<view class="container">
  <view class="page-body">
    <view class="page-section message">
      <text wx:if="{{appear}}">
        小球出现
      </text>
      <text wx:else>
        小球消失
      </text>
    </view>
    <view class="page-section">
      <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>
          <view class="filling"></view>
          <view class="ball"></view>
        </view>
      </scroll-view>
    </view>
  </view>
</view>

.scroll-view {
  height: 400rpx;
  background: #fff;
  border: 1px solid #ccc;
  box-sizing: border-box;
}

.scroll-area {
  height: 1300rpx;
  display: flex;
  flex-direction: column;
  align-items: center;
  transition: .5s;
}

.notice {
  margin-top: 150rpx;
}

.ball {
  width: 200rpx;
  height: 200rpx;
  background: #1AAD19;
  border-radius: 50%;
}

.filling {
  height: 400rpx;
}

.message {
  width: 100%;
  display: flex;
  justify-content: center;
}

.message text {
  font-size: 40rpx;
  font-family: -apple-system-font, Helvetica Neue,Helvetica,sans-serif;
}

Page({
  data: {
    appear: false
  },
  onLoad() {
    this._observer = wx.createIntersectionObserver(this)
    this._observer
      .relativeTo('.scroll-view')
      .observe('.ball', (res) => {
        console.log(res);
        this.setData({
          appear: res.intersectionRatio > 0
        })
      })
  },
  onUnload() {
    if (this._observer) this._observer.disconnect()
  }
})