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[Java教程]React直出实现与原理


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.jsvar 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:
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:
/** * 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过innerHTML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));  setInnerHTML(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'));