-dev/arkts-utils/taskpool-waterflow.md

# ArkUI瀑布流渲染场景
<!--Kit: ArkTS-->
<!--Subsystem: CommonLibrary-->
<!--Owner: @lijiamin2025-->
<!--Designer: @weng-changcheng-->
<!--Tester: @kirl75; @zsw_zhushiwei-->
<!--Adviser: @ge-yafang-->

此处提供使用任务池[TaskPool](../reference/apis-arkts/js-apis-taskpool.md)提升[WaterFlow瀑布流](../reference/apis-arkui/arkui-ts/ts-container-waterflow.md)渲染性能的开发指导。UI线程查询数据库数据，并将数据渲染到瀑布流组件，数据过大时会导致UI线程长时间等待，影响用户体验。因此，我们可以将数据查询操作放到子线程中，并通过TaskPool的接口返回数据给UI线程。

本示例说明以下场景：
- 模拟子线程[读取数据库数据](batch-database-operations-guide.md)并返回给UI线程。
- UI线程感知到数据更新，将子线程返回的数据渲染到瀑布流组件。

1. 定义一个接口，用于子线程查询数据库并将数据返回给UI线程。

    ```ts
    // Mock.ets
    import { taskpool } from '@kit.ArkTS';
    import { fillImg } from './Index';

    @Concurrent
    function query() {
      console.info("TaskPoolTest-this is query");
      let result = new Array<string>(33);
      for (let i = 0; i < 33; i++) {
        result[i] = 'Image' + i;
      }
      taskpool.Task.sendData(result);
    }


    export function getImgFromDB() {
      //此处模拟查询数据库，并返回数据
      let task = new taskpool.Task(query);
      task.onReceiveData(fillImg);
      taskpool.execute(task);
    }
    ```
    <!-- @[query_database_return_main_thread](https://gitcode.com/openharmony/applications_app_samples/blob/master/code/DocsSample/ArkTS/ArkTsConcurrent/ApplicationMultithreadingDevelopment/PracticalCases/entry/src/main/ets/managers/Mock.ets) -->

2. 封装一个瀑布流组件数据源，用于瀑布流组件加载数据。

    ```ts
    // WaterFlowDataSource.ets

    // 实现IDataSource接口的对象，用于瀑布流组件加载数据
    export class WaterFlowDataSource implements IDataSource {
      private dataArray: number[] = [];
      private listeners: DataChangeListener[] = [];

      constructor() {
        for (let i = 0; i < 100; i++) {
          this.dataArray.push(i);
        }
      }

      // 获取索引对应的数据
      public getData(index: number): number {
        return this.dataArray[index];
      }

      // 通知控制器数据重新加载
      notifyDataReload(): void {
        this.listeners.forEach(listener => {
          listener.onDataReloaded();
        })
      }

      // 通知控制器数据增加
      notifyDataAdd(index: number): void {
        this.listeners.forEach(listener => {
          listener.onDataAdd(index);
        })
      }

      // 通知控制器数据变化
      notifyDataChange(index: number): void {
        this.listeners.forEach(listener => {
          listener.onDataChange(index);
        })
      }

      // 通知控制器数据删除
      notifyDataDelete(index: number): void {
        this.listeners.forEach(listener => {
          listener.onDataDelete(index);
        })
      }

      // 通知控制器数据位置变化
      notifyDataMove(from: number, to: number): void {
        this.listeners.forEach(listener => {
          listener.onDataMove(from, to);
        })
      }

      //通知控制器数据批量修改
      notifyDatasetChange(operations: DataOperation[]): void {
        this.listeners.forEach(listener => {
          listener.onDatasetChange(operations);
        })
      }

      // 获取数据总数
      public totalCount(): number {
        return this.dataArray.length;
      }

      // 注册改变数据的控制器
      registerDataChangeListener(listener: DataChangeListener): void {
        if (this.listeners.indexOf(listener) < 0) {
          this.listeners.push(listener);
        }
      }

      // 注销改变数据的控制器
      unregisterDataChangeListener(listener: DataChangeListener): void {
        const pos = this.listeners.indexOf(listener);
        if (pos >= 0) {
          this.listeners.splice(pos, 1);
        }
      }

      // 增加数据
      public add1stItem(): void {
        this.dataArray.splice(0, 0, this.dataArray.length);
        this.notifyDataAdd(0);
      }

      // 在数据尾部增加一个元素
      public addLastItem(): void {
        this.dataArray.splice(this.dataArray.length, 0, this.dataArray.length);
        this.notifyDataAdd(this.dataArray.length - 1);
      }

      // 在指定索引位置增加一个元素
      public addItem(index: number): void {
        this.dataArray.splice(index, 0, this.dataArray.length);
        this.notifyDataAdd(index);
      }

      // 删除第一个元素
      public delete1stItem(): void {
        this.dataArray.splice(0, 1);
        this.notifyDataDelete(0);
      }

      // 删除第二个元素
      public delete2ndItem(): void {
        this.dataArray.splice(1, 1);
        this.notifyDataDelete(1);
      }

      // 删除最后一个元素
      public deleteLastItem(): void {
        this.dataArray.splice(-1, 1);
        this.notifyDataDelete(this.dataArray.length - 1);
      }

      // 在指定索引位置删除一个元素
      public deleteItem(index: number): void {
        this.dataArray.splice(index, 1);
        this.notifyDataDelete(index);
      }

      // 重新加载数据
      public reload(): void {
        this.dataArray.splice(1, 1);
        this.dataArray.splice(3, 2);
        this.notifyDataReload();
      }
    }
    ```
    <!-- @[encapsulate_waterfall_data_source](https://gitcode.com/openharmony/applications_app_samples/blob/master/code/DocsSample/ArkTS/ArkTsConcurrent/ApplicationMultithreadingDevelopment/PracticalCases/entry/src/main/ets/managers/WaterFlowDataSource.ets) -->

3. 在应用冷启动阶段，调用`getImgFromDB()`接口，将数据查询操作放到子线程中。在`img`接收到子线程返回的数据后，将数据渲染到瀑布流组件。

    ```ts
    // Index.ets
    import { WaterFlowDataSource } from './WaterFlowDataSource';
    import { getImgFromDB } from './Mock';

    // 模拟图片数组
    let img = new Array<string>(33);
    export function fillImg(imgArr : Array<string>) {
      img = imgArr;
    }

    @Entry
    @Component
    struct WaterFlowDemo {
      @State minSize: number = 80;
      @State maxSize: number = 180;
      @State fontSize: number = 24;
      @State colors: number[] = [0xFFC0CB, 0xDA70D6, 0x6B8E23, 0x6A5ACD, 0x00FFFF, 0x00FF7F];
      scroller: Scroller = new Scroller();
      dataSource: WaterFlowDataSource = new WaterFlowDataSource();
      private itemWidthArray: number[] = [];
      private itemHeightArray: number[] = [];
      // 计算FlowItem宽/高
      getSize() {
        let ret = Math.floor(Math.random() * this.maxSize);
        return (ret > this.minSize ? ret : this.minSize);
      }

      // 设置FlowItem的宽/高数组
      setItemSizeArray() {
        for (let i = 0; i < 100; i++) {
          this.itemWidthArray.push(this.getSize());
          this.itemHeightArray.push(this.getSize());
        }
      }

      aboutToAppear() {
        this.setItemSizeArray();
      }

      @Builder
      itemFoot() {
        Column() {
          Text(`Footer`)
            .fontSize(10)
            .backgroundColor(Color.Red)
            .width(50)
            .height(50)
            .align(Alignment.Center)
            .margin({ top: 2 });
        }
      }

      build() {
        Column({ space: 2 }) {
          Text("ArkUI WaterFlow Demo")
            .onAppear(()=>{
              getImgFromDB();
            })
          WaterFlow() {
            LazyForEach(this.dataSource, (item: number) => {
              FlowItem() {
                Column() {
                  Text("N" + item)
                    .fontSize(12)
                    .height('16')
                    .onClick(()=>{

                    });
                  // 为了模拟图片加载，使用Text组件显示，正常加载jpg文件时，可以直接使用Image组件，参考Image(this.img[item % 33]).objectFit(ImageFit.Contain).width('100%').layoutWeight(1)
                  if (img[item % 33] == null) {
                    Text("图片加载中...")
                      .width('100%')
                      .layoutWeight(1);
                  }
                  Text(img[item % 33])
                    .width('100%')
                    .layoutWeight(1);
                }
              }
              .onAppear(() => {
                // 即将触底时提前增加数据
                if (item + 20 == this.dataSource.totalCount()) {
                  for (let i = 0; i < 100; i++) {
                    this.dataSource.addLastItem();
                  }
                }
              })
              .width('100%')
              .height(this.itemHeightArray[item % 100])
              .backgroundColor(this.colors[item % 5])
            }, (item: string) => item)
          }
          .columnsTemplate("1fr 1fr")
          .columnsGap(10)
          .rowsGap(5)
          .backgroundColor(0xFAEEE0)
          .width('100%')
          .height('100%')
          .onReachStart(() => {
            console.info('TaskPoolTest-waterFlow reach start');
          })
          .onScrollStart(() => {
            console.info('TaskPoolTest-waterFlow scroll start');
          })
          .onScrollStop(() => {
            console.info('TaskPoolTest-waterFlow scroll stop');
          })
          .onScrollFrameBegin((offset: number, state: ScrollState) => {
            console.info('TaskPoolTest-waterFlow scrollFrameBegin offset: ' + offset + ' state: ' + state.toString());
            return { offsetRemain: offset };
          })
        }
      }
    }
    ```
    <!-- @[receive_child_thread_data_render_waterfall_component](https://gitcode.com/openharmony/applications_app_samples/blob/master/code/DocsSample/ArkTS/ArkTsConcurrent/ApplicationMultithreadingDevelopment/PracticalCases/entry/src/main/ets/managers/WaterfallRendering.ets) -->