1# Visible Area Change Event 2 3The 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. 4 5> **NOTE** 6> 7> The APIs of this module are supported since API version 9. Updates will be marked with a superscript to indicate their earliest API version. 8 9## onVisibleAreaChange 10 11onVisibleAreaChange(ratios: Array<number>, event: VisibleAreaChangeCallback): T 12 13Called when the visible area of the component changes. 14 15**Atomic service API**: This API can be used in atomic services since API version 11. 16 17**System capability**: SystemCapability.ArkUI.ArkUI.Full 18 19**Parameters** 20 21| Name| Type | Mandatory| Description | 22| ------ | --------------------------------------------------- | ---- | ------------------------------------------------------------ | 23| ratios | Array<number> | Yes | 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; only the area within the parent component is counted) 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.<br>**NOTE**<br>When the value is close to the boundary 0 or 1, it is rounded off with a round-off error not greater than 0.001. For example, 0.9997 is rounded off to 1.| 24| event | [VisibleAreaChangeCallback](ts-types.md#visibleareachangecallback12) | Yes | Callback for visible area changes of the component.| 25 26**Return value** 27 28| Type| Description| 29| -------- | -------- | 30| T | Current component.| 31 32> **NOTE** 33> 34> 35>- This API only takes into account the relative clipped area ratio of the component with respect to all ancestor nodes (up to the window boundary) and its own area. 36> 37>- It does not support calculations for obstructions caused by sibling components or by sibling components of any ancestors, such as those managed by [Stack](ts-container-stack.md) or [z-order control](ts-universal-attributes-z-order.md). 38> 39>- It does not support visibility change calculations for nodes that are not in the component tree. For example, preloaded nodes or custom nodes mounted using the [overlay](ts-universal-attributes-overlay.md#overlay) capability. 40 41## onVisibleAreaApproximateChange<sup>18+</sup> 42 43onVisibleAreaApproximateChange(options: VisibleAreaEventOptions, event: VisibleAreaChangeCallback | undefined): void 44 45Called to set the callback parameters for the [onVisibleAreaChange](./ts-universal-component-visible-area-change-event.md#onvisibleareachange) event and restrict its execution interval. 46 47**Atomic service API**: This API can be used in atomic services since API version 18. 48 49**System capability**: SystemCapability.ArkUI.ArkUI.Full 50 51**Parameters** 52 53| Name| Type | Mandatory| Description | 54| ------ | ------ | ---- | -------------------------- | 55| options | [VisibleAreaEventOptions](./ts-types.md#visibleareaeventoptions12) | Yes | Options of visible area changes.| 56| event | [VisibleAreaChangeCallback](./ts-types.md#visibleareachangecallback12) \| undefined | Yes | Callback for the **onVisibleAreaChange** event. This callback is triggered when the ratio of the component's visible area to its total area approaches the threshold set in **options**.| 57 58>**NOTE** 59> 60> This callback is not a real-time callback. The actual callback interval may differ from the expected interval. 61> 62> The interval between two visible area change callbacks will not be less than the expected update interval. If the provided expected interval is too short, the actual callback interval will be determined by the system load. 63> 64> By default, the interval threshold of the visible area change callback includes 0. This means that, if the provided threshold is [0.5], the effective threshold will be [0.0, 0.5]. 65 66## Example 67 68### Example 1: Using onVisibleAreaChange to Listen for Visible Area Changes 69 70This example demonstrates how to set an **onVisibleAreaChange** event for a component, which triggers the callback when the component is fully displayed or completely hidden. 71 72```ts 73// xxx.ets 74@Entry 75@Component 76struct ScrollExample { 77 scroller: Scroller = new Scroller() 78 private arr: number[] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] 79 @State testTextStr: string = 'test' 80 @State testRowStr: string = 'test' 81 82 build() { 83 Column() { 84 Column() { 85 Text(this.testTextStr) 86 .fontSize(20) 87 88 Text(this.testRowStr) 89 .fontSize(20) 90 } 91 .height(100) 92 .backgroundColor(Color.Gray) 93 .opacity(0.3) 94 95 Scroll(this.scroller) { 96 Column() { 97 Text("Test Text Visible Change") 98 .fontSize(20) 99 .height(200) 100 .margin({ top: 50, bottom: 20 }) 101 .backgroundColor(Color.Green) 102 // Set ratios to [0.0, 1.0] to invoke the callback when the component is fully visible or invisible on screen. 103 .onVisibleAreaChange([0.0, 1.0], (isExpanding: boolean, currentRatio: number) => { 104 console.info('Test Text isExpanding: ' + isExpanding + ', currentRatio:' + currentRatio) 105 if (isExpanding && currentRatio >= 1.0) { 106 console.info('Test Text is fully visible. currentRatio:' + currentRatio) 107 this.testTextStr = 'Test Text is fully visible' 108 } 109 110 if (!isExpanding && currentRatio <= 0.0) { 111 console.info('Test Text is completely invisible.') 112 this.testTextStr = 'Test Text is completely invisible' 113 } 114 }) 115 116 Row() { 117 Text('Test Row Visible Change') 118 .fontSize(20) 119 .margin({ bottom: 20 }) 120 121 } 122 .height(200) 123 .backgroundColor(Color.Yellow) 124 .onVisibleAreaChange([0.0, 1.0], (isExpanding: boolean, currentRatio: number) => { 125 console.info('Test Row isExpanding:' + isExpanding + ', currentRatio:' + currentRatio) 126 if (isExpanding && currentRatio >= 1.0) { 127 console.info('Test Row is fully visible.') 128 this.testRowStr = 'Test Row is fully visible' 129 } 130 131 if (!isExpanding && currentRatio <= 0.0) { 132 console.info('Test Row is completely invisible.') 133 this.testRowStr = 'Test Row is completely invisible' 134 } 135 }) 136 137 ForEach(this.arr, (item:number) => { 138 Text(item.toString()) 139 .width('90%') 140 .height(150) 141 .backgroundColor(0xFFFFFF) 142 .borderRadius(15) 143 .fontSize(16) 144 .textAlign(TextAlign.Center) 145 .margin({ top: 10 }) 146 }, (item:number) => (item.toString())) 147 148 }.width('100%') 149 } 150 .backgroundColor(0x317aff) 151 .scrollable(ScrollDirection.Vertical) 152 .scrollBar(BarState.On) 153 .scrollBarColor(Color.Gray) 154 .scrollBarWidth(10) 155 .onWillScroll((xOffset: number, yOffset: number, scrollState: ScrollState) => { 156 console.info(xOffset + ' ' + yOffset) 157 }) 158 .onScrollEdge((side: Edge) => { 159 console.info('To the edge') 160 }) 161 .onScrollStop(() => { 162 console.info('Scroll Stop') 163 }) 164 165 }.width('100%').height('100%').backgroundColor(0xDCDCDC) 166 } 167} 168``` 169 170
