杨文坚

2015-05-04 22:58

React直出实现与原理

本文作者:IMWeb 杨文坚 原文出处:IMWeb社区 未经同意,禁止转载

前一篇文章我们介绍了虚拟DOM的实现与原理,这篇文章我们来讲讲React的直出。 比起MVVM,React比较容易实现直出,那么React的直出是如何实现,有什么值得我们学习的呢?

为什么MVVM不能做直出?

对于MVVM,HTML片段即为配置,而直出后的HTML无法还原配置,所以问题不是MVVM能否直出,而是在于直出后的片段能否还原原来的配置。下面是一个简单的例子:

<sapn>Hello {name}!</span>

上面这段HTML配置和数据在一起,直出后会变成:

<span>Hello world!</span>

这时候当我们失去了name的值改变的时候会导致页面渲染这个细节。当然,如果为了实现MVVM直出我们可能有另外的方法来解决,例如直出结果变成这样:

<span>Hello <span q-text="name">world</span>!</span>

这时候我们是可以把丢失的信息找回来的,当然结构可能和我们想象的有些差别。当然还有其他问题,例如直出HTML不一定能反向还原数据,由于篇幅问题,这里不展开讨论。

React如何直出?

2

如图:

  • React的虚拟DOM的生成是可以在任何支持Javascript的环境生成的,所以可以在NodeJS或Iojs环境生成
  • 虚拟DOM可以直接转成String
  • 然后插入到html文件中输出给浏览器便可

具体例子可以参考,https://github.com/DavidWells/isomorphic-react-example/,下面是其渲染路由的写法:

// https://github.com/DavidWells/isomorphic-react-example/blob/master/app/routes/coreRoutes.js

var React = require('react/addons');
var ReactApp = React.createFactory(require('../components/ReactApp').ReactApp);

module.exports = function(app) {

    app.get('/', function(req, res){
        // React.renderToString takes your component
        // and generates the markup
        var reactHtml = React.renderToString(ReactApp({}));
        // Output html rendered by react
        // console.log(myAppHtml);
        res.render('index.ejs', {reactOutput: reactHtml});
    });

};

OK,我们现在知道如何利用React实现直出,以及如何前后端代码复用。

但还有下面几个问题有待解决:

  • 如何渲染文字节点,每个虚拟DOM节点是需要对应实际的节点,但无法通过html文件生成相邻的Text Node,例如下面例子应当如何渲染:
    React.createClass({
      render: function () {
          return (
              <p>
                  Hello {name}!            
              </p>
          );
      }
    })
    
  • 如何避免直出的页面被React重新渲染一遍?或者直出的页面和前端的数据是不对应的怎么办?

相邻的Text Node,想多了相邻的span而已

1

通过一个简单的例子,我们可以发现,实际上React根本没用Text Node,而是使用span来代替Text Node,这样就可以实现虚拟DOM和直出DOM的一一映射关系。

重复渲染?没门

刚刚的例子,如果我们通过React.renderToString拿到<Test />可以发现是:

<p data-reactid=".0" data-react-checksum="-793171045"><span data-reactid=".0.0">Hello </span><span data-reactid=".0.1">world</span><span data-reactid=".0.2">!</span></p>

我们可以发现一个有趣的属性data-react-checksum,这是啥?实际上这是上面这段HTML片段的adler32算法值。实际上调用React.render(<MyComponent />, container);时候做了下面一些事情:

  • 看看container是否为空,不为空则认为有可能是直出了结果。
  • 接下来第一个元素是否有data-react-checksum属性,如果有则通过React.renderToString拿到前端的,通过adler32算法得到的值和data-react-checksum对比,如果一致则表示,无需渲染,否则重新渲染,下面是adler32算法实现: ```javascript var MOD = 65521;

// This is a clean-room implementation of adler32 designed for detecting // if markup is not what we expect it to be. It does not need to be // cryptographically strong, only reasonably good at detecting if markup // generated on the server is different than that on the client. function adler32(data) { var a = 1; var b = 0; for (var i = 0; i < data.length; i++) { a = (a + data.charCodeAt(i)) % MOD; b = (b + a) % MOD; } return a | (b << 16); }

* 如果需要重新渲染,先通过下面简单的差异算法找到差异在哪里,打印出错误:
```javascript/**
 * Finds the index of the first character
 * that's not common between the two given strings.
 *
 * @return {number} the index of the character where the strings diverge
 */
function firstDifferenceIndex(string1, string2) {
  var minLen = Math.min(string1.length, string2.length);
  for (var i = 0; i < minLen; i++) {
    if (string1.charAt(i) !== string2.charAt(i)) {
      return i;
    }
  }
  return string1.length === string2.length ? -1 : minLen;
}

下面是首屏渲染时的主要逻辑,可以发现React对首屏实际上也是通来渲染的:

_mountImageIntoNode: function(markup, container, shouldReuseMarkup) {
    ("production" !== process.env.NODE_ENV ? invariant(
      container && (
        (container.nodeType === ELEMENT_NODE_TYPE || container.nodeType === DOC_NODE_TYPE)
      ),
      'mountComponentIntoNode(...): Target container is not valid.'
    ) : invariant(container && (
      (container.nodeType === ELEMENT_NODE_TYPE || container.nodeType === DOC_NODE_TYPE)
    )));

    if (shouldReuseMarkup) {
      var rootElement = getReactRootElementInContainer(container);
      if (ReactMarkupChecksum.canReuseMarkup(markup, rootElement)) {
        return;
      } else {
        var checksum = rootElement.getAttribute(
          ReactMarkupChecksum.CHECKSUM_ATTR_NAME
        );
        rootElement.removeAttribute(ReactMarkupChecksum.CHECKSUM_ATTR_NAME);

        var rootMarkup = rootElement.outerHTML;
        rootElement.setAttribute(
          ReactMarkupChecksum.CHECKSUM_ATTR_NAME,
          checksum
        );

        var diffIndex = firstDifferenceIndex(markup, rootMarkup);
        var difference = ' (client) ' +
          markup.substring(diffIndex - 20, diffIndex + 20) +
          '\n (server) ' + rootMarkup.substring(diffIndex - 20, diffIndex + 20);

        ("production" !== process.env.NODE_ENV ? invariant(
          container.nodeType !== DOC_NODE_TYPE,
          'You\'re trying to render a component to the document using ' +
          'server rendering but the checksum was invalid. This usually ' +
          'means you rendered a different component type or props on ' +
          'the client from the one on the server, or your render() ' +
          'methods are impure. React cannot handle this case due to ' +
          'cross-browser quirks by rendering at the document root. You ' +
          'should look for environment dependent code in your components ' +
          'and ensure the props are the same client and server side:\n%s',
          difference
        ) : invariant(container.nodeType !== DOC_NODE_TYPE));

        if ("production" !== process.env.NODE_ENV) {
          ("production" !== process.env.NODE_ENV ? warning(
            false,
            'React attempted to reuse markup in a container but the ' +
            'checksum was invalid. This generally means that you are ' +
            'using server rendering and the markup generated on the ' +
            'server was not what the client was expecting. React injected ' +
            'new markup to compensate which works but you have lost many ' +
            'of the benefits of server rendering. Instead, figure out ' +
            'why the markup being generated is different on the client ' +
            'or server:\n%s',
            difference
          ) : null);
        }
      }
    }

    ("production" !== process.env.NODE_ENV ? invariant(
      container.nodeType !== DOC_NODE_TYPE,
      'You\'re trying to render a component to the document but ' +
        'you didn\'t use server rendering. We can\'t do this ' +
        'without using server rendering due to cross-browser quirks. ' +
        'See React.renderToString() for server rendering.'
    ) : invariant(container.nodeType !== DOC_NODE_TYPE));

    se(container, markup);
  }

最后

尝试一下下面的代码,想想React为啥认为这是错误的?

var Test = React.createClass({
  getInitialState: function() {
    return {name: 'world'};
  },
  render: function() {
    return (
        <p>Hello</p>
        <p>
            Hello {this.state.name}!
        </p>
    );
  }
});

React.render(
  <Test />,
  document.getElementById('content')
);
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