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React框架核心原理全面深入解析

2022-11-16 00:16

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前言

这篇文章循序渐进地介绍实现以下几个概念,遵循本篇文章基本就能搞懂为啥需要fiber,为啥需要commit和phases、reconciliation阶段等原理。本篇文章又不完全和原文一致,这里会加入我自己的一些思考,比如经过performUnitOfWork处理后fiber tree和element tree的联系等。

第一章 基本概念

以下面代码为例

// 1.jsx语法不是合法的js语法
// const element = <h1 title="foo">Hello</h1>
// 2.经babel等编译工具将jsx转换成js,将jsx转换成createElement函数调用的方式
// const element = React.createElement(
//   "h1",
//   { title: "foo" },
//   "Hello"
// )
// 3.React.createElement返回的最终对象大致如下:
const element = {
  type: "h1",
  props: {
    title: "foo",
    children: "Hello",
  },
}
const container = document.getElementById("root")
// ReactDOM.render(element, container)
// 4.替换ReactDOM.render函数的逻辑,ReactDOM.render大致处理逻辑:
const node = document.createElement(element.type)
node['title'] = element.props.title
const text = document.createTextNode("")
text["nodeValue"] = element.props.children
node.appendChild(text)
container.appendChild(node)

为了避免歧义,这里使用 element 表示 React elementsnode 表示真实的DOM元素节点。

至此,这段代码无需经过任何编译已经能够在浏览器上跑起来了,不信你可以复制到浏览器控制台试试

这里有几点需要注意:

第二章 createElement 函数

以下面的代码为例

React.createElement接收的children有可能是原子值,比如字符串或者数字等,React.createElement('h1', {title: 'foo'}, 'Hello')。为了简化我们的代码,创建一个特殊的TEXT_ELEMENT 类型将其转换成对象

参考React实战视频讲解:进入学习

React.createElement = (type, props, ...children) => {
  return {
    type,
    props: {
      ...props,
      children: children.map(child => {
        if(typeof child === 'object'){
          return child
        }
        return {
          type: 'TEXT_ELEMENT',
          props: {
            nodeValue: child,
            children: [],
          }
        }
      })
    }
  }
}
// const element = (
//   <div id="foo">
//     <a>bar</a>
//     <b />
//   </div>
// )
// 将jsx转换成js语法
const element = React.createElement(
  "div",
  { id: "foo" },
  React.createElement("a", null, "bar"),
  React.createElement("b")
)
const container = document.getElementById("root")
ReactDOM.render(element, container)

好了,现在我们已经实现了一个简单的createElement函数,我们可以通过一段特殊的注释来告诉babel在将jsx转换成js时使用我们自己的createElement函数:

const MiniReact = {
  createElement:  (type, props, ...children) => {
    return {
      type,
      props: {
        ...props,
        children: children.map(child => {
          if(typeof child === 'object'){
            return child
          }
          return {
            type: 'TEXT_ELEMENT',
            props: {
              nodeValue: child,
              children: [],
            }
          }
        })
      }
    }
  }
}

const element = (
  <div id="foo">
    <a>bar</a>
    <b />
  </div>
)
console.log('element======', element)
const container = document.getElementById("root")
ReactDOM.render(element, container)

第三章 render函数

import React from 'react';
function render(element, container) {
  const dom = element.type === 'TEXT_ELEMENT' ? document.createTextNode("") : document.createElement(element.type)
  const isProperty = key => key !== 'children'
  Object.keys(element.props)
    .filter(isProperty)
    .forEach(name => {
      dom[name] = element.props[name]
    })
  element.props.children.forEach(child => {
    render(child, dom)
  });
  container.appendChild(dom)
}
const MiniReact = {
  createElement:  (type, props, ...children) => {
    return {
      type,
      props: {
        ...props,
        children: children.map(child => {
          if(typeof child === 'object'){
            return child
          }
          return {
            type: 'TEXT_ELEMENT',
            props: {
              nodeValue: child,
              children: [],
            }
          }
        })
      }
    }
  },
  render
}

const element = (
  <div id="foo">
    <a>bar</a>
    <b />
  </div>
)
console.log('element======', element)
const container = document.getElementById("root")
MiniReact.render(element, container)

render函数递归创建真实的dom元素,然后将各个元素append到其父元素中,最后整个dom树append到root container中,渲染完成,这个过程一旦开始,中间是无法打断的,直到整个应用渲染完成。这也是React16版本以前的渲染过程

注意,只有当整个dom树append到root container中时,页面才会显示

第四章 Concurrent Mode

在第三章中可以看到,当前版本的render函数是递归构建dom树,最后才append到root container,最终页面才渲染出来。这里有个问题,如果dom节点数量庞大,递归层级过深,这个过程其实是很耗时的,导致render函数长时间占用主线程,浏览器无法响应用户输入等事件,造成卡顿的现象。

因此我们需要将render过程拆分成小的任务单元,每执行完一个单元,都允许浏览器打断render过程并执行高优先级的任务,等浏览器得空再继续执行render过程

如果对requestIdleCallback不熟悉的,可以自行了解一下。真实React代码中并没有使用这个api,因为有兼容性问题。因此React使用scheduler package模拟这个调度过程

let nextUnitOfWork = null
function workLoop(deadline) {
  let shouldYield = false
  while (nextUnitOfWork && !shouldYield) {
    nextUnitOfWork = performUnitOfWork(
      nextUnitOfWork
    )
    shouldYield = deadline.timeRemaining() < 1
  }
  requestIdleCallback(workLoop)
}
requestIdleCallback(workLoop)
function performUnitOfWork(nextUnitOfWork) {
  // TODO
}

performUnitOfWork接收当前工作单元,并返回下一个工作单元。工作单元可以理解为就是一个fiber对象节点

workLoop循环里会循环调用performUnitOfWork,直到所有工作单元都已经处理完毕,或者当前帧浏览器已经没有空闲时间,则循环终止。等下次浏览器空闲时间再接着继续执行

因此我们需要一种数据结构,能够支持任务打断并且可以接着继续执行,很显然,链表就非常适合

第五章 Fibers

Fibers就是一种数据结构,支持将渲染过程拆分成工作单元,本质上就是一个双向链表。这种数据结构的好处就是方便找到下一个工作单元

Fiber包含三层含义:

Fiber的几点冷知识:

以下面代码为例:

MiniReact.render(
  <div>
    <h1>
      <p />
      <a />
    </h1>
    <h2 />
  </div>,
  container
)

对应的fiber tree如下:

import React from 'react';
// 根据fiber节点创建真实的dom节点
function createDom(fiber) {
  const dom = fiber.type === 'TEXT_ELEMENT' ? document.createTextNode("") : document.createElement(fiber.type)
  const isProperty = key => key !== 'children'
  Object.keys(fiber.props)
    .filter(isProperty)
    .forEach(name => {
      dom[name] = fiber.props[name]
    })
  return dom
}
let nextUnitOfWork = null
// render函数主要逻辑:
//   根据root container容器创建root fiber
//   将nextUnitOfWork指针指向root fiber
//   element是react element tree
function render(element, container){
  nextUnitOfWork = {
    dom: container,
    props: {
      children: [element], // 此时的element还只是React.createElement函数创建的virtual dom树
    },
  }
}
function workLoop(deadline) {
  let shouldYield = false
  while (nextUnitOfWork && !shouldYield) {
    nextUnitOfWork = performUnitOfWork(nextUnitOfWork)
    shouldYield = deadline.timeRemaining() < 1
  }
  requestIdleCallback(workLoop)
}
requestIdleCallback(workLoop)
// performUnitOfWork函数主要逻辑:
//   将element元素添加到DOM
//   给element的子元素创建对应的fiber节点
//   返回下一个工作单元,即下一个fiber节点,查找过程:
//      1.如果有子元素,则返回子元素的fiber节点
//      2.如果没有子元素,则返回兄弟元素的fiber节点
//      3.如果既没有子元素又没有兄弟元素,则往上查找其父节点的兄弟元素的fiber节点
//      4.如果往上查找到root fiber节点,说明render过程已经结束
function performUnitOfWork(fiber) {
  // 第一步 根据fiber节点创建真实的dom节点,并保存在fiber.dom属性中
  if(!fiber.dom){
    fiber.dom = createDom(fiber)
  }
  // 第二步 将当前fiber节点的真实dom添加到父节点中,注意,这一步是会触发浏览器回流重绘的!!!
  if(fiber.parent){
    fiber.parent.dom.appendChild(fiber.dom)
  }
  // 第三步 给子元素创建对应的fiber节点
  const children = fiber.props.children
  let prevSibling
  children.forEach((child, index) => {
    const newFiber = {
      type: child.type,
      props: child.props,
      parent: fiber,
      dom: null
    }
    if(index === 0){
      fiber.child = newFiber
    } else {
      prevSibling.sibling = newFiber
    }
    prevSibling = newFiber
  })
  // 第四步,查找下一个工作单元
  if(fiber.child){
    return fiber.child
  }
  let nextFiber = fiber
  while(nextFiber){
    if(nextFiber.sibling){
      return nextFiber.sibling
    }
    nextFiber = nextFiber.parent
  }
}
const MiniReact = {
  createElement:  (type, props, ...children) => {
    return {
      type,
      props: {
        ...props,
        children: children.map(child => {
          if(typeof child === 'object'){
            return child
          }
          return {
            type: 'TEXT_ELEMENT',
            props: {
              nodeValue: child,
              children: [],
            }
          }
        })
      }
    }
  },
  render
}

const element = (
  <div>
    <h1>
      <p />
      <a />
    </h1>
    <h2 />
  </div>
)
// const element = (
//   <div id="foo">
//     <a>bar</a>
//     <b />
//   </div>
// )
console.log('element======', element)
const container = document.getElementById("root")
MiniReact.render(element, container)

这里有一点值得细品,React.createElement返回的element treeperformUnitOfWork创建的fiber tree有什么联系。如下图所示:

第六章 Render and Commit Phases

第五章的performUnitOfWork有些问题,在第二步中我们直接将新创建的真实dom节点挂载到了容器上,这样会带来两个问题:

我们需要改造一下我们的代码,在performUnitOfWork阶段不把真实的dom节点挂载到容器上。保存fiber tree根节点的引用。等到fiber tree构建完成,再一次性提交真实的dom节点并且添加到容器上。

import React from 'react';
function createDom(fiber) {
  const dom = fiber.type === 'TEXT_ELEMENT' ? document.createTextNode("") : document.createElement(fiber.type)
  const isProperty = key => key !== 'children'
  Object.keys(fiber.props)
    .filter(isProperty)
    .forEach(name => {
      dom[name] = fiber.props[name]
    })
  return dom
}
let nextUnitOfWork = null
let wipRoot = null
function render(element, container){
  wipRoot = {
    dom: container,
    props: {
      children: [element], // 此时的element还只是React.createElement函数创建的virtual dom树
    },
  }
  nextUnitOfWork = wipRoot
}
function commitRoot(){
  commitWork(wipRoot.child)
  wipRoot = null
}
function commitWork(fiber){
  if(!fiber){
    return
  }
  const domParent = fiber.parent.dom;
  domParent.appendChild(fiber.dom)
  commitWork(fiber.child)
  commitWork(fiber.sibling)
}
function workLoop(deadline) {
  let shouldYield = false
  while (nextUnitOfWork && !shouldYield) {
    nextUnitOfWork = performUnitOfWork(nextUnitOfWork)
    shouldYield = deadline.timeRemaining() < 1
  }
  if(!nextUnitOfWork && wipRoot){
    commitRoot()
  }
  requestIdleCallback(workLoop)
}
requestIdleCallback(workLoop)
function performUnitOfWork(fiber) {
  // 第一步 根据fiber节点创建真实的dom节点,并保存在fiber.dom属性中
  if(!fiber.dom){
    fiber.dom = createDom(fiber)
  }
  // 第二步 将当前fiber节点的真实dom添加到父节点中,注意,这一步是会触发浏览器回流重绘的!!!
  // if(fiber.parent){
  //   fiber.parent.dom.appendChild(fiber.dom)
  // }
  // 第三步 给子元素创建对应的fiber节点
  const children = fiber.props.children
  let prevSibling
  children.forEach((child, index) => {
    const newFiber = {
      type: child.type,
      props: child.props,
      parent: fiber,
      dom: null
    }
    if(index === 0){
      fiber.child = newFiber
    } else {
      prevSibling.sibling = newFiber
    }
    prevSibling = newFiber
  })
  // 第四步,查找下一个工作单元
  if(fiber.child){
    return fiber.child
  }
  let nextFiber = fiber
  while(nextFiber){
    if(nextFiber.sibling){
      return nextFiber.sibling
    }
    nextFiber = nextFiber.parent
  }
}
const MiniReact = {
  createElement:  (type, props, ...children) => {
    return {
      type,
      props: {
        ...props,
        children: children.map(child => {
          if(typeof child === 'object'){
            return child
          }
          return {
            type: 'TEXT_ELEMENT',
            props: {
              nodeValue: child,
              children: [],
            }
          }
        })
      }
    }
  },
  render
}

const element = (
  <div>
    <h1>
      <p />
      <a />
    </h1>
    <h2 />
  </div>
)
// const element = (
//   <div id="foo">
//     <a>bar</a>
//     <b />
//   </div>
// )
console.log('element======', element)
const container = document.getElementById("root")
MiniReact.render(element, container)

第七章 Reconciliation

目前为止,我们只考虑添加dom节点到容器上这一单一场景,更新删除还没实现。

我们需要对比最新的React Element Tree和最近一次的Fiber Tree的差异

我们需要给每个fiber节点添加一个alternate属性来保存旧的fiber节点

alternate保存的旧的fiber节点主要有以下几个用途:

  const children = fiber.props.children
  reconcileChildren(fiber, children)
  function reconcileChildren(wipFiber, elements) {
    let index = 0
    let oldFiber = wipFiber.alternate && wipFiber.alternate.child
    let prevSibling = null
    while (index < elements.length || oldFiber != null) {
      const element = elements[index]
      let newFiber = null
      const sameType = oldFiber && element && element.type == oldFiber.type
      if (sameType) {
        newFiber = {
          type: oldFiber.type,
          props: element.props,
          dom: oldFiber.dom,
          parent: wipFiber,
          alternate: oldFiber,
          effectTag: "UPDATE",
        }
      }
      if (element && !sameType) {
        newFiber = {
          type: element.type,
          props: element.props,
          dom: null,
          parent: wipFiber,
          alternate: null,
          effectTag: "PLACEMENT",
        }
      }
      if (oldFiber && !sameType) {
        oldFiber.effectTag = "DELETION"
        deletions.push(oldFiber)
      }
      if (oldFiber) {
        oldFiber = oldFiber.sibling
      }
      if (index === 0) {
        wipFiber.child = newFiber
      } else if (element) {
        prevSibling.sibling = newFiber
      }
      prevSibling = newFiber
      index++
    }
}

如上代码所示:

协调过程:

注意,协调过程,还是以最新的React Element Tree为主去创建一个新的fiber tree,只不过是新的fiber节点复用旧的fiber节点的真实dom元素,毕竟频繁创建真实dom是很消耗内存的。新的fiber节点还是会保存着对旧的fiber节点的引用,方便在commit阶段进行新属性和旧属性的比较。这里会有个问题,如果新fiber节点保留旧fiber节点的引用,那么随着更新次数越来越多,旧的fiber tree是不是也会越来越多,如何销毁?

import React from 'react';
function createDom(fiber) {
  const dom = fiber.type === 'TEXT_ELEMENT' ? document.createTextNode("") : document.createElement(fiber.type)
  updateDom(dom, {}, fiber.props)
  return dom
}
let nextUnitOfWork = null
let wipRoot = null // 保存着对root fiber的引用
let currentRoot = null // 保存着当前页面对应的fiber tree
let deletions = null
function render(element, container){
  wipRoot = {
    dom: container,
    props: {
      children: [element], // 此时的element还只是React.createElement函数创建的virtual dom树
    },
    alternate: currentRoot,
  }
  deletions = []
  nextUnitOfWork = wipRoot
}
function commitRoot(){
  deletions.forEach(commitWork)
  commitWork(wipRoot.child)
  currentRoot = wipRoot
  wipRoot = null
}
const isEvent = key => key.startsWith("on")
const isProperty = key => key !== "children" && !isEvent(key)
const isNew = (prev, next) => key => prev[key] !== next[key]
const isGone = (prev, next) => key => !(key in next)
function updateDom(dom, prevProps, nextProps) {
  //Remove old or changed event listeners
  Object.keys(prevProps)
    .filter(isEvent)
    .filter(
      key =>
        !(key in nextProps) ||
        isNew(prevProps, nextProps)(key)
    )
    .forEach(name => {
      const eventType = name
        .toLowerCase()
        .substring(2)
      dom.removeEventListener(
        eventType,
        prevProps[name]
      )
    })
  // Remove old properties
  Object.keys(prevProps)
    .filter(isProperty)
    .filter(isGone(prevProps, nextProps))
    .forEach(name => {
      dom[name] = ""
    })
  // Set new or changed properties
  Object.keys(nextProps)
    .filter(isProperty)
    .filter(isNew(prevProps, nextProps))
    .forEach(name => {
      dom[name] = nextProps[name]
    })
  // Add event listeners
  Object.keys(nextProps)
    .filter(isEvent)
    .filter(isNew(prevProps, nextProps))
    .forEach(name => {
      const eventType = name
        .toLowerCase()
        .substring(2)
      dom.addEventListener(
        eventType,
        nextProps[name]
      )
    })
}
function commitWork(fiber){
  if(!fiber){
    return
  }
  const domParent = fiber.parent.dom;
  if (fiber.effectTag === "PLACEMENT" && fiber.dom != null) {
    domParent.appendChild(fiber.dom)
  } else if (fiber.effectTag === "UPDATE" && fiber.dom != null) {
    updateDom(
      fiber.dom,
      fiber.alternate.props,
      fiber.props
    )
  } else if (fiber.effectTag === "DELETION") {
    domParent.removeChild(fiber.dom)
  }
  commitWork(fiber.child)
  commitWork(fiber.sibling)
}
function workLoop(deadline) {
  let shouldYield = false
  while (nextUnitOfWork && !shouldYield) {
    nextUnitOfWork = performUnitOfWork(nextUnitOfWork)
    shouldYield = deadline.timeRemaining() < 1
  }
  if(!nextUnitOfWork && wipRoot){
    commitRoot()
  }
  requestIdleCallback(workLoop)
}
requestIdleCallback(workLoop)
function reconcileChildren(wipFiber, elements) {
  let index = 0
  let oldFiber =
      wipFiber.alternate && wipFiber.alternate.child
  let prevSibling = null
  while (index < elements.length || oldFiber != null) {
    const element = elements[index]
    let newFiber = null
    const sameType = oldFiber && element && element.type == oldFiber.type
    if (sameType) {
      newFiber = {
        type: oldFiber.type,
        props: element.props,
        dom: oldFiber.dom,
        parent: wipFiber,
        alternate: oldFiber,
        effectTag: "UPDATE",
      }
    }
    if (element && !sameType) {
      newFiber = {
        type: element.type,
        props: element.props,
        dom: null,
        parent: wipFiber,
        alternate: null,
        effectTag: "PLACEMENT",
      }
    }
    if (oldFiber && !sameType) {
      oldFiber.effectTag = "DELETION"
      deletions.push(oldFiber)
    }
    if (oldFiber) {
      oldFiber = oldFiber.sibling
    }
    if (index === 0) {
      wipFiber.child = newFiber
    } else if (element) {
      prevSibling.sibling = newFiber
    }
    prevSibling = newFiber
    index++
  }
}
function performUnitOfWork(fiber) {
  // 第一步 根据fiber节点创建真实的dom节点,并保存在fiber.dom属性中
  if(!fiber.dom){
    fiber.dom = createDom(fiber)
  }
  // 第二步 将当前fiber节点的真实dom添加到父节点中,注意,这一步是会触发浏览器回流重绘的!!!
  // if(fiber.parent){
  //   fiber.parent.dom.appendChild(fiber.dom)
  // }
  // 第三步 给子元素创建对应的fiber节点
  const children = fiber.props.children
  // let prevSibling
  // children.forEach((child, index) => {
  //   const newFiber = {
  //     type: child.type,
  //     props: child.props,
  //     parent: fiber,
  //     dom: null
  //   }
  //   if(index === 0){
  //     fiber.child = newFiber
  //   } else {
  //     prevSibling.sibling = newFiber
  //   }
  //   prevSibling = newFiber
  // })
  reconcileChildren(fiber, children)
  // 第四步,查找下一个工作单元
  if(fiber.child){
    return fiber.child
  }
  let nextFiber = fiber
  while(nextFiber){
    if(nextFiber.sibling){
      return nextFiber.sibling
    }
    nextFiber = nextFiber.parent
  }
}
const MiniReact = {
  createElement:  (type, props, ...children) => {
    return {
      type,
      props: {
        ...props,
        children: children.map(child => {
          if(typeof child === 'object'){
            return child
          }
          return {
            type: 'TEXT_ELEMENT',
            props: {
              nodeValue: child,
              children: [],
            }
          }
        })
      }
    }
  },
  render
}

const container = document.getElementById("root")
const updateValue = e => {
  rerender(e.target.value)
}
const rerender = value => {
  const element = (
    <div>
      <input onInput={updateValue} value={value} />
      <h2>Hello {value}</h2>
    </div>
  )
  MiniReact.render(element, container)
}
rerender("World")

第八章 Function Components

本章以下面的代码为例:


const container = document.getElementById("root")
function App(props){
  return <h1>Hi { props.name }</h1>
}
const element = <App name="foo" />
MiniReact.render(element, container)

jsx经过babel编译后:

function App(props) {
  return MiniReact.createElement("h1", null, "Hi ", props.name);
}
const element = MiniReact.createElement(App, {
  name: "foo"
});

函数组件有两点不同的地方:

由于函数组件没有对应的fiber节点,因此在commit阶段在找父fiber节点对应的dom时,需要判断是否存在该dom元素

本章完整代码:

import React from 'react';
function createDom(fiber) {
  const dom = fiber.type === 'TEXT_ELEMENT' ? document.createTextNode("") : document.createElement(fiber.type)
  updateDom(dom, {}, fiber.props)
  return dom
}
let nextUnitOfWork = null
let wipRoot = null // 保存着对root fiber的引用
let currentRoot = null // 保存着当前页面对应的fiber tree
let deletions = null
function render(element, container){
  wipRoot = {
    dom: container,
    props: {
      children: [element], // 此时的element还只是React.createElement函数创建的virtual dom树
    },
    alternate: currentRoot,
  }
  deletions = []
  nextUnitOfWork = wipRoot
}
function commitRoot(){
  deletions.forEach(commitWork)
  commitWork(wipRoot.child)
  currentRoot = wipRoot
  wipRoot = null
}
const isEvent = key => key.startsWith("on")
const isProperty = key => key !== "children" && !isEvent(key)
const isNew = (prev, next) => key => prev[key] !== next[key]
const isGone = (prev, next) => key => !(key in next)
function updateDom(dom, prevProps, nextProps) {
  //Remove old or changed event listeners
  Object.keys(prevProps)
    .filter(isEvent)
    .filter(
      key =>
        !(key in nextProps) ||
        isNew(prevProps, nextProps)(key)
    )
    .forEach(name => {
      const eventType = name
        .toLowerCase()
        .substring(2)
      dom.removeEventListener(
        eventType,
        prevProps[name]
      )
    })
  // Remove old properties
  Object.keys(prevProps)
    .filter(isProperty)
    .filter(isGone(prevProps, nextProps))
    .forEach(name => {
      dom[name] = ""
    })
  // Set new or changed properties
  Object.keys(nextProps)
    .filter(isProperty)
    .filter(isNew(prevProps, nextProps))
    .forEach(name => {
      dom[name] = nextProps[name]
    })
  // Add event listeners
  Object.keys(nextProps)
    .filter(isEvent)
    .filter(isNew(prevProps, nextProps))
    .forEach(name => {
      const eventType = name
        .toLowerCase()
        .substring(2)
      dom.addEventListener(
        eventType,
        nextProps[name]
      )
    })
}
function commitWork(fiber){
  if(!fiber){
    return
  }
  let domParentFiber = fiber.parent
  while(!domParentFiber.dom){
    domParentFiber = domParentFiber.parent
  }
  const domParent = domParentFiber.dom;
  if (fiber.effectTag === "PLACEMENT" && fiber.dom != null) {
    domParent.appendChild(fiber.dom)
  } else if (fiber.effectTag === "UPDATE" && fiber.dom != null) {
    updateDom(fiber.dom, fiber.alternate.props, fiber.props)
  } else if (fiber.effectTag === "DELETION") {
    // domParent.removeChild(fiber.dom)
    commitDeletion(fiber, domParent)
  }
  commitWork(fiber.child)
  commitWork(fiber.sibling)
}
function commitDeletion(fiber, domParent){
  if(fiber.dom){
    domParent.removeChild(fiber.dom)
  } else {
    commitDeletion(fiber.child, domParent)
  }
}
function workLoop(deadline) {
  let shouldYield = false
  while (nextUnitOfWork && !shouldYield) {
    nextUnitOfWork = performUnitOfWork(nextUnitOfWork)
    shouldYield = deadline.timeRemaining() < 1
  }
  if(!nextUnitOfWork && wipRoot){
    commitRoot()
  }
  requestIdleCallback(workLoop)
}
requestIdleCallback(workLoop)
function reconcileChildren(wipFiber, elements) {
  let index = 0
  let oldFiber =
      wipFiber.alternate && wipFiber.alternate.child
  let prevSibling = null
  while (index < elements.length || oldFiber != null) {
    const element = elements[index]
    let newFiber = null
    const sameType = oldFiber && element && element.type == oldFiber.type
    if (sameType) {
      newFiber = {
        type: oldFiber.type,
        props: element.props,
        dom: oldFiber.dom,
        parent: wipFiber,
        alternate: oldFiber,
        effectTag: "UPDATE",
      }
    }
    if (element && !sameType) {
      newFiber = {
        type: element.type,
        props: element.props,
        dom: null,
        parent: wipFiber,
        alternate: null,
        effectTag: "PLACEMENT",
      }
    }
    if (oldFiber && !sameType) {
      oldFiber.effectTag = "DELETION"
      deletions.push(oldFiber)
    }
    if (oldFiber) {
      oldFiber = oldFiber.sibling
    }
    if (index === 0) {
      wipFiber.child = newFiber
    } else if (element) {
      prevSibling.sibling = newFiber
    }
    prevSibling = newFiber
    index++
  }
}
function performUnitOfWork(fiber) {
  // 1.函数组件对应的fiber节点没有真实dom元素
  // 2.函数组件需要运行函数获取children
  const isFunctionComponent = fiber.type instanceof Function
  if(!isFunctionComponent && !fiber.dom){
    fiber.dom = createDom(fiber)
  }
  const children = isFunctionComponent ? [fiber.type(fiber.props)] : fiber.props.children
  // 第二步 为每一个新的react element节点创建对应的fiber节点,并判断旧的fiber节点上的真实dom元素是否可以复用。
  // 节省创建真实dom元素的开销
  reconcileChildren(fiber, children)
  // 第三步,查找下一个工作单元
  if(fiber.child){
    return fiber.child
  }
  let nextFiber = fiber
  while(nextFiber){
    if(nextFiber.sibling){
      return nextFiber.sibling
    }
    nextFiber = nextFiber.parent
  }
}
const MiniReact = {
  createElement:  (type, props, ...children) => {
    return {
      type,
      props: {
        ...props,
        children: children.map(child => {
          if(typeof child === 'object'){
            return child
          }
          return {
            type: 'TEXT_ELEMENT',
            props: {
              nodeValue: child,
              children: [],
            }
          }
        })
      }
    }
  },
  render
}

const container = document.getElementById("root")
function App(props){
  return <h1>Hi { props.name }</h1>
}
const element = <App name="foo" />
MiniReact.render(element, container)

第九章 Hooks

本章完整代码

import React from 'react';
function createDom(fiber) {
  const dom = fiber.type === 'TEXT_ELEMENT' ? document.createTextNode("") : document.createElement(fiber.type)
  updateDom(dom, {}, fiber.props)
  return dom
}
let nextUnitOfWork = null
let wipRoot = null // 保存着对root fiber的引用
let currentRoot = null // 保存着当前页面对应的fiber tree
let deletions = null
function render(element, container){
  wipRoot = {
    dom: container,
    props: {
      children: [element], // 此时的element还只是React.createElement函数创建的virtual dom树
    },
    alternate: currentRoot,
  }
  deletions = []
  nextUnitOfWork = wipRoot
}
function commitRoot(){
  deletions.forEach(commitWork)
  commitWork(wipRoot.child)
  currentRoot = wipRoot
  wipRoot = null
}
const isEvent = key => key.startsWith("on")
const isProperty = key => key !== "children" && !isEvent(key)
const isNew = (prev, next) => key => prev[key] !== next[key]
const isGone = (prev, next) => key => !(key in next)
function updateDom(dom, prevProps, nextProps) {
  //Remove old or changed event listeners
  Object.keys(prevProps)
    .filter(isEvent)
    .filter(
      key =>
        !(key in nextProps) ||
        isNew(prevProps, nextProps)(key)
    )
    .forEach(name => {
      const eventType = name
        .toLowerCase()
        .substring(2)
      dom.removeEventListener(
        eventType,
        prevProps[name]
      )
    })
  // Remove old properties
  Object.keys(prevProps)
    .filter(isProperty)
    .filter(isGone(prevProps, nextProps))
    .forEach(name => {
      dom[name] = ""
    })
  // Set new or changed properties
  Object.keys(nextProps)
    .filter(isProperty)
    .filter(isNew(prevProps, nextProps))
    .forEach(name => {
      dom[name] = nextProps[name]
    })
  // Add event listeners
  Object.keys(nextProps)
    .filter(isEvent)
    .filter(isNew(prevProps, nextProps))
    .forEach(name => {
      const eventType = name
        .toLowerCase()
        .substring(2)
      dom.addEventListener(
        eventType,
        nextProps[name]
      )
    })
}
function commitWork(fiber){
  if(!fiber){
    return
  }
  let domParentFiber = fiber.parent
  while(!domParentFiber.dom){
    domParentFiber = domParentFiber.parent
  }
  const domParent = domParentFiber.dom;
  if (fiber.effectTag === "PLACEMENT" && fiber.dom != null) {
    domParent.appendChild(fiber.dom)
  } else if (fiber.effectTag === "UPDATE" && fiber.dom != null) {
    updateDom(fiber.dom, fiber.alternate.props, fiber.props)
  } else if (fiber.effectTag === "DELETION") {
    // domParent.removeChild(fiber.dom)
    commitDeletion(fiber, domParent)
  }
  commitWork(fiber.child)
  commitWork(fiber.sibling)
}
function commitDeletion(fiber, domParent){
  if(fiber.dom){
    domParent.removeChild(fiber.dom)
  } else {
    commitDeletion(fiber.child, domParent)
  }
}
function workLoop(deadline) {
  let shouldYield = false
  while (nextUnitOfWork && !shouldYield) {
    nextUnitOfWork = performUnitOfWork(nextUnitOfWork)
    shouldYield = deadline.timeRemaining() < 1
  }
  if(!nextUnitOfWork && wipRoot){
    commitRoot()
  }
  requestIdleCallback(workLoop)
}
requestIdleCallback(workLoop)
function reconcileChildren(wipFiber, elements) {
  let index = 0
  let oldFiber =
      wipFiber.alternate && wipFiber.alternate.child
  let prevSibling = null
  while (index < elements.length || oldFiber != null) {
    const element = elements[index]
    let newFiber = null
    const sameType = oldFiber && element && element.type == oldFiber.type
    if (sameType) {
      newFiber = {
        type: oldFiber.type,
        props: element.props,
        dom: oldFiber.dom,
        parent: wipFiber,
        alternate: oldFiber,
        effectTag: "UPDATE",
      }
    }
    if (element && !sameType) {
      newFiber = {
        type: element.type,
        props: element.props,
        dom: null,
        parent: wipFiber,
        alternate: null,
        effectTag: "PLACEMENT",
      }
    }
    if (oldFiber && !sameType) {
      oldFiber.effectTag = "DELETION"
      deletions.push(oldFiber)
    }
    if (oldFiber) {
      oldFiber = oldFiber.sibling
    }
    if (index === 0) {
      wipFiber.child = newFiber
    } else if (element) {
      prevSibling.sibling = newFiber
    }
    prevSibling = newFiber
    index++
  }
}
function performUnitOfWork(fiber) {
  // 1.函数组件对应的fiber节点没有真实dom元素
  // 2.函数组件需要运行函数获取children
  const isFunctionComponent = fiber.type instanceof Function
  if(!isFunctionComponent && !fiber.dom){
    fiber.dom = createDom(fiber)
  }
  const children = isFunctionComponent ? updateFunctionComponent(fiber) : fiber.props.children
  // 第二步 为每一个新的react element节点创建对应的fiber节点,并判断旧的fiber节点上的真实dom元素是否可以复用。
  // 节省创建真实dom元素的开销
  reconcileChildren(fiber, children)
  // 第三步,查找下一个工作单元
  if(fiber.child){
    return fiber.child
  }
  let nextFiber = fiber
  while(nextFiber){
    if(nextFiber.sibling){
      return nextFiber.sibling
    }
    nextFiber = nextFiber.parent
  }
}
let wipFiber = null
let hookIndex = null
function updateFunctionComponent(fiber){
  wipFiber = fiber
  hookIndex = 0
  wipFiber.hooks = []
  return [fiber.type(fiber.props)]
}
function useState(initial){
  const oldHook = wipFiber.alternate && wipFiber.alternate.hooks && wipFiber.alternate.hooks[hookIndex]
  const hook = {
    state: oldHook ? oldHook.state : initial,
    queue: [],
  }
  const actions = oldHook ? oldHook.queue : []
  actions.forEach(action => {
    hook.state = action(hook.state)
  })
  const setState = action => {
    hook.queue.push(action)
    wipRoot = {
      dom: currentRoot.dom,
      props: currentRoot.props,
      alternate: currentRoot,
    }
    nextUnitOfWork = wipRoot
    deletions = []
  }
  wipFiber.hooks.push(hook)
  hookIndex++
  return [hook.state, setState]
}
const MiniReact = {
  createElement:  (type, props, ...children) => {
    return {
      type,
      props: {
        ...props,
        children: children.map(child => {
          if(typeof child === 'object'){
            return child
          }
          return {
            type: 'TEXT_ELEMENT',
            props: {
              nodeValue: child,
              children: [],
            }
          }
        })
      }
    }
  },
  render,
  useState,
}

const container = document.getElementById("root")
function Counter(){
  const [state, setState] = MiniReact.useState(1)
  return (
    <h1 onClick={() => setState(c => c + 1)}>
      Count: { state }
    </h1>
  )
}
const element = <Counter />
MiniReact.render(element, container)

到此这篇关于React框架核心原理全面深入解析的文章就介绍到这了,更多相关React框架核心内容请搜索编程网以前的文章或继续浏览下面的相关文章希望大家以后多多支持编程网!

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