通过源码学习React.createElement

概述

本文主要学习React.createElement()API的应用。在学习之前，我们需要知道React的几个基础核心概念，React Element，React Component，React Instance不熟悉的童鞋推荐阅读下文。

React Components, Elements, and Instances。

Babel与JSX

我们都知道JSX是React中的一种标签语法，它是一个语法糖，主要用来取代React.createElement()产生React Element。但是浏览器呢不支持JSX，因此我们需要一个工具Babel来进行翻译。因此Babel就是把JSX转为浏览器相兼容的格式。

React.createElement

源码位置在createelement。

// ReactSymbols.js

// The Symbol used to tag the ReactElement-like types. If there is no native Symbol
// nor polyfill, then a plain number is used for performance.
const hasSymbol = typeof Symbol === 'function' && Symbol.for;

export const REACT_ELEMENT_TYPE = hasSymbol
  ? Symbol.for('react.element')
  : 0xeac7;
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// ReactElement.js

/**
 * Create and return a new ReactElement of the given type.
 * See https://reactjs.org/docs/react-api.html#createelement
 * type: 节点类型，可以是原生DOM节点或者React Component。React Component的type需要大写，否则当作普通DOM处理。
 * config: 对应的属性集合。
 * children: 子节点。
 */
export 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;

  if (config != null) {
    // 找出对应ref
    if (hasValidRef(config)) {
      ref = config.ref;
    }
    // 找出对应key
    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
    // 遍历取出propName挂载props
    for (propName in config) {
      if (
        hasOwnProperty.call(config, propName) &&
        !RESERVED_PROPS.hasOwnProperty(propName)
      ) {
        props[propName] = config[propName];
      }
    }
  }

  // Children can be more than one argument, and those are transferred onto
  // the newly allocated props object.
  // 子节点长度
  const childrenLength = arguments.length - 2;
  if (childrenLength === 1) {
    props.children = children;
  } else if (childrenLength > 1) {
    // 找出对应所有子节点children, 转为数组挂载props
    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;
  }

  // Resolve default props
  // defaultProps 可以为 Class 组件添加默认 props。这一般用于 props 未赋值，但又不能为 null 的情况
  // 为props添加defaultProps
  if (type && type.defaultProps) {
    const defaultProps = type.defaultProps;
    for (propName in defaultProps) {
      // 如果 props.xxx 被设置为 null，则它将保持为 null
      if (props[propName] === undefined) {
        props[propName] = defaultProps[propName];
      }
    }
  }
  if (__DEV__) {
    if (key || ref) {
      const displayName =
        typeof type === 'function'
          ? type.displayName || type.name || 'Unknown'
          : type;
      if (key) {
        defineKeyPropWarningGetter(props, displayName);
      }
      if (ref) {
        defineRefPropWarningGetter(props, displayName);
      }
    }
  }
  return ReactElement(
    type,
    key,
    ref,
    self,
    source,
    ReactCurrentOwner.current,
    props,
  );
}

/**
 * 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 $$typeof field against Symbol.for('react.element') to check
 * if something is a React Element.
 *
 * @param {*} type
 * @param {*} props
 * @param {*} key
 * @param {string|object} ref
 * @param {*} owner
 * @param {*} self A *temporary* helper to detect places where `this` is
 * different from the `owner` when React.createElement is called, so that we
 * can warn. We want to get rid of owner and replace string `ref`s with arrow
 * functions, and as long as `this` and owner are the same, there will be no
 * change in behavior.
 * @param {*} source An annotation object (added by a transpiler or otherwise)
 * indicating filename, line number, and/or other information.
 * @internal
 */
const ReactElement = function(type, key, ref, self, source, owner, props) {
  const element = {
    // This tag allows us to uniquely identify this as a React Element
    // 通过Symbol作为独一无二标识符 Symbol.for进行复用
    $$typeof: REACT_ELEMENT_TYPE,

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

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

  if (__DEV__) {
    // The validation flag is currently mutative. We put it on
    // an external backing store so that we can freeze the whole object.
    // This can be replaced with a WeakMap once they are implemented in
    // commonly used development environments.
    element._store = {};

    // To make comparing ReactElements easier for testing purposes, we make
    // the validation flag non-enumerable (where possible, which should
    // include every environment we run tests in), so the test framework
    // ignores it.
    Object.defineProperty(element._store, 'validated', {
      configurable: false,
      enumerable: false,
      writable: true,
      value: false,
    });
    // self and source are DEV only properties.
    Object.defineProperty(element, '_self', {
      configurable: false,
      enumerable: false,
      writable: false,
      value: self,
    });
    // Two elements created in two different places should be considered
    // equal for testing purposes and therefore we hide it from enumeration.
    Object.defineProperty(element, '_source', {
      configurable: false,
      enumerable: false,
      writable: false,
      value: source,
    });
    if (Object.freeze) {
      Object.freeze(element.props);
      Object.freeze(element);
    }
  }

  return element;
};
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demo

我们将下面代码在React中执行看看打印出来的React Element是什么样子。

function Comp() {
	return React.createElement("button", null, "btn");
}

const a = React.createElement("Comp", {
	className: "demo",
	attr: "12"
}, "inner", React.createElement("p", {
	id: "p"
}, "p"), React.createElement("span", {
	id: "span"
}, "span"));

console.log(a)
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我们可以对比下真正打出的React Element。