React源码解析系列(一) —— React.createElement

在开始看react源码前觉得代码量应该会非常庞大，然而在看完react源码后发现实际上react的源码只是一些API和数据结构的定义，真正的更新渲染逻辑是react-dom这块源码里控制的。

本此源码解析系列不包括dev环境下的代码，只对主流程代码进行讲解。

react的源码地址为：react

我们先来看下react的目录结构：

下面让我们看下react对我们暴露的API：/packages/react/src/React.js

const React = {
  Children: {
    map,
    forEach,
    count,
    toArray,
    only,
  },

  createRef,
  Component,
  PureComponent,

  createContext,
  forwardRef,
  lazy,
  memo,

  Fragment: REACT_FRAGMENT_TYPE,
  StrictMode: REACT_STRICT_MODE_TYPE,
  Suspense: REACT_SUSPENSE_TYPE,

  createElement: __DEV__ ? createElementWithValidation : createElement,
  cloneElement: __DEV__ ? cloneElementWithValidation : cloneElement,
  createFactory: __DEV__ ? createFactoryWithValidation : createFactory,
  isValidElement: isValidElement,

  version: ReactVersion,

  unstable_ConcurrentMode: REACT_CONCURRENT_MODE_TYPE,
  unstable_Profiler: REACT_PROFILER_TYPE,

  __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED: ReactSharedInternals,
};

createElement

React.createElement这个方法用于创建一个ReactElement，也就是jsx语法解析后的对象，如

// jsx
msg

// 上述jsx解析完成后的结构
React.createElement('div', { id: 'app' }, 'msg')

下面让我们看下createElement这个函数：

const RESERVED_PROPS = {
  key: true,
  ref: true,
  __self: true,
  __source: true,
};

function hasValidRef(config) {
  return config.ref !== undefined;
}
function hasValidKey(config) {
  return config.key !== undefined;
}

// React.createElement('div', { id: 'btn' }, '提交');
// config包含组件上的所有props，包括：事件、key、ref、各种属性
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) {
    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
    // 这里遍历传入的config
    for (propName in config) {
      if (
        hasOwnProperty.call(config, propName) &&
        // 不是key和ref
        !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.
  // 这里对children特殊处理，如果传入多个则props.children为数组，否则直接等于传入的children
  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];
    }
    props.children = childArray;
  }

  // Resolve default props
  // 对默认值的特殊处理
  if (type && type.defaultProps) {
    const defaultProps = type.defaultProps;
    for (propName in defaultProps) {
      if (props[propName] === undefined) {
        props[propName] = defaultProps[propName];
      }
    }
  }
  // props不包含key和ref
  return ReactElement(
    type,
    key,
    ref,
    self,
    source,
    ReactCurrentOwner.current,
    props,
  );
}

让我们再看几个例子来理解下

从上面几个例子可以看出type的几种取值（这里一定要理解type指向的值，后续更新流程中会有很多地方用到type）：

• 当为原生标签时type为标签对应的字符串，如 'div'。
• 当为function或者class等自定义组件时，type表示组件的function或者class。

现在让我们看下返回的element的结构

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;
};

上面还剩两个属性没讲到

1、：REACT_ELEMENT_TYPE，表示这是通过createElement创建的。在react中还有一种情况是通过ReactDOM.createPoratl()创建，这时它的'$$typeof'为REACT_PORTAL_TYPE。

2、'_owner'：表示创建当前组件的class组件的实例，用于后续ref时使用。因为字符串类型的ref是要绑定到class实例的this.refs.xxx上的。

注意：这里留意一下$$typeof和type这两个属性，后续更新逻辑中会有大量对这两个属性的判断，比较容易混淆。

createFactory

这个方法就是将createElement的type绑定一个固定的类型，后续不需要传type了。

export function createFactory(type) {
  const factory = createElement.bind(null, type);
  factory.type = type;
  return factory;
}

cloneElement

用于复制一个element

export function cloneElement(element, config, children) {
  // 校验下element不能为空，若为空则throw new Error
  invariant(
    !(element === null || element === undefined),
    'React.cloneElement(...): The argument must be a React element, but you passed %s.',
    element,
  );

  let propName;

  // Original props are copied
  const props = Object.assign({}, element.props);

  // Reserved names are extracted
  let key = element.key;
  let ref = element.ref;
  // Self is preserved since the owner is preserved.
  const self = element._self;
  // Source is preserved since cloneElement is unlikely to be targeted by a
  // transpiler, and the original source is probably a better indicator of the
  // true owner.
  const source = element._source;

  // Owner will be preserved, unless ref is overridden
  let owner = element._owner;

  // 复制config到element.props里，并更新key和ref
  if (config != null) {
    if (hasValidRef(config)) {
      // Silently steal the ref from the parent.
      ref = config.ref;
      owner = ReactCurrentOwner.current;
    }
    if (hasValidKey(config)) {
      key = '' + config.key;
    }

    // Remaining properties override existing props
    let defaultProps;
    if (element.type && element.type.defaultProps) {
      defaultProps = element.type.defaultProps;
    }
    for (propName in config) {
      if (
        hasOwnProperty.call(config, propName) &&
        !RESERVED_PROPS.hasOwnProperty(propName)
      ) {
        if (config[propName] === undefined && defaultProps !== undefined) {
          // Resolve default props
          props[propName] = defaultProps[propName];
        } else {
          props[propName] = config[propName];
        }
      }
    }
  }

  // Children can be more than one argument, and those are transferred onto
  // the newly allocated props object.
  const childrenLength = arguments.length - 2;
  // 如果传入child，则更新props.children
  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];
    }
    props.children = childArray;
  }

  return ReactElement(element.type, key, ref, self, source, owner, props);
}

isValidElement

根据$$typeof来判断是否是正确的element格式

export function isValidElement(object) {
  return (
    typeof object === 'object' &&
    object !== null &&
    object.$$typeof === REACT_ELEMENT_TYPE
  );
}