Create Intersection Observer

<template>
	<view>
		<page-head :title="data.title"></page-head>
		<view class="uni-padding-wrap uni-common-mt">
			<view class="uni-title uni-common-mt">
				{{data.appear ? '小球出现' : '小球消失'}}
			</view>
			<scroll-view class="scroll-view" :scrollTop="data.scrollTop">
				<view class="scroll-area">
					<text class="notice">向下滚动让小球出现</text>
					<view class="ball"></view>
				</view>
			</scroll-view>
		</view>
	</view>
</template>
<script setup lang="uts">
  let observer: IntersectionObserver | null = null;

  type DataType = {
    appear: boolean;
    title: string;
    testRes: ObserveResult | null;
    scrollTop: number;
  }

  const data = reactive({
    appear: false,
    title: 'intersectionObserver',
    testRes: null,
    scrollTop: 0
  } as DataType)

  onReady(() => {
    observer = uni.createIntersectionObserver(getCurrentInstance()!.proxy!, {});
    observer.relativeTo('.scroll-view').observe('.ball', (res: ObserveResult) => {
      data.testRes = res;
      if (res.intersectionRatio > 0 && !data.appear) {
        data.appear = true;
      } else if (res.intersectionRatio <= 0 && data.appear) {
        data.appear = false;
      }
    })
  })

  onUnload(() => {
    if (observer) {
      observer.disconnect()
    }
  })

  defineExpose({
    data
  })
</script>
<style>
	.scroll-view {
		height: 200px;
		background: #fff;
		border: 1px solid #ccc;
		box-sizing: border-box;
	}

	.scroll-area {
		height: 650px;
		display: flex;
		flex-direction: column;
		align-items: center;
	}

	.notice {
		margin-top: 75px;
		margin:75px 0 200px 0;
	}

	.ball {
		width: 100px;
		height: 100px;
		background: #4cd964;
		border-radius: 100px;
	}
</style>