javascript - 渲染后调用react componentDidMount执行

Question

我是 React JS 的新手，现在对组件生命周期方法的工作方式感到困惑；在我的例子中，这与 ReactJS.org 文档完全矛盾。预期的行为是，componentDidMount() 应该在 render() 之前执行，但是 render() 在 componentDidMount() 之前执行。

我的源代码:

import React from 'react';
import '../main.css';
//import sam from "../images/gallery/Farmer1.jpg";
import galleries from './GalleryImages.js';
class Gallery extends React.Component {
  constructor(props) {
    super(props);
  }
  cache = {};
  dummy = './Farmer1.jpg';
  importAll = (r, ping) => {
    console.log(ping + ' called');
    r.keys().forEach(key => {
      this.cache[key] = r(key);
      console.log(`key is ${key} and value : ${r(key)}`);
      this.dummy = r(key);
    });
    console.log(this.cache);
  };
  componentDidMount() {
    this.importAll(
      require.context('../images/gallery/', false, /\.jpg$/),
      'mount'
    );
  }
  render() {
    //this.importAll(require.context('../images/gallery/', false, /\.jpg$/),"render");
    console.log(this.cache);
    const gallery = galleries.map(gallery => {
      return (
        <a
          className="spotlight"
          href={gallery.src}
          data-title={gallery.title}
          data-description={gallery.description}
        >
          {console.log(this.cache[gallery.src])}
          <img src={this.cache[gallery.src]} />
        </a>
      );
    });
    return (
      <div className="container margintop150">
        <div className="greentext center">
          <h4>
            {' '}
            <b>Gallery</b>
          </h4>
        </div>
        <h5 className="type_5" />
        <div className="row">
          <div
            className="spotlight-group"
            data-title="Untitled"
            data-animation="fade"
            data-fullscreen="false"
            data-maximize="false"
            data-minimize="false"
          >
            {gallery}
          </div>
        </div>
      </div>
    );
  }
}
export default Gallery;

//GalleryImages.js
const galleries = [
  {
    src: './Farmer2.jpg',
    title: 'Farmer2',
    description: 'farmer2'
  },
  {
    src: './Farmer1.jpg',
    title: 'Farmer1',
    description: 'farmer1'
  },
  {
    src: './Farmer3.jpg',
    title: 'Farmer3',
    description: 'farmer3'
  }
];
export default galleries;

如果在 componentDidMount() 中调用 this.importAll()，则类变量 this.cache 为 null，但它会占用如果在 render() 中调用 this.importAll() 的值。

Answer 1

这是正确的行为，在渲染后调用 componentDidMount()。

These methods are called in the following order when an instance of a component is being created and inserted into the DOM:
* constructor()
* static getDerivedStateFromProps()
* render()
* componentDidMount()

之前调用了已弃用的 componentWillMount()。

Note: These methods are considered legacy and you should avoid them in new code:

UNSAFE_componentWillMount()