React源码学习（三）：React.createElement

一、前言

【在正式学习该文章前，可以先学习VirtualDOM系列文章：VirtualDOM】

上篇文章（从 JSX 到 React.createElement ），以及 Babel 的 babel-standalone.js 分析，告诉我们，无论是手动调用，还是由babel来调用，在真正调用 ReactDOM.render 渲染之前，都会去调用 React.createElement 方法，然后再塞到 div.container 中，最后才渲染。

二、React.createElement

[代码所在文件：react/src/ReactElement.s]

const React = {
  ......
  createElement: __DEV__ ? createElementWithValidation : createElement,
  ......
};

在React.js中，如果当前运行的环境是DEV，则会先调用 createElementWithValidation ，对传入的 type 进行检查，然后再调用 createElement ，而生产环境则是直接调用 createElement ；

三、再谈JSX到React.createElement

当我们有很多嵌套HTML标签，再被加载运行时，babel会将整个JSX都transform为React.createElement，且保持嵌套顺序：

<script type="text/babel">
  ReactDOM.render(
    <div id='app'>
      Hello, chris!
      <h1>
        title
      </h1>
    </div>,
    document.getElementById('container')
  );
</script>

转换后的结果如下：

trans.png

四、React.createElement执行顺序（ReactElement 生成顺序）

以上节第三点 div 与 h1 嵌套为例，babel 转换代码后，将依次从里到外，从上至下（若有兄弟节点）调用 React.createElement，创建 ReactElement对象，即可以理解为最终到根时，就生成了一棵 ReactElement Tree ：

1. 先调用 React.createElement 创建 h1 的 ReactElement对象；

create-1.png

2. 然后再调用 React.createElement 创建 div 的 ReactElement对象；

create-2.png

五、React.createElement源码分析

源码稍微较长，这里将分段分析！

5.1 局部变量初始化

/**
 * type = 'div'
 * config = {id: 'app'}
 * children = 'Hello, chris!'
 * arguments[3] = {$$typeof: Symbol(react.element), type: 'h1',...}
 */
function createElement(type, config, children) {
  let propName;

  // Reserved names are extracted
  const props = {};

  let key = null;
  let ref = null;
  let self = null;
  let source = null;
  ......
}

5.2 config 解析

/**
 * config = {id: 'app'}
 */
function createElement(type, config, children) {
  ......
  if (config != null) {
    if (hasValidRef(config)) {
      ref = config.ref;
    }
    if (hasValidKey(config)) {
      key = '' + config.key;
    }

    self = config.__self === undefined ? null : config.__self;
    source = config.__source === undefined ? null : config.__source;
    // Remaining properties are added to a new props object
    for (propName in config) {
      if (
        hasOwnProperty.call(config, propName) &&
        !RESERVED_PROPS.hasOwnProperty(propName)
      ) {
        props[propName] = config[propName];
      }
    }
  }
  ......
}

config对象中，没有 ref, key, __self, __source，只有 id: 'app'，所以，props['id'] = 'app'；

5.3 判断真实children个数

/**
 * children = 'Hello, chris!'
 * argument[3] = {$$typeof: Symbol(react.element), type: 'h1',...}
 */
function createElement(type, config, children) {
  ......
  // Children can be more than one argument, and those are transferred onto
  // the newly allocated props object.
  const childrenLength = arguments.length - 2;
  if (childrenLength === 1) {
    props.children = children;
  } else if (childrenLength > 1) {
    const childArray = Array(childrenLength);
    for (let i = 0; i < childrenLength; i++) {
      childArray[i] = arguments[i + 2];
    }
    if (__DEV__) {
      if (Object.freeze) {
        Object.freeze(childArray);
      }
    }
    props.children = childArray;
  }
  ......
}

arguments默认是3个参数：type、config、children（这里的是字符串 'Hello, chris!' ）；但是如果有嵌套HTML，则 arguments 就不止3个无数，会更多，所以，代码中，减去 arguments 前两个，判断剩下的个数，并最后赋值给 props（props['children'] = children 或 childArray ）；

5.4 如果 type 是对象，则判断是否有 defaultProps，有则赋值给 props

function createElement(type, config, children) {
  ......
  // Resolve default props
  if (type && type.defaultProps) {
    const defaultProps = type.defaultProps;
    for (propName in defaultProps) {
      if (props[propName] === undefined) {
        props[propName] = defaultProps[propName];
      }
    }
  }
  ......
}

5.5 返回 ReactElement 对象

function createElement(type, config, children) {
  ......
  return ReactElement(
    type,
    key,
    ref,
    self,
    source,
    ReactCurrentOwner.current,
    props,
  );
}

5.6 ReactElement 方法（简单的封装成 REACT_ELEMENT_TYPE 对象）

/**
  * Factory method to create a new React element. This no longer adheres to
  * the class pattern, so do not use new to call it. Also, no instanceof check
  * will work. Instead test $$typeof field against Symbol.for('react.element') to check
  * if something is a React Element.
  *
  * @param {*} type
  * @param {*} props
  * @param {*} key
 * @param {string | object} ref
 * @param {*} owner
 * @param {*} self A *temporary* helper to detect places where `this` is
 * different from the `owner` when React.createElement is called, so that we
 * can warn. We want to get rid of owner and replace string `ref`s with arrow
 * functions, and as long as `this` and owner are the same, there will be no
 * change in behavior.
 * @param {*} source An annotation object (added by a transpiler or otherwise)
 * indicating filename, line number, and/or other information.
 * @internal
 */
const ReactElement = function(type, key, ref, self, source, owner, props) {
  const element = {
    // This tag allows us to uniquely identify this as a React Element
    $$typeof: REACT_ELEMENT_TYPE,

    // Built-in properties that belong on the element
    type: type,
    key: key,
    ref: ref,
    props: props,

    // Record the component responsible for creating this element.
    _owner: owner,
  };

  return element;
};