React再思考

import React from 'react';

/**
 * Create and return a new ReactElement of the given type.
 * See https://reactjs.org/docs/react-api.html#createelement
 * type: 节点类型
 * 如果是原生的节点类型，那么就是一个字符串 tagName
 * 如果是一个组件(我们自己定义的、或者React内置提供的)那么就是一个变量
 *
 * config: 就是写在jsx上的属性，它们都会转为key: value格式存到config对象上
 *
 * children: 所有后代元素
 */
 export function createElement(type, config, children) {
  let propName;

  // Reserved names are extracted
  // 对于组件内部来说传入的属性就是props
  const props = {};

  // 预定义key ref等变量用于后面存值
  // key ref属于特殊的attrs需要剔除
  let key = null;
  let ref = null;
  let self = null;
  let source = null;

  if (config != null) {
    if (hasValidRef(config)) {
      // 存储合法的ref
      ref = config.ref;
    }
    if (hasValidKey(config)) {
      // 存储合法的key => string
      key = '' + config.key;
    }

    self = config.__self === undefined ? null : config.__self;
    source = config.__source === undefined ? null : config.__source;
    // 其他属性枚举到props对象
    for (propName in config) {
      if (
        // 仅限config自身的属性 && 排除React预留的props
        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.
  // 获取children的个数，一个节点下的children是可以有多个的
  const childrenLength = arguments.length - 2;

  // 赋值处理props.children
  if (childrenLength === 1) {
    props.children = children;
  } else if (childrenLength > 1) {
    // 定长数组、再遍历拷贝
    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
  // 处理defaultProps，另外需要处理defaultProps的顺序
  /**
   * class CustomButton extends React.component{}
   * CustomButton.defaultProps = {}
   * 可见type是一个组件
   */
  if (type && type.defaultProps) {
    const defaultProps = type.defaultProps;
    for (propName in defaultProps) {
      // 如果 props.color 被设置为 null，则它将保持为 null
      // 关于这里为什么是undefined、而不是null，其实很好理解，定义了一个遍历而不初始化那它就是undefined，而null需要手动初始化。
      if (props[propName] === undefined) {
        props[propName] = defaultProps[propName];
      }
    }
  }

 if (__DEV__) {
    if (key || ref) {
      // 处理displayName，即推导过程
      // 如果是组件、先从组件的displayName取、其次是组件的名字、最后是默认值
      const displayName =
        typeof type === 'function'
          ? type.displayName || type.name || 'Unknown'
          : type;
      if (key) {
        defineKeyPropWarningGetter(props, displayName);
      }
      if (ref) {
        defineRefPropWarningGetter(props, displayName);
      }
    }
  }

  // 返回一个真正的ReactElement
  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;
};