摘要：仅针对数据属性描述有效获取该属性的访问器函数。当且仅当指定对象的属性可以被枚举出时，为。冻结及其对象主要目的是为提高测试环境下效率，将的一些属性配置为不可枚举，进行遍历的时候跳过这些属性。

React系列 --- 简单模拟语法(一)
React系列 --- Jsx, 合成事件与Refs(二)
React系列 --- virtualdom diff算法实现分析(三)
React系列 --- 从Mixin到HOC再到HOOKS(四)
React系列 --- createElement, ReactElement与Component部分源码解析(五)
React系列 --- 从使用React了解Css的各种使用方案(六)

因为之前写过一些文章分别关于怎么模拟React语法,React基本知识和virtualdom diff实现思路,接下来就跟着React源码大概了解一下怎么一个过程,只是主逻辑代码,忽略部分开发环境的代码.
以下解析仅限于我当时理解,不一定准确.

还是用之前的例子

  123456

编译成

React.createElement("div", {
  className: "num",
  index: 1
}, React.createElement("span", null, "123456"));

我们看下API的语法

React.createElement(
  type,
  [props],
  [...children]
)

创建并返回给定类型的新 React element 。

因为有babel会编译JSX,所以一般很少会直接调用这个方法.

然后我们进入找到对应的源码位置查看代码 react/packages/react/src/ReactElement.js

/**
 * Create and return a new ReactElement of the given type.
 * See https://reactjs.org/docs/react-api.html#createelement
 */
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;

  // 有传config的情况下
  if (config != null) {
    // 是否有有效的Ref
    if (hasValidRef(config)) {
      ref = config.ref;
    }
    // 是否有有效的Key
    if (hasValidKey(config)) {
      key = "" + config.key;
    }

    // 暂时还没联系上下文,保存self和source
    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) {
      // 符合情况拷贝属性
      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.
  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;
  }

  // Resolve default props
  if (type && type.defaultProps) {
    const defaultProps = type.defaultProps;
    for (propName in defaultProps) {
      if (props[propName] === undefined) {
        props[propName] = defaultProps[propName];
      }
    }
  }
  if (__DEV__) {
    if (key || ref) {
      // 如果type是函数说明不是原生dom,所以可以取一下几个值 
      const displayName =
        typeof type === "function"
          ? type.displayName || type.name || "Unknown"
          : type;

      // 定义key属性的取值器,添加对应警告
      if (key) {
        defineKeyPropWarningGetter(props, displayName);
      }
      // 定义ref属性的取值器,添加对应警告
      if (ref) {
        defineRefPropWarningGetter(props, displayName);
      }
    }
  }
  return ReactElement(
    type,
    key,
    ref,
    self,
    source,
    ReactCurrentOwner.current,
    props,
  );
}

代码还比较简单,可以看出就是传入参数之后它会帮你做些特殊处理然后导出给ReactElement方法使用,如果有部分代码还不知道是干嘛的话也不用担心,下面会有说到

function hasValidRef(config) {
  // 开发环境下
  if (__DEV__) {
    // 自身是否含有ref字段
    if (hasOwnProperty.call(config, "ref")) {
      // 获取它的取值器
      const getter = Object.getOwnPropertyDescriptor(config, "ref").get;
      // 满足条件的话为非法ref
      if (getter && getter.isReactWarning) {
        return false;
      }
    }
  }
  // 直接和undefined作比较判断是否合法
  return config.ref !== undefined;
}

同上
function hasValidKey(config) {
  // 开发环境
  if (__DEV__) {
    if (hasOwnProperty.call(config, "key")) {
      const getter = Object.getOwnPropertyDescriptor(config, "key").get;
      if (getter && getter.isReactWarning) {
        return false;
      }
    }
  }
  return config.key !== undefined;
}

// 初始化标记
let specialPropKeyWarningShown, specialPropRefWarningShown;

// 定义key的取值器
function defineKeyPropWarningGetter(props, displayName) {
  const warnAboutAccessingKey = function() {
    if (!specialPropKeyWarningShown) {
      specialPropKeyWarningShown = true;
      // 目测是警告提示
      warningWithoutStack(
        false,
        "%s: `key` is not a prop. Trying to access it will result " +
          "in `undefined` being returned. If you need to access the same " +
          "value within the child component, you should pass it as a different " +
          "prop. (https://fb.me/react-special-props)",
        displayName,
      );
    }
  };
  // 是否已经警告过
  warnAboutAccessingKey.isReactWarning = true;
  // 定义key字段
  Object.defineProperty(props, "key", {
    get: warnAboutAccessingKey,
    configurable: true,
  });
}

// 同上
function defineRefPropWarningGetter(props, displayName) {
  const warnAboutAccessingRef = function() {
    if (!specialPropRefWarningShown) {
      specialPropRefWarningShown = true;
      warningWithoutStack(
        false,
        "%s: `ref` is not a prop. Trying to access it will result " +
          "in `undefined` being returned. If you need to access the same " +
          "value within the child component, you should pass it as a different " +
          "prop. (https://fb.me/react-special-props)",
        displayName,
      );
    }
  };
  warnAboutAccessingRef.isReactWarning = true;
  Object.defineProperty(props, "ref", {
    get: warnAboutAccessingRef,
    configurable: true,
  });
}

代码来看是开发模式下限制了对应key和ref的取值器,使用时会执行对应方法进行报错不让读取.

至此相关源码基本了解了

上面其中核心方法介绍是这个

Object.getOwnPropertyDescriptor(obj, prop)

方法返回指定对象上一个自有属性对应的属性描述符。（自有属性指的是直接赋予该对象的属性，不需要从原型链上进行查找的属性）

该方法允许对一个属性的描述进行检索。在 Javascript 中， 属性 由一个字符串类型的“名字”（name）和一个“属性描述符”（property descriptor）对象构成。

一个属性描述符是一个记录，由下面属性当中的某些组成的：

然后再看看ReactElement的源码

/**
 * 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,
  };

  // 开发模式下增改部分属性
  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;
};

整段代码来看它是在开发环境下对字段作处理:

创建React元素,设置对应属性值

开发环境下

创建_store属性并配置其validated的属性描述符对象,达到方便调试React元素的目的

配置_self的属性描述符对象,self和source只是DEV的属性

配置_source的属性描述符对象,出于测试的目的，应该将在两个不同位置创建的两个元素视为相等的，因此我们将它从枚举中隐藏起来。

冻结element及其props对象

主要目的是为提高测试环境下效率，将element的一些属性配置为不可枚举，进行遍历的时候跳过这些属性。

其中REACT_ELEMENT_TYPE是

// The Symbol used to tag the ReactElement type. If there is no native Symbol
// nor polyfill, then a plain number is used for performance.
var REACT_ELEMENT_TYPE =
  (typeof Symbol === "function" && Symbol.for && Symbol.for("react.element")) ||
  0xeac7;

用来标识这个对象是一个ReactElement对象,至此Jsx编译成ReactElement对象的相关源码大概知道了.

16.0.0以后已经废弃,可忽略

从官方例子来看React.Component

class Welcome extends React.Component {
  render() {
    return 
Hello, {this.props.name}
;
  }
}

从源码看这个类写法做了什么 react/packages/react/src/ReactBaseClasses.js

const emptyObject = {};
if (__DEV__) {
  Object.freeze(emptyObject);
}

/**
 * Base class helpers for the updating state of a component.
 */
function Component(props, context, updater) {
  this.props = props;
  this.context = context;
  // If a component has string refs, we will assign a different object later.
  this.refs = emptyObject;
  // We initialize the default updater but the real one gets injected by the
  // renderer.
  this.updater = updater || ReactNoopUpdateQueue;
}

Component.prototype.isReactComponent = {};

结构比较简单,结合注释可以知道只是基本赋值,里面有个更新器后面再说,现在先记住是用来更新State就行了.

/**
 * Sets a subset of the state. Always use this to mutate
 * state. You should treat `this.state` as immutable.
 *
 * There is no guarantee that `this.state` will be immediately updated, so
 * accessing `this.state` after calling this method may return the old value.
 *
 * There is no guarantee that calls to `setState` will run synchronously,
 * as they may eventually be batched together.  You can provide an optional
 * callback that will be executed when the call to setState is actually
 * completed.
 *
 * When a function is provided to setState, it will be called at some point in
 * the future (not synchronously). It will be called with the up to date
 * component arguments (state, props, context). These values can be different
 * from this.* because your function may be called after receiveProps but before
 * shouldComponentUpdate, and this new state, props, and context will not yet be
 * assigned to this.
 *
 * @param {object|function} partialState Next partial state or function to
 *        produce next partial state to be merged with current state.
 * @param {?function} callback Called after state is updated.
 * @final
 * @protected
 */
Component.prototype.setState = function(partialState, callback) {
  invariant(
    typeof partialState === "object" ||
      typeof partialState === "function" ||
      partialState == null,
    "setState(...): takes an object of state variables to update or a " +
      "function which returns an object of state variables.",
  );
  this.updater.enqueueSetState(this, partialState, callback, "setState");
};

注释很长实际代码很短,大概意思就是

不保证this.state会立即更新,所以调用方法之后可能获取的旧数据

不保证this.state会同步运行,可能最终他们会批量组合执行,可以提供一个可选完成回调当更新之后再执行

回调会在未来某个点执行,可以拿到最新的入参(state, props, context),不同于this.XX,因为它会在shouldComponentUpdate之前接收新的props属性之后执行,此时还没赋值给this.

/**
 * Forces an update. This should only be invoked when it is known with
 * certainty that we are **not** in a DOM transaction.
 *
 * You may want to call this when you know that some deeper aspect of the
 * component"s state has changed but `setState` was not called.
 *
 * This will not invoke `shouldComponentUpdate`, but it will invoke
 * `componentWillUpdate` and `componentDidUpdate`.
 *
 * @param {?function} callback Called after update is complete.
 * @final
 * @protected
 */
Component.prototype.forceUpdate = function(callback) {
  this.updater.enqueueForceUpdate(this, callback, "forceUpdate");
};

这是强制更新视图的方法

这应该只在确定我们不是在一个DOM事务中时调用

这应该在当你知道某些深层嵌套组件状态已变但是没有执行setState的时候调用

它不会执行shouldComponentUpdate但会执行componentWillUpdate 和componentDidUpdate生命周期

接着我们看源码 react/packages/shared/invariant.js做了什么

/**
 * Use invariant() to assert state which your program assumes to be true.
 *
 * Provide sprintf-style format (only %s is supported) and arguments
 * to provide information about what broke and what you were
 * expecting.
 *
 * The invariant message will be stripped in production, but the invariant
 * will remain to ensure logic does not differ in production.
 */

export default function invariant(condition, format, a, b, c, d, e, f) {
  throw new Error(
    "Internal React error: invariant() is meant to be replaced at compile " +
      "time. There is no runtime version.",
  );
}

使用constant()断言程序假定为真的状态,仅用于开发,会从生产环境中剥离保证不受影响

接下来我们再看看 react/packages/react/src/ReactNoopUpdateQueue.js

/**
 * This is the abstract API for an update queue.
 */
const ReactNoopUpdateQueue = {
  /**
   * Checks whether or not this composite component is mounted.
   * @param {ReactClass} publicInstance The instance we want to test.
   * @return {boolean} True if mounted, false otherwise.
   * @protected
   * @final
   */
  isMounted: function(publicInstance) {
    return false;
  },
  /**
   * Forces an update. This should only be invoked when it is known with
   * certainty that we are **not** in a DOM transaction.
   *
   * You may want to call this when you know that some deeper aspect of the
   * component"s state has changed but `setState` was not called.
   *
   * This will not invoke `shouldComponentUpdate`, but it will invoke
   * `componentWillUpdate` and `componentDidUpdate`.
   *
   * @param {ReactClass} publicInstance The instance that should rerender.
   * @param {?function} callback Called after component is updated.
   * @param {?string} callerName name of the calling function in the public API.
   * @internal
   */
  enqueueForceUpdate: function(publicInstance, callback, callerName) {
    warnNoop(publicInstance, "forceUpdate");
  },
  /**
   * Replaces all of the state. Always use this or `setState` to mutate state.
   * You should treat `this.state` as immutable.
   *
   * There is no guarantee that `this.state` will be immediately updated, so
   * accessing `this.state` after calling this method may return the old value.
   *
   * @param {ReactClass} publicInstance The instance that should rerender.
   * @param {object} completeState Next state.
   * @param {?function} callback Called after component is updated.
   * @param {?string} callerName name of the calling function in the public API.
   * @internal
   */
  enqueueReplaceState: function(
    publicInstance,
    completeState,
    callback,
    callerName,
  ) {
    warnNoop(publicInstance, "replaceState");
  },
  /**
   * Sets a subset of the state. This only exists because _pendingState is
   * internal. This provides a merging strategy that is not available to deep
   * properties which is confusing. TODO: Expose pendingState or don"t use it
   * during the merge.
   *
   * @param {ReactClass} publicInstance The instance that should rerender.
   * @param {object} partialState Next partial state to be merged with state.
   * @param {?function} callback Called after component is updated.
   * @param {?string} Name of the calling function in the public API.
   * @internal
   */
  enqueueSetState: function(
    publicInstance,
    partialState,
    callback,
    callerName,
  ) {
    warnNoop(publicInstance, "setState");
  },
};
export default ReactNoopUpdateQueue;

里面提供了三个函数,分别是

enqueueForceUpdate: 强制更新队列包装器

enqueueReplaceState: 状态替换队列包装器

enqueueSetState: 状态更新队列包装器

实际里面都是调用同一个方法warnNoop,设置首参数都一样.

const didWarnStateUpdateForUnmountedComponent = {};

function warnNoop(publicInstance, callerName) {
  if (__DEV__) {
    const constructor = publicInstance.constructor;
    const componentName =
      (constructor && (constructor.displayName || constructor.name)) ||
      "ReactClass";
    const warningKey = `${componentName}.${callerName}`;
    if (didWarnStateUpdateForUnmountedComponent[warningKey]) {
      return;
    }
    warningWithoutStack(
      false,
      "Can"t call %s on a component that is not yet mounted. " +
        "This is a no-op, but it might indicate a bug in your application. " +
        "Instead, assign to `this.state` directly or define a `state = {};` " +
        "class property with the desired state in the %s component.",
      callerName,
      componentName,
    );
    didWarnStateUpdateForUnmountedComponent[warningKey] = true;
  }
}

目测应该是给传入的React组件实例设置componentName和KEY, 在里面我们再次看到同一个方法warningWithoutStack,我们直接看看他究竟做了什么. react/packages/shared/warningWithoutStack.js

/**
 * Similar to invariant but only logs a warning if the condition is not met.
 * This can be used to log issues in development environments in critical
 * paths. Removing the logging code for production environments will keep the
 * same logic and follow the same code paths.
 */

let warningWithoutStack = () => {};

if (__DEV__) {
  warningWithoutStack = function(condition, format, ...args) {
    if (format === undefined) {
      throw new Error(
        "`warningWithoutStack(condition, format, ...args)` requires a warning " +
          "message argument",
      );
    }
    if (args.length > 8) {
      // Check before the condition to catch violations early.
      throw new Error(
        "warningWithoutStack() currently supports at most 8 arguments.",
      );
    }
    if (condition) {
      return;
    }
    if (typeof console !== "undefined") {
      const argsWithFormat = args.map(item => "" + item);
      argsWithFormat.unshift("Warning: " + format);

      // We intentionally don"t use spread (or .apply) directly because it
      // breaks IE9: https://github.com/facebook/react/issues/13610
      Function.prototype.apply.call(console.error, console, argsWithFormat);
    }
    try {
      // --- Welcome to debugging React ---
      // This error was thrown as a convenience so that you can use this stack
      // to find the callsite that caused this warning to fire.
      let argIndex = 0;
      const message =
        "Warning: " + format.replace(/%s/g, () => args[argIndex++]);
      throw new Error(message);
    } catch (x) {}
  };
}

export default warningWithoutStack;

类似invariant但是只有不满足条件的时候才会打印出警告.这可以用于在关键路径中记录开发环境中的问题.生产环境下会移除日志代码保证正常逻辑.代码只是一些基本的条件设定和优雅降级代码.

还有一个类似的继承类PureComponent,可以用于组件进行浅对比决定是否需要更新

function ComponentDummy() {}
ComponentDummy.prototype = Component.prototype;

/**
 * Convenience component with default shallow equality check for sCU.
 */
function PureComponent(props, context, updater) {
  this.props = props;
  this.context = context;
  // If a component has string refs, we will assign a different object later.
  this.refs = emptyObject;
  this.updater = updater || ReactNoopUpdateQueue;
}

const pureComponentPrototype = (PureComponent.prototype = new ComponentDummy());
pureComponentPrototype.constructor = PureComponent;
// Avoid an extra prototype jump for these methods.
Object.assign(pureComponentPrototype, Component.prototype);
pureComponentPrototype.isPureReactComponent = true;

基本代码和Component相似,也继承自它的原型.但不继承其自身的属性方法.