getSnapshotBeforeUpdate的使用场景

Tutorial: React基础 Category: React Published: 2026-04-07 13:58:26 Views: 20 Likes: 0 Comments: 0

29_getSnapshotBeforeUpdate的使用场景

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <title>4_getSnapShotBeforeUpdate的使用场景</title>
    <style>
      .list {
        width: 200px;
        height: 150px;
        background-color: skyblue;
        overflow: auto;
      }

      .news {
        height: 30px;
      }
    </style>
  </head>

  <body>
    <!-- 准备好一个"容器" -->
    <div id="test"></div>

    <!-- 引入react核心库 -->
    <script
      type="text/javascript"
      src="./js/17.0.1/react.development.js"
    ></script>

    <!-- 引入react-dom, 用于支持react操作DOM -->
    <script
      type="text/javascript"
      src="./js/17.0.1/react-dom.development.js"
    ></script>

    <!-- 引入babel, 用于将jsx转为js -->
    <script type="text/javascript" src="./js/17.0.1/babel.min.js"></script>

    <script type="text/babel">
      class NewsList extends React.Component {
        state = { newsArr: [] };

        componentDidMount() {
          setInterval(() => {
            // 获取原状态
            const { newsArr } = this.state;
            // 模拟一条新闻
            const news = "新闻" + (newsArr.length + 1);
            // 更新状态
            this.setState({ newsArr: [news, ...newsArr] });
          }, 1000);
        }

        getSnapshotBeforeUpdate() {
          return this.refs.list.scrollHeight;
        }

        componentDidUpdate(preProps, preState, height) {
          this.refs.list.scrollTop += this.refs.list.scrollHeight - height;
        }

        render() {
          return (
            <div className="list" ref="list">
              {this.state.newsArr.map((n, index) => {
                return (
                  <div key={index} className="news">
                    {n}
                  </div>
                );
              })}
            </div>
          );
        }
      }

      ReactDOM.render(<NewsList />, document.getElementById("test"));
    </script>
  </body>
</html>
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