深入解析 PReact 10.5.13 源码：Context 与 Diff 机制

import { EMPTY_OBJ, EMPTY_ARR } from './constants';
import { commitRoot, diff } from './diff/index';
import { createElement, Fragment } from './create-element';
import options from './options';

/**
 * Render a Preact virtual node into a DOM element
 * @param {import('./internal').ComponentChild} vnode The virtual node to render
 * @param {import('./internal').PreactElement} parentDom The DOM element to
 * render into
 * @param {import('./internal').PreactElement | object} [replaceNode] Optional: Attempt to re-use an
 * existing DOM tree rooted at `replaceNode`
 */
export function render(vnode, parentDom, replaceNode) {
 if (options._root) options._root(vnode, parentDom);

 // We abuse the `replaceNode` parameter in `hydrate()` to signal if we are in
 // hydration mode or not by passing the `hydrate` function instead of a DOM
 // element..
 let isHydrating = typeof replaceNode === 'function';

 // To be able to support calling `render()` multiple times on the same
 // DOM node, we need to obtain a reference to the previous tree. We do
 // this by assigning a new `_children` property to DOM nodes which points
 // to the last rendered tree. By default this property is not present, which
 // means that we are mounting a new tree for the first time.
  // 为了支持多次在一个dom节点上调用render函数，需要在dom节点上添加一个饮用，用来获取指向上一次渲染的虚拟dom树。
  // 这个属性默认是指向空的，也意味着我们第一次正在装备一颗新的树
  // 所以开始时这里的oldVNode是空（不论isHydrating的值），但是如果重复在这个节点上调用render那oldVNode是有值的
 let oldVNode = isHydrating
  ? null
  : (replaceNode && replaceNode._children) || parentDom._children;

 // 用Fragment包裹一下vnode，同时给replaceNode和parentDom的_children赋值
  vnode = (
  (!isHydrating && replaceNode) ||
  parentDom
 )._children = createElement(Fragment, null, [vnode]);

 // List of effects that need to be called after diffing.
  // 用来放置diff之后需要进行各种生命周期处理的Component，比如cdm、cdu；componentWillUnmount在diffChildren的unmount函数中执行不在commitRoot时执行
 let commitQueue = [];
 diff(
  parentDom, // 这个使用parentDom的_children属性已经指向[vnode]了
  // Determine the new vnode tree and store it on the DOM element on
  // our custom `_children` property.
  vnode,
  oldVNode || EMPTY_OBJ, // 旧的树
  EMPTY_OBJ,
  parentDom.ownerSVGElement !== undefined,
    // excessDomChildren，这个参数用来做dom复用的作用
  !isHydrating && replaceNode
   ? [replaceNode]
   : oldVNode
   ? null
   : parentDom.firstChild // 如果parentDom有子节点就会把整个子节点作为待复用的节点使用
   ? EMPTY_ARR.slice.call(parentDom.childNodes)
   : null,
  commitQueue,
    // oldDom，在后续方法中用来做标记插入位置使用
  !isHydrating && replaceNode
   ? replaceNode
   : oldVNode
   ? oldVNode._dom
   : parentDom.firstChild,
  isHydrating
 );

 // Flush all queued effects
  // 调用所有commitQueue中的节点_renderCallbacks中的方法
 commitRoot(commitQueue, vnode);
}

/**
 * Update an existing DOM element with data from a Preact virtual node
 * @param {import('./internal').ComponentChild} vnode The virtual node to render
 * @param {import('./internal').PreactElement} parentDom The DOM element to
 * update
 */
export function hydrate(vnode, parentDom) {
 render(vnode, parentDom, hydrate);
}

import { createContext, h, render } from 'preact';

const FontContext = createContext(20);

function Child() {
 return <FontContext.Consumer>
 {fontSize=><div style={{fontSize:fontSize}}>child</div>}
 </FontContext.Consumer>
}
function App(){
 return <Child/>
}
render(
 <FontContext.Provider value={26}>
 <App/>
 </FontContext.Provider>,
 document.getElementById('app')
);

import { enqueueRender } from './component';

export let i = 0;

export function createContext(defaultValue, contextId) {
 contextId = '__cC' + i++; // 生成一个唯一ID

 const context = {
  _id: contextId,
  _defaultValue: defaultValue,
  /** @type {import('./internal').FunctionComponent} */
  Consumer(props, contextValue) {
   // return props.children(
   //  context[contextId] ? context[contextId].props.value : defaultValue
   // );
   return props.children(contextValue);
  },
  /** @type {import('./internal').FunctionComponent} */
  Provider(props) {
   if (!this.getChildContext) { // 第一次调用时进行一些初始化操作
    let subs = [];
    let ctx = {};
    ctx[contextId] = this;
       
       // 在diff操作用，如果判断一个组件在Comsumer中，会调用sub进行订阅；
       // 同时这个节点后续所有diff的地方都会带上这个context，调用sub方法进行调用
       // context具有层级优先级，组件会先加入最近的context中
    this.getChildContext = () => ctx; 

    this.shouldComponentUpdate = function(_props) {
     if (this.props.value !== _props.value) {
      // I think the forced value propagation here was only needed when `options.debounceRendering` was being bypassed:
      // https://github.com/preactjs/preact/commit/4d339fb803bea09e9f198abf38ca1bf8ea4b7771#diff-54682ce380935a717e41b8bfc54737f6R358
      // In those cases though, even with the value corrected, we're double-rendering all nodes.
      // It might be better to just tell folks not to use force-sync mode.
      // Currently, using `useContext()` in a class component will overwrite its `this.context` value.
      // subs.some(c => {
      //  c.context = _props.value;
      //  enqueueRender(c);
      // });

      // subs.some(c => {
      //  c.context[contextId] = _props.value;
      //  enqueueRender(c);
      // });
            // enqueueRender最终会进入renderComponent函数，进行diff、commitRoot、updateParentDomPointers等操作
      subs.some(enqueueRender);
     }
    };

    this.sub = c => {
     subs.push(c);// 进入订阅数组，
     let old = c.componentWillUnmount;
     c.componentWillUnmount = () => { // 重写componentWillUnmount
      subs.splice(subs.indexOf(c), 1);
      if (old) old.call(c);
     };
    };
   }

   return props.children;
  }
 };

 // Devtools needs access to the context object when it
 // encounters a Provider. This is necessary to support
 // setting `displayName` on the context object instead
 // of on the component itself. See:
 // https://reactjs.org/docs/context.html#contextdisplayname
 // createContext最终返回的是一个context对象，带着Provider和Consumer两个函数
 // 同时Consumber函数的contextType和Provider函数的_contextRef属性都指向context
 return (context.Provider._contextRef = context.Consumer.contextType = context);
}

if (c.getChildContext != null) {
    globalContext = assign(assign({}, globalContext), c.getChildContext());
   }

   if (!isNew && c.getSnapshotBeforeUpdate != null) {
    snapshot = c.getSnapshotBeforeUpdate(oldProps, oldState);
   }

   let isTopLevelFragment =
    tmp != null && tmp.type === Fragment && tmp.key == null;
   let renderResult = isTopLevelFragment ? tmp.props.children : tmp;

   diffChildren(
    parentDom,
    Array.isArray(renderResult) ? renderResult : [renderResult],
    newVNode,
    oldVNode,
    globalContext,
    isSvg,
    excessDomChildren,
    commitQueue,
    oldDom,
    isHydrating
   );

// Instantiate the new component
    if ('prototype' in newType && newType.prototype.render) {
     // @ts-ignore The check above verifies that newType is suppose to be constructed
     newVNode._component = c = new newType(newProps, componentContext); // eslint-disable-line new-cap
    } else {
     // @ts-ignore Trust me, Component implements the interface we want
     newVNode._component = c = new Component(newProps, componentContext);
     c.constructor = newType;
     c.render = doRender;
    }

/** The `.render()` method for a PFC backing instance. */
function doRender(props, state, context) {
 return this.constructor(props, context);
}