React源码解读【一】API复习与基础

人生就是你全部选择的总和。那么，你今天要作什么？——阿尔贝·加缪node

四年，如人生小溪中的一洼清水，如历史长河中的一点水滴，而却就是这四年，我完成了从懵懂到成熟的蜕变。回首这四年，有过创业，有过生病，有过说不出的苦楚，也有过让我笑不间断的喜悦。react

那年的背包，依然在背着；那年的代码，依然还在用相似的逻辑实现着；一件好的东西总会让我爱不释手，react就是其中一个，从React.createClass到React.createElement到React.Component；从Mixin到class component到functional component；从flux到redux、mobx到hooks；每一次更进一步，每一次爱更深一筹。就在这个时间节点，我以为我做为一个禅意开发者，应该记念一下我这位老情人了。git

这一系列文章与视频讲解（微信公众号：《JavaScript全栈》）将深刻剖析React源码。github

为了保证源码一致，请阅读与本文及视频相同版本，可到github下载，地址：github.com/Walker-Leee…redux

解读安排以下react-native

好了，感慨发完，咱们来一块儿揭开React神秘面纱吧！api

React基础与API

早期作react开发的同窗应该都知道，最开始react和react-dom在同一个包，后来为了作平台移植性，将react与react-dom分拆，相信作过react-native的同窗都知道，咱们写react-native项目时，也是用的react，只是表现层用了react-native的组件与api。因此看react源码咱们先分析react对于api的定义。数组

我将react中的部分代码片断展现于此promise

import {Component, PureComponent} from './ReactBaseClasses';
import {createRef} from './ReactCreateRef';
import {forEach, map, count, toArray, only} from './ReactChildren';
import {
  createElement,
  createFactory,
  cloneElement,
  isValidElement,
  jsx,
} from './ReactElement';
import {createContext} from './ReactContext';
import {lazy} from './ReactLazy';
import forwardRef from './forwardRef';
import memo from './memo';
import {
  useCallback,
  useContext,
  useEffect,
  useImperativeHandle,
  useDebugValue,
  useLayoutEffect,
  useMemo,
  useReducer,
  useRef,
  useState,
  useResponder,
  useTransition,
  useDeferredValue,
} from './ReactHooks';
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Component 与 PureComponent

二者的区别在于，PureComponent多给了一个标识，经过该标识在ReactFiberClassComponent中处理，决定是否进行shalloEqual。浏览器

if (ctor.prototype && ctor.prototype.isPureReactComponent) {
  return (
    !shallowEqual(oldProps, newProps) || !shallowEqual(oldState, newState)
  );
}
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比较state和props的值，来判断是否须要更新。

相似改对比的地方还有一个，就是在 shouldComponentUpdate 。

createRef

更新后的ref用法，咱们能够看到React即将抛弃<div>123</div> ，之后只能使用如下两种方式使用ref。

class App extends React.Component{

  constructor() {
    this.ref = React.createRef()
  }

  render() {
    return <div ref={this.ref} />
    // 或者是
    return <div ref={(node) => this.ref = node} />
  }

}
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forwardRef

用来解决组件封装时，ref 的传递问题，你们看过antd源码的应该知道，不少组件使用到了 forwardRef 。好比form组件中，@Form.create() 将form组件相关的props绑定到组件上，this.props.validate

ReactChildren

该文件中包含api有：forEach, map, count, toArray, only ，这些方法都是对于reactChildren的处理。

createElement 与 cloneElement

咱们在使用react彷佛少见createElement方法，由于在咱们如今项目中大多用上了jsx，大多时候是babel帮咱们将jsx转换为createElement，React.createElement('h1', {id: 'title'}, 'hello world') 。

cloneElement顾名思义，拷贝已有元素。

memo

函数组件中相似pureComponent的用法，浅比较函数式组件的props，肯定是否须要更新。

export default function memo<Props>( type: React$ElementType, compare?: (oldProps: Props, newProps: Props) => boolean, ) {
  return {
    ?typeof: REACT_MEMO_TYPE,
    type,
    compare: compare === undefined ? null : compare,
  };
}
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ReactElement

在react中，调用createElement方法，返回值为ReactElement 。

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

// 与createElement相比较，预先定义ReactElement的type值，并返回ReactElement
export function createFactory(type) {
  const factory = createElement.bind(null, type);
  factory.type = type;
  return factory;
}
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咱们再来看看ReactElement的定义

const ReactElement = function(type, key, ref, self, source, owner, props) {
  const element = {
    // This tag allows us to uniquely identify this as a React Element
    // 该参数指明React节点类型
    ?typeof: REACT_ELEMENT_TYPE,

    // Built-in properties that belong on the element
    // 标识改ReactElement属于什么类型
    type: type,
    key: key,
    ref: ref,
    props: props,

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

  return element;
};
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咱们能够发现，ReactElement 只是一个用来记录节点相关信息的对象，在后续的操做中经过该对象中的这些属性值，执行不一样类型逻辑。同时，这些信息在不一样平台渲染时，提供了脱离平台的能力。

Fiber、FiberRoot

FiberRoot

type BaseFiberRootProperties = {|
  // 挂载节点，在ReactDOM.render方法接收的第二个参数
  containerInfo: any,
  // 在持久更新时用到该属性，换言之不支持增量更新平台，在react-dom中不涉及
  pendingChildren: any,
  // 当前应用对应的Fiber，即Root Fiber
  current: Fiber,

  // 如下顺序表示优先级
  // 1) 还没提交(committed)的任务
  // 2) 还未提交的挂起任务
  // 3) 未提交的可能被挂起的任务
  // 在提交时被挂起最老和最新任务
  earliestSuspendedTime: ExpirationTime,
  latestSuspendedTime: ExpirationTime,
  // The earliest and latest priority levels that are not known to be suspended.
  // 不肯定是否会挂起的最老和最新任务（全部任务初始化都是该状态）
  earliestPendingTime: ExpirationTime,
  latestPendingTime: ExpirationTime,
  // The latest priority level that was pinged by a resolved promise and can be retried.
  latestPingedTime: ExpirationTime,

  // 若是有抛出错误且此时没有更多更新，此时咱们将尝试在处理错误前同步从头渲染
  // 在renderRoot出现没法处理的错误时，该值会被置为`true`
  didError: boolean,

  // 等待提交任务的`expirationTime`属性
  pendingCommitExpirationTime: ExpirationTime,
  // 已经完成的任务的FiberRoot对象，若是你只有一个Root，那他永远只多是这个Root对应的Fiber，或者是null
  // 在commit阶段，只会处理这个值对应的任务
  finishedWork: Fiber | null,
  // 在任务被挂起时，经过setTimeout设置的返回内容，用来下一次若是有新的任务挂起时清理还没触发的timeout
  timeoutHandle: TimeoutHandle | NoTimeout,
  // 顶层context对象，只有主动调用renderSubtreeIntoContainer时才会使用到
  context: Object | null,
  pendingContext: Object | null,
  // 用来肯定在第一次渲染时，是否须要合并
  hydrate: boolean,
  // 当前root对象上所剩余的过时时间
  nextExpirationTimeToWorkOn: ExpirationTime,
  // 当前更新对应的过时时间
  expirationTime: ExpirationTime,
  // List of top-level batches. This list indicates whether a commit should be
  // deferred. Also contains completion callbacks.
  // 顶层批处理任务，该变量指明一个commit是否应该被推迟处理，同时包含了完成处理后的回调
  firstBatch: Batch | null,
  // root之间关联的链表结构
  nextScheduledRoot: FiberRoot | null,
|};
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Fiber

// Fiber对应一个须要被处理或者已经处理的组件，组件与Fiber能够是一对多关系
type Fiber = {|
  // 不一样的组件类型
  tag: WorkTag,

  // ReactElement里面的key
  key: null | string,

  // ReactElement.type，咱们调用`createElement`的第一个参数
  elementType: any,

  // The resolved function/class/ associated with this fiber.
  // 异步组件resolved以后返回的内容，通常是`function`或者`class`，表示函数或class
  type: any,

  // The local state associated with this fiber.
  // 跟当前Fiber相关本地状态（若在浏览器环境中，该值为DOM节点）
  stateNode: any,

  // 指向他在Fiber节点树中的`parent`，用来在处理完这个节点以后向上返回
  return: Fiber | null,

  // 指向自身的第一个子节点
  // 单链表树结构
  child: Fiber | null,
  // 指向自身的兄弟节点
  // 兄弟节点的return与之指向同一个父节点
  sibling: Fiber | null,
  index: number,

  // ref属性
  ref: null | (((handle: mixed) => void) & {_stringRef: ?string}) | RefObject,

  // 新的更新带来的props
  pendingProps: any, 
  // 上次渲染完成后的props
  memoizedProps: any,

  // 队列，存放该Fiber对应的组件产生的Update
  updateQueue: UpdateQueue<any> | null,

  // 上一次渲染时的state
  memoizedState: any,

  // 列表，存放这个Fiber依赖的context
  firstContextDependency: ContextDependency<mixed> | null,

  // 用来描述当前Fiber和他子树的`Bitfield`
  // 共存的模式表示这个子树是否默认是异步渲染的
  // Fiber被建立的时候他会继承父Fiber
  // 其余的标识也能够在建立的时候被设置
  // 可是在建立以后不该该再被修改，特别是他的子Fiber建立以前
  mode: TypeOfMode,

  // Effect
  // 用来记录Side Effect
  effectTag: SideEffectTag,

  // 单链表用来快速查找下一个side effect
  nextEffect: Fiber | null,

  // 子树中第一个side effect
  firstEffect: Fiber | null,
  // 子树中最后一个side effect
  lastEffect: Fiber | null,

  // 表明任务在将来的哪一个时间点应该被完成
  // 不包括他的子树产生的任务
  expirationTime: ExpirationTime,

  // 快速肯定子树中是否有不在等待的变化
  childExpirationTime: ExpirationTime,

  // 在Fiber树更新的过程当中，每一个Fiber都会有一个跟其对应的Fiber，current <==> workInProgress
  //在渲染完成后，保存fiber
  alternate: Fiber | null,

  // 调试相关，收集每一个Fiber和子树渲染时间

  actualDuration?: number,
  actualStartTime?: number,
  selfBaseDuration?: number,
  treeBaseDuration?: number,
  _debugID?: number,
  _debugSource?: Source | null,
  _debugOwner?: Fiber | null,
  _debugIsCurrentlyTiming?: boolean,
|};
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effectTags、ReactWorkTag、sideEffects

这三个文件主要定义了react中操做相关的类型，值得一提的是，react中类型的定义与组合很巧妙，若是同窗以前未使用过这种思路，能够在权限设计系统中试用该方法。

effectTags

/** * Copyright (c) Facebook, Inc. and its affiliates. * * This source code is licensed under the MIT license found in the * LICENSE file in the root directory of this source tree. * * @flow */

export type SideEffectTag = number;

// Don't change these two values. They're used by React Dev Tools.
export const NoEffect = /* */ 0b00000000000;
export const PerformedWork = /* */ 0b00000000001;

// You can change the rest (and add more).
export const Placement = /* */ 0b00000000010;
export const Update = /* */ 0b00000000100;
export const PlacementAndUpdate = /* */ 0b00000000110;
export const Deletion = /* */ 0b00000001000;
export const ContentReset = /* */ 0b00000010000;
export const Callback = /* */ 0b00000100000;
export const DidCapture = /* */ 0b00001000000;
export const Ref = /* */ 0b00010000000;
export const Snapshot = /* */ 0b00100000000;

// Update & Callback & Ref & Snapshot
export const LifecycleEffectMask = /* */ 0b00110100100;

// Union of all host effects
export const HostEffectMask = /* */ 0b00111111111;

export const Incomplete = /* */ 0b01000000000;
export const ShouldCapture = /* */ 0b10000000000;
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ReactWorkTag

export const FunctionComponent = 0;
export const ClassComponent = 1;
export const IndeterminateComponent = 2; // Before we know whether it is function or class
export const HostRoot = 3; // Root of a host tree. Could be nested inside another node.
export const HostPortal = 4; // A subtree. Could be an entry point to a different renderer.
export const HostComponent = 5;
export const HostText = 6;
export const Fragment = 7;
export const Mode = 8;
export const ContextConsumer = 9;
export const ContextProvider = 10;
export const ForwardRef = 11;
export const Profiler = 12;
export const SuspenseComponent = 13;
export const MemoComponent = 14;
export const SimpleMemoComponent = 15;
export const LazyComponent = 16;
export const IncompleteClassComponent = 17;
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sideEffects

/** * Copyright (c) Facebook, Inc. and its affiliates. * * This source code is licensed under the MIT license found in the * LICENSE file in the root directory of this source tree. * * @flow */

export type SideEffectTag = number;

// Don't change these two values. They're used by React Dev Tools.
export const NoEffect = /* */ 0b00000000000;
export const PerformedWork = /* */ 0b00000000001;

// You can change the rest (and add more).
export const Placement = /* */ 0b00000000010;
export const Update = /* */ 0b00000000100;
export const PlacementAndUpdate = /* */ 0b00000000110;
export const Deletion = /* */ 0b00000001000;
export const ContentReset = /* */ 0b00000010000;
export const Callback = /* */ 0b00000100000;
export const DidCapture = /* */ 0b00001000000;
export const Ref = /* */ 0b00010000000;
export const Snapshot = /* */ 0b00100000000;

// Update & Callback & Ref & Snapshot
export const LifecycleEffectMask = /* */ 0b00110100100;

// Union of all host effects
export const HostEffectMask = /* */ 0b00111111111;

export const Incomplete = /* */ 0b01000000000;
export const ShouldCapture = /* */ 0b10000000000;
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Update、UpdateQueue

export type Update<State> = {
  // 更新的过时时间
  expirationTime: ExpirationTime,

  // 该tag标识更新类型
  // UpdateState -> 0;
  // ReplaceState -> 1;
  // ForceUpdate -> 2;
  // CaptureUpdate -> 3;
  tag: 0 | 1 | 2 | 3,
  // 更新内容，如调用setState时接收的第一个参数
  payload: any,
  // 对应的回调函数，调用setState或render时
  callback: (() => mixed) | null,

  // 指向下一个更新
  next: Update<State> | null,
  // 指向下一个side effect
  nextEffect: Update<State> | null,
};

export type UpdateQueue<State> = {
  // 每次操做完更新后的state
  baseState: State,

  // 队首的Update
  firstUpdate: Update<State> | null,
  // 队尾的Update
  lastUpdate: Update<State> | null,

  firstCapturedUpdate: Update<State> | null,
  lastCapturedUpdate: Update<State> | null,

  firstEffect: Update<State> | null,
  lastEffect: Update<State> | null,

  firstCapturedEffect: Update<State> | null,
  lastCapturedEffect: Update<State> | null,
};
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React.Children

数据结构中有一个结构——链表，不知能否记得链表的遍历？最多见链表的遍历使用递归实现，该api实现就是借助递归。咱们以forEach为例来看看代码片断实现。

function forEachChildren(children, forEachFunc, forEachContext) {
  if (children == null) {
    return children;
  }
  const traverseContext = getPooledTraverseContext(
    null,
    null,
    forEachFunc,
    forEachContext,
  );
  traverseAllChildren(children, forEachSingleChild, traverseContext);
  releaseTraverseContext(traverseContext);
}
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function traverseAllChildrenImpl( children, nameSoFar, callback, traverseContext, ) {
  const type = typeof children;

  if (type === 'undefined' || type === 'boolean') {
    // All of the above are perceived as null.
    children = null;
  }

  let invokeCallback = false;

  if (children === null) {
    invokeCallback = true;
  } else {
    switch (type) {
      case 'string':
      case 'number':
        invokeCallback = true;
        break;
      case 'object':
        switch (children.?typeof) {
          case REACT_ELEMENT_TYPE:
          case REACT_PORTAL_TYPE:
            invokeCallback = true;
        }
    }
  }

  if (invokeCallback) {
    callback(
      traverseContext,
      children,
      // If it's the only child, treat the name as if it was wrapped in an array
      // so that it's consistent if the number of children grows.
      nameSoFar === '' ? SEPARATOR + getComponentKey(children, 0) : nameSoFar,
    );
    return 1;
  }

  let child;
  let nextName;
  let subtreeCount = 0; // Count of children found in the current subtree.
  const nextNamePrefix =
    nameSoFar === '' ? SEPARATOR : nameSoFar + SUBSEPARATOR;

  if (Array.isArray(children)) {
    for (let i = 0; i < children.length; i++) {
      child = children[i];
      nextName = nextNamePrefix + getComponentKey(child, i);
      subtreeCount += traverseAllChildrenImpl(
        child,
        nextName,
        callback,
        traverseContext,
      );
    }
  } else {
    const iteratorFn = getIteratorFn(children);
    if (typeof iteratorFn === 'function') {

      const iterator = iteratorFn.call(children);
      let step;
      let ii = 0;
      while (!(step = iterator.next()).done) {
        child = step.value;
        nextName = nextNamePrefix + getComponentKey(child, ii++);
        subtreeCount += traverseAllChildrenImpl(
          child,
          nextName,
          callback,
          traverseContext,
        );
      }
    } else if (type === 'object') {
      let addendum = '';
      const childrenString = '' + children;
      invariant(
        false,
        'Objects are not valid as a React child (found: %s).%s',
        childrenString === '[object Object]'
          ? 'object with keys {' + Object.keys(children).join(', ') + '}'
          : childrenString,
        addendum,
      );
    }
  }

  return subtreeCount;
}

function traverseAllChildren(children, callback, traverseContext) {
  if (children == null) {
    return 0;
  }

  return traverseAllChildrenImpl(children, '', callback, traverseContext);
}
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const POOL_SIZE = 10;
const traverseContextPool = [];
function releaseTraverseContext(traverseContext) {
  traverseContext.result = null;
  traverseContext.keyPrefix = null;
  traverseContext.func = null;
  traverseContext.context = null;
  traverseContext.count = 0;
  if (traverseContextPool.length < POOL_SIZE) {
    traverseContextPool.push(traverseContext);
  }
}
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