# WaterFlow


The **\<WaterFlow>** component is a container that consists of cells formed by rows and columns and arranges items of different sizes from top to bottom according to the preset rules.


> **NOTE**
>
> This component is supported since API version 9. Updates will be marked with a superscript to indicate their earliest API version.


## Child Components


This component can contain the [\<FlowItem>](ts-container-flowitem.md) child component.

>  **NOTE**
>
>  If the **visibility** attribute of a **\<FlowItem>** is set to **None**, the component is not displayed in the container, but the gaps around the component are still effective.

## APIs


WaterFlow(options?: {footer?: CustomBuilder, scroller?: Scroller})

**Parameters**

| Name    | Type                                       | Mandatory| Description                                    |
| ---------- | ----------------------------------------------- | ------ | -------------------------------------------- |
| footer |  [CustomBuilder](ts-types.md#custombuilder8) | No  | Footer of the **\<WaterFlow>** component. |
| scroller | [Scroller](ts-container-scroll.md#scroller) | No  | Controller, which can be bound to scrollable components.<br>**NOTE**<br>The scroller cannot be bound to other scrollable components, such as [\<List>](ts-container-list.md), [\<Grid>](ts-container-grid.md), or [\<Scroll>](ts-container-scroll.md).|


## Attributes


In addition to the [universal attributes](ts-universal-attributes-size.md), the following attributes are supported.

| Name| Type| Description|
| -------- | -------- | -------- |
| columnsTemplate | string | Number of columns in the layout. If this attribute is not set, one column is used by default.<br>For example, **'1fr 1fr 2fr'** indicates three columns, with the first column taking up 1/4 of the parent component's full width, the second column 1/4, and the third column 2/4.<br>You can use **columnsTemplate('repeat(auto-fill,track-size)')** to automatically calculate the number of columns based on the specified column width **track-size**. **repeat** and **auto-fill** are keywords. The units for **track-size** can be px, vp, %, or a valid number. For details, see Example 2.<br>Default value: **'1fr'**|
| rowsTemplate | string | Number of rows in the layout. If this attribute is not set, one row is used by default.<br>For example, **'1fr 1fr 2fr'** indicates three rows, with the first row taking up 1/4 of the parent component's full height, the second row 1/4, and the third row 2/4.<br>You can use **rowsTemplate('repeat(auto-fill,track-size)')** to automatically calculate the number of rows based on the specified row height **track-size**. **repeat** and **auto-fill** are keywords. The units for **track-size** can be px, vp, %, or a valid number.<br>Default value: **'1fr'**|
| itemConstraintSize | [ConstraintSizeOptions](ts-types.md#constraintsizeoptions) | Size constraints of the child components during layout.              |
| columnsGap | [Length](ts-types.md#length) |Gap between columns.<br>Default value: **0**|
| rowsGap | [Length](ts-types.md#length) |Gap between rows.<br> Default value: **0**|
| layoutDirection | [FlexDirection](ts-appendix-enums.md#flexdirection) |Main axis direction of the layout.<br>Default value: **FlexDirection.Column**|
| enableScrollInteraction<sup>10+</sup>  |  boolean  |   Whether to support scroll gestures. When this attribute is set to **false**, scrolling by finger or mouse is not supported, but the scrolling controller API is not affected.<br>Default value: **true**     |
| nestedScroll<sup>10+</sup>                 | [NestedScrollOptions](ts-container-scroll.md#nestedscrolloptions10)         | Nested scrolling options. You can set the nested scrolling mode in the forward and backward directions to implement scrolling linkage with the parent component.|
| friction<sup>10+</sup> | number \| [Resource](ts-types.md#resource)    | Friction coefficient. It applies only to gestures in the scrolling area, and it affects only indirectly the scroll chaining during the inertial scrolling process.<br>Default value: **0.9** for wearable devices and **0.6** for non-wearable devices<br>**NOTE**<br>A value less than or equal to 0 evaluates to the default value.|
| cachedCount<sup>11+</sup> | number | Number of items to be cached. This attribute is effective only in **LazyForEach**.<br> Default value: **1**<br>**NOTE**<br>Items<br>that exceed the display and cache range are released.<br>A value less than 0 evaluates to the default value.|
| scrollBar<sup>11+</sup>            | [BarState](ts-appendix-enums.md#barstate) | Scrollbar status.<br>Default value: **BarState.Off**<br>**NOTE**<br>Until the layout of the **\<WaterFlow>** component is complete, the position and length of the scrollbar are subject to the total height and current offset of the items that have been laid out.|
| scrollBarWidth<sup>11+</sup> | string \| number         | Width of the scrollbar. This attribute cannot be set in percentage.<br>Default value: **4**<br>Unit: vp<br>**NOTE**<br>If the width of the scrollbar exceeds its height, it will change to the default value.|
| scrollBarColor<sup>11+</sup> | string \| number \| [Color](ts-appendix-enums.md#color)   | Color of the scrollbar.|
| edgeEffect<sup>11+</sup>     | value:[EdgeEffect](ts-appendix-enums.md#edgeeffect), <br>options?:[EdgeEffectOptions<sup>11+</sup>](ts-container-scroll.md#edgeeffectoptions11)        | Effect used at the edges of the component when the boundary of the scrollable content is reached.<br>\- **value**: effect used at the edges of the **\<WaterFlow>** component when the boundary of the scrollable content is reached. The spring effect and shadow effect are supported.<br>Default value: **EdgeEffect.None**<br>\- **options**: whether to enable the scroll effect when the component content size is smaller than the component itself.<br>Default value: **false**|

The priority of **layoutDirection** is higher than that of **rowsTemplate** and **columnsTemplate**. Depending on the **layoutDirection** settings, there are three layout modes:

- **layoutDirection** is set to **FlexDirection.Column** or **FlexDirection.ColumnReverse**

	In this case, **columnsTemplate** is valid. If it is not set, the default value is used. For example, if **columnsTemplate** is set to **"1fr 1fr"** and **rowsTemplate** **"1fr 1fr 1fr"**, child components are arranged in vertical layout, with the cross axis equally divided into two columns.

- **layoutDirection** set to **FlexDirection.Row** or **FlexDirection.RowReverse**

	In this case, **rowsTemplate** is valid. If it is not set, the default value is used. For example, if **columnsTemplate** is set to **"1fr 1fr"** and **rowsTemplate** **"1fr 1fr 1fr"**, child components are arranged in horizontal layout, with the cross axis equally divided into three columns.

- **layoutDirection** is not set

	In this case, the default value of **layoutDirection** is used, which is **FlexDirection.Column**, and **columnsTemplate** is valid. For example, if **columnsTemplate** is set to **"1fr 1fr"** and **rowsTemplate** **"1fr 1fr 1fr"**, child components are arranged in vertical layout, with the cross axis equally divided into two columns.

## Events


In addition to the [universal events](ts-universal-events-click.md), the following events are supported.


| Name| Description|
| -------- | -------- |
| onReachStart(event: () => void) | Triggered when the component reaches the start.|
| onReachEnd(event: () => void)   | Triggered when the component reaches the end position.|
| onScrollFrameBegin<sup>10+</sup>(event: (offset: number, state: ScrollState) => { offsetRemain }) | Triggered when the component starts to scroll. The input parameters indicate the amount by which the component will scroll. The event handler then works out the amount by which the component needs to scroll based on the real-world situation and returns the result.<br>\- **offset**: amount to scroll by, in vp.<br>\- **state**: current scrolling state.<br>- **offsetRemain**: actual amount by which the component scrolls, in vp.<br>This event is triggered when the user starts dragging the component or the component starts inertial scrolling. It is not triggered when the component rebounds, the scrolling controller is used, or the scrollbar is dragged. |
| onScroll<sup>11+</sup>(event: (scrollOffset: number, scrollState: [ScrollState](ts-container-list.md#scrollstate)) => void) | Triggered when the component starts to scroll.<br>- **scrollOffset**: scroll offset of each frame. The offset is positive when the component is scrolled up and negative when the list is scrolled down.<br>- **scrollState**: current scroll state.|
| onScrollIndex<sup>11+</sup>(event: (first: number, last: number) => void) | Triggered when the first or last item displayed in the component changes. It is triggered once when the component is initialized.<br>- **first**: index of the first item of the component.<br>- **last**: index of the last item of the component.<br>This event is triggered when either of the preceding indexes changes.|
| onScrollStart<sup>11+</sup>(event: () => void) | Triggered when the component starts scrolling initiated by the user's finger dragging the component or its scrollbar. This event is also triggered when the animation contained in the scrolling triggered by [Scroller](ts-container-scroll.md#scroller) starts.|
| onScrollStop<sup>11+</sup>(event: () => void)          | Triggered when the component stops scrolling after the user's finger leaves the screen. This event is also triggered when the animation contained in the scrolling triggered by [Scroller](ts-container-scroll.md#scroller) stops.|


## Example

### Example 1
Basic usage of **\<WaterFlow>**:
```ts
// WaterFlowDataSource.ets

// Object that implements the IDataSource API, which is used by the <WaterFlow> component to load data.
export class WaterFlowDataSource implements IDataSource {
  private dataArray: number[] = []
  private listeners: DataChangeListener[] = []

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

  // Obtain the data corresponding to the specified index.
  public getData(index: number): number {
    return this.dataArray[index]
  }

  // Notify the controller of data reloading.
  notifyDataReload(): void {
    this.listeners.forEach(listener => {
      listener.onDataReloaded()
    })
  }

  // Notify the controller of data addition.
  notifyDataAdd(index: number): void {
    this.listeners.forEach(listener => {
      listener.onDataAdd(index)
    })
  }

  // Notify the controller of data changes.
  notifyDataChange(index: number): void {
    this.listeners.forEach(listener => {
      listener.onDataChange(index)
    })
  }

  // Notify the controller of data deletion.
  notifyDataDelete(index: number): void {
    this.listeners.forEach(listener => {
      listener.onDataDelete(index)
    })
  }

  // Notify the controller of the data location change.
  notifyDataMove(from: number, to: number): void {
    this.listeners.forEach(listener => {
      listener.onDataMove(from, to)
    })
  }

  // Obtain the total number of data records.
  public totalCount(): number {
    return this.dataArray.length
  }

  // Register the data change listener.
  registerDataChangeListener(listener: DataChangeListener): void {
    if (this.listeners.indexOf(listener) < 0) {
      this.listeners.push(listener)
    }
  }

  // Unregister the data change listener.
  unregisterDataChangeListener(listener: DataChangeListener): void {
    const pos = this.listeners.indexOf(listener)
    if (pos >= 0) {
      this.listeners.splice(pos, 1)
    }
  }

  // Add data.
  public add1stItem(): void {
    this.dataArray.splice(0, 0, this.dataArray.length)
    this.notifyDataAdd(0)
  }

  // Add an item to the end of the data.
  public addLastItem(): void {
    this.dataArray.splice(this.dataArray.length, 0, this.dataArray.length)
    this.notifyDataAdd(this.dataArray.length - 1)
  }

  // Add an item to the position corresponding to the specified index.
  public addItem(index: number): void {
    this.dataArray.splice(index, 0, this.dataArray.length)
    this.notifyDataAdd(index)
  }

  // Delete the first item.
  public delete1stItem(): void {
    this.dataArray.splice(0, 1)
    this.notifyDataDelete(0)
  }

  // Delete the second item.
  public delete2ndItem(): void {
    this.dataArray.splice(1, 1)
    this.notifyDataDelete(1)
  }

  // Delete the last item.
  public deleteLastItem(): void {
    this.dataArray.splice(-1, 1)
    this.notifyDataDelete(this.dataArray.length)
  }

  // Reload data.
  public reload(): void {
    this.dataArray.splice(1, 1)
    this.dataArray.splice(3, 2)
    this.notifyDataReload()
  }
}
```

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

@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[] = []

  // Calculate the width and height of a flow item.
  getSize() {
    let ret = Math.floor(Math.random() * this.maxSize)
    return (ret > this.minSize ? ret : this.minSize)
  }

  // Save the width and height of the flow item.
  getItemSizeArray() {
    for (let i = 0; i < 100; i++) {
      this.itemWidthArray.push(this.getSize())
      this.itemHeightArray.push(this.getSize())
    }
  }

  aboutToAppear() {
    this.getItemSizeArray()
  }

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

  build() {
    Column({ space: 2 }) {
      WaterFlow() {
        LazyForEach(this.dataSource, (item: number) => {
          FlowItem() {
            Column() {
              Text("N" + item).fontSize(12).height('16')
              Image('res/waterFlowTest(' + item % 5 + ').jpg')
                .objectFit(ImageFit.Fill)
                .width('100%')
                .layoutWeight(1)
            }
          }
          .onAppear(() => {
            // Add data in advance when scrolling is about to end.
            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%')
    }
  }
}
```

![en-us_image_WaterFlow.gif](figures/waterflow-perf-demo.gif)

### Example 2
Example of using **auto-fill**:
```ts
//index.ets
import { WaterFlowDataSource } from './WaterFlowDataSource'

@Entry
@Component
struct WaterFlowDemo {
  @State minSize: number = 80
  @State maxSize: number = 180
  @State colors: number[] = [0xFFC0CB, 0xDA70D6, 0x6B8E23, 0x6A5ACD, 0x00FFFF, 0x00FF7F]
  dataSource: WaterFlowDataSource = new WaterFlowDataSource()
  private itemWidthArray: number[] = []
  private itemHeightArray: number[] = []

  // Calculate the width and height of a flow item.
  getSize() {
    let ret = Math.floor(Math.random() * this.maxSize)
    return (ret > this.minSize ? ret : this.minSize)
  }

  // Save the width and height of the flow item.
  getItemSizeArray() {
    for (let i = 0; i < 100; i++) {
      this.itemWidthArray.push(this.getSize())
      this.itemHeightArray.push(this.getSize())
    }
  }

  aboutToAppear() {
    this.getItemSizeArray()
  }

  build() {
    Column({ space: 2 }) {
      WaterFlow() {
        LazyForEach(this.dataSource, (item: number) => {
          FlowItem() {
            Column() {
              Text("N" + item).fontSize(12).height('16')
            }
          }
          .width('100%')
          .height(this.itemHeightArray[item % 100])
          .backgroundColor(this.colors[item % 5])
        }, (item: string) => item)
      }
      .columnsTemplate('repeat(auto-fill,80)')
      .columnsGap(10)
      .rowsGap(5)
      .padding({left:5})
      .backgroundColor(0xFAEEE0)
      .width('100%')
      .height('100%')
    }
  }
}
```

![waterflow_auto-fill.png](figures/waterflow_auto-fill.png)