IntersectionObserver.observe

解释：指定目标节点并开始监听相交状态变化情况。

方法参数

String targetSelector, Function callback

targetSelector  参数说明：选择器。
callback  参数说明：监听相交状态变化的回调函数。
callback  返回参数说明

intersectionRect 参数说明

boundingClientRect 参数说明

relativeRect 参数说明

示例 

在开发者工具中打开

图片示例

代码示例

<view class="wrap">
    <scroll-view class="scroll-view" scroll-y>
        <view class="scroll-area" style="{{appear ? 'background: #ccc' : ''}}">
            <text class="notice">向下滚动让小球出现</text>
            <!-- 占位元素 -->
            <view class="filling"></view> 
            <!-- 小球 -->
            <view class="ball"></view>
        </view>
    </scroll-view>
    <view class="list-area border-bottom">
        <view class="list-item-key-4">top:</view>
        <view class="list-item-value">{{intersectionRect.top}}</view>
    </view>
    <view class="list-area border-bottom">
        <view class="list-item-key-4">right:</view>
        <view class="list-item-value">{{intersectionRect.right}}</view>
    </view>
    <view class="list-area border-bottom">
        <view class="list-item-key-4">bottom:</view>
        <view class="list-item-value">{{intersectionRect.bottom}}</view>
    </view>
    <view class="list-area border-bottom">
        <view class="list-item-key-4">left:</view>
        <view class="list-item-value">{{intersectionRect.left}}</view>
    </view>
    <view class="list-area border-bottom">
        <view class="list-item-key-4">width</view>
        <view class="list-item-value">{{intersectionRect.width}}</view>
    </view>
    <view class="list-area border-bottom">
        <view class="list-item-key-4">height</view>
        <view class="list-item-value">{{intersectionRect.height}}</view>
    </view>
</view>

Page({
    data: {
        intersectionRect: ''
    },
    onReady() {
        swan.createIntersectionObserver(this)
            .relativeTo('.scroll-view')
            .observe('.ball', res => {
                this.setData('intersectionRect', res.intersectionRect);
                console.log(res.intersectionRect.left); // 相交区域的左边界坐标
                console.log(res.intersectionRect.right); // 相交区域的右边界坐标
                console.log(res.intersectionRect.top); // 相交区域的上边界坐标
                console.log(res.intersectionRect.bottom); // 相交区域的下边界坐标
                console.log(res.intersectionRect.width); // 相交区域的宽度
                console.log(res.intersectionRect.height); // 相交区域的高度
            });
    }
});