React入门

React入门

　　自从有了Node.js后，JAVASCRIPT已经变成了一个与JavaEE不相上下的企业级语言了。React 是一个用于构建用户界面的 JAVASCRIPT 库，通常也运行在Node.js的平台之下。React主要用于构建UI，很多人认为 React 是 MVC 中的 V（视图）。React 起源于 Facebook 的内部项目，用来架设 Instagram 的网站，并于 2013 年 5 月开源。React 拥有较高的性能，代码逻辑非常简单，越来越多的人已开始关注和使用它。结合个人使用JavaEE多年的体会，React就象当年sun主推的JSF框架的作用有类似之处，是一个组件型的V端框架。作为入门，必须先要去React官网看一下，官网地址：https://reactjs.org ，源码地址：https://github.com/facebook/react/ ，编译好的React版本的下载地址：https://github.com/facebook/react/releases

　　目前个人的理解是React可以基于Node.js进行使用（这是主流的应用方式）。也可以基于浏览器的使用，这种形式则跟传统js项目的形式差不多。下面将分别演示这两种项目的形式，同时也通过代码熟悉一下React的一些概念。

　　一、先来体验传统的方式，即仅基于浏览器使用React的方式：（在官网上正好有这样的例子 https://reactjs.org/docs/add-react-to-a-website.html）

代码只有两个文件，一个index.html和like_button.js ，在这个项目上我们仅用了 react.development.js 和 react-dom.development.js

index.html

<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8" />
    <title>Add React in One Minute</title>
  </head>
  <body>

    <h2>Add React in One Minute</h2>
    <p>This page demonstrates using React with no build tooling.</p>
    <p>React is loaded as a script tag.</p>

    <!-- 将放置React组件在这个div中 -->
    <div ></div>

    <!-- 加载React. -->
    <!-- 注意: 在生产环境部署时, 需要替换 "development.js" 为"production.min.js". -->
    <script src="https://unpkg.com/react@16/umd/react.development.js" crossorigin></script>
    <script src="https://unpkg.com/react-dom@16/umd/react-dom.development.js" crossorigin></script>

    <!-- 加载我们的 React组件. -->
    <script src="like_button.js"></script>

  </body>
</html>

like_button.js

'use strict';

const e = React.createElement;  //创建React Element的一种简写形式

class LikeButton extends React.Component {
  constructor(props) {
    super(props);
    this.state = { liked: false };
  }

  render() {
    if (this.state.liked) {
      return 'You liked this.';
    }

    return e(
      'button',
      { onClick: () => this.setState({ liked: true }) },
      'Like'
    );
  }
}

const domContainer = document.querySelector('#like_button_container');
ReactDOM.render(e(LikeButton), domContainer);

通过浏览器访问index.html页面即可看到效果。

二. 结合node.js进行使用

执行命令npx create-react-app my-app创建一个基于Node.js的React项目，其中my-app是项目名，执行后会在项目的目录下创建以my-app命名的目录，进入my-app目录后，项目结构如下：

React入门

启动项目，默认会启动服务监听3000端口：
cd my-app
npm start

访问 http://127.0.0.1:3000，即会触发src/index.js的代码，只不过此时是由Node.js来执行该代码，在浏览器上展示界面如下：

React入门

项目运行说明：

npm start命令实示是启动了项目下的package.json文件中定义的脚本，如图：

React入门

即react-scripts模块下的start脚本，在项目下的node_modules/scripts/目录下的start.js 代码有引用到路径配置的一段代码：

React入门

然后打开config/paths.js文件，可以看到项目内的相关文件的设置：

React入门

这样也就大致能理解项目下生成的这些文件和目录的作用了。

再打开src/index.js文件，如下：

import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
import App from './App';
import registerServiceWorker from './registerServiceWorker';

ReactDOM.render(<App />, document.getElementById('root'));
registerServiceWorker();

再打开src/registerServiceWorker.js文件，如下：

// In production, we register a service worker to serve assets from local cache.

// This lets the app load faster on subsequent visits in production, and gives
// it offline capabilities. However, it also means that developers (and users)
// will only see deployed updates on the "N+1" visit to a page, since previously
// cached resources are updated in the background.

// To learn more about the benefits of this model, read https://goo.gl/KwvDNy.
// This link also includes instructions on opting out of this behavior.

const isLocalhost = Boolean(
  window.location.hostname === 'localhost' ||
    // [::1] is the IPv6 localhost address.
    window.location.hostname === '[::1]' ||
    // 127.0.0.1/8 is considered localhost for IPv4.
    window.location.hostname.match(
      /^127(?:.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)){3}$/
    )
);

export default function register() {
  if (process.env.NODE_ENV === 'production' && 'serviceWorker' in navigator) {
    // The URL constructor is available in all browsers that support SW.
    const publicUrl = new URL(process.env.PUBLIC_URL, window.location);
    if (publicUrl.origin !== window.location.origin) {
      // Our service worker won't work if PUBLIC_URL is on a different origin
      // from what our page is served on. This might happen if a CDN is used to
      // serve assets; see https://github.com/facebookincubator/create-react-app/issues/2374
      return;
    }

    window.addEventListener('load', () => {
      const swUrl = `${process.env.PUBLIC_URL}/service-worker.js`;

      if (isLocalhost) {
        // This is running on localhost. Lets check if a service worker still exists or not.
        checkValidServiceWorker(swUrl);

        // Add some additional logging to localhost, pointing developers to the
        // service worker/PWA documentation.
        navigator.serviceWorker.ready.then(() => {
          console.log(
            'This web app is being served cache-first by a service ' +
              'worker. To learn more, visit https://goo.gl/SC7cgQ'
          );
        });
      } else {
        // Is not local host. Just register service worker
        registerValidSW(swUrl);
      }
    });
  }
}

function registerValidSW(swUrl) {
  navigator.serviceWorker
    .register(swUrl)
    .then(registration => {
      registration.onupdatefound = () => {
        const installingWorker = registration.installing;
        installingWorker.onstatechange = () => {
          if (installingWorker.state === 'installed') {
            if (navigator.serviceWorker.controller) {
              // At this point, the old content will have been purged and
              // the fresh content will have been added to the cache.
              // It's the perfect time to display a "New content is
              // available; please refresh." message in your web app.
              console.log('New content is available; please refresh.');
            } else {
              // At this point, everything has been precached.
              // It's the perfect time to display a
              // "Content is cached for offline use." message.
              console.log('Content is cached for offline use.');
            }
          }
        };
      };
    })
    .catch(error => {
      console.error('Error during service worker registration:', error);
    });
}

function checkValidServiceWorker(swUrl) {
  // Check if the service worker can be found. If it can't reload the page.
  fetch(swUrl)
    .then(response => {
      // Ensure service worker exists, and that we really are getting a JS file.
      if (
        response.status === 404 ||
        response.headers.get('content-type').indexOf('javascript') === -1
      ) {
        // No service worker found. Probably a different app. Reload the page.
        navigator.serviceWorker.ready.then(registration => {
          registration.unregister().then(() => {
            window.location.reload();
          });
        });
      } else {
        // Service worker found. Proceed as normal.
        registerValidSW(swUrl);
      }
    })
    .catch(() => {
      console.log(
        'No internet connection found. App is running in offline mode.'
      );
    });
}

export function unregister() {
  if ('serviceWorker' in navigator) {
    navigator.serviceWorker.ready.then(registration => {
      registration.unregister();
    });
  }
}

可以看到start.js启动了http服务在监听3000端口，node.js以package.json文件中的入口index.js上面的React代码来渲染组件，通过public/index.html作为模板，最终返回给浏览器进行展示。

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