harmony 鸿蒙Visible Area Change Event

2022-08-09
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Visible Area Change Event

The visible area change event of a component refers to the change in the visual portion of the component on the screen. It can be used to determine whether the component is completely or partially displayed on the screen. It is usually applicable to scenarios such as advertisement exposure tracing.

NOTE

The APIs of this module are supported since API version 9. Updates will be marked with a superscript to indicate their earliest API version.

Events

Name	Description
onVisibleAreaChange(ratios: Array<number>, event: (isVisible: boolean, currentRatio: number) => void)	Called when the visual area of the component changes. - ratios: threshold array. Each threshold represents a ratio of the component’s visible area (that is, the area of the component that is visible on screen) to the component’s total area. This callback is invoked when the ratio of the component’s visible area to its total area is greater than or less than the threshold. The value range of the threshold is [0.0, 1.0]. If the threshold set exceeds this range, the value 0.0 or 1.0 will be used. - isVisible: whether the ratio of the component’s visible area to its total area is greater than the threshold. The value true means that the ratio is greater than the threshold, and false means that the ratio is less than the threshold. - currentRatio: ratio of the component’s visible area to its total area when this callback is invoked. NOTE This API applies only to the scenario where the component layout area exceeds or is not within the current screen display area. It does not apply to the scenario where the area becomes invisible due to component stacking or the visible area exceeds the allowed range as a result of calling transformation APIs such as offset or translate.

Name

Description

onVisibleAreaChange(ratios: Array<number>, event: (isVisible: boolean, currentRatio: number) => void)

Called when the visual area of the component changes.
- ratios: threshold array. Each threshold represents a ratio of the component’s visible area (that is, the area of the component that is visible on screen) to the component’s total area. This callback is invoked when the ratio of the component’s visible area to its total area is greater than or less than the threshold. The value range of the threshold is [0.0, 1.0]. If the threshold set exceeds this range, the value 0.0 or 1.0 will be used.
- isVisible: whether the ratio of the component’s visible area to its total area is greater than the threshold. The value true means that the ratio is greater than the threshold, and false means that the ratio is less than the threshold.
- currentRatio: ratio of the component’s visible area to its total area when this callback is invoked.
NOTE
This API applies only to the scenario where the component layout area exceeds or is not within the current screen display area. It does not apply to the scenario where the area becomes invisible due to component stacking or the visible area exceeds the allowed range as a result of calling transformation APIs such as offset or translate.

Example

// xxx.ets
@Entry
@Component
struct ScrollExample {
  scroller: Scroller = new Scroller()
  private arr: number[] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
  @State testTextStr: string = 'test'
  @State testRowStr: string = 'test'

  build() {
    Column() {
      Column() {
        Text(this.testTextStr)
          .fontSize(20)

        Text(this.testRowStr)
          .fontSize(20)
      }
      .height(100)
      .backgroundColor(Color.Gray)
      .opacity(0.3)

      Scroll(this.scroller) {
        Column() {
          Text("Test Text Visible Change")
            .fontSize(20)
            .height(200)
            .margin({ top: 50, bottom: 20 })
            .backgroundColor(Color.Green)
              // Set ratios to [0.0, 1.0] to invoke the callback when the component is fully visible or invisible on screen.
            .onVisibleAreaChange([0.0, 1.0], (isVisible: boolean, currentRatio: number) => {
              console.info('Test Text isVisible: ' + isVisible + ', currentRatio:' + currentRatio)
              if (isVisible && currentRatio >= 1.0) {
                console.info('Test Text is fully visible. currentRatio:' + currentRatio)
                this.testTextStr = 'Test Text is fully visible'
              }

              if (!isVisible && currentRatio <= 0.0) {
                console.info('Test Text is completely invisible.')
                this.testTextStr = 'Test Text is completely invisible'
              }
            })

          Row() {
            Text('Test Row Visible  Change')
              .fontSize(20)
              .margin({ bottom: 20 })

          }
          .height(200)
          .backgroundColor(Color.Yellow)
          .onVisibleAreaChange([0.0, 1.0], (isVisible: boolean, currentRatio: number) => {
            console.info('Test Row isVisible:' + isVisible + ', currentRatio:' + currentRatio)
            if (isVisible && currentRatio >= 1.0) {
              console.info('Test Row is fully visible.')
              this.testRowStr = 'Test Row is fully visible'
            }

            if (!isVisible && currentRatio <= 0.0) {
              console.info('Test Row is is completely invisible.')
              this.testRowStr = 'Test Row is is completely invisible'
            }
          })

          ForEach(this.arr, (item:number) => {
            Text(item.toString())
              .width('90%')
              .height(150)
              .backgroundColor(0xFFFFFF)
              .borderRadius(15)
              .fontSize(16)
              .textAlign(TextAlign.Center)
              .margin({ top: 10 })
          }, (item:number) => (item.toString()))

        }.width('100%')
      }
      .backgroundColor(0x317aff)
      .scrollable(ScrollDirection.Vertical)
      .scrollBar(BarState.On)
      .scrollBarColor(Color.Gray)
      .scrollBarWidth(10)
      .onScroll((xOffset: number, yOffset: number) => {
        console.info(xOffset + ' ' + yOffset)
      })
      .onScrollEdge((side: Edge) => {
        console.info('To the edge')
      })
      .onScrollStop(() => {
        console.info('Scroll Stop')
      })

    }.width('100%').height('100%').backgroundColor(0xDCDCDC)
  }
}