This article helps you understand PReact10.5.13 source code

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.
  // In order to support multiple calls to the render function on a DOM node, you need to add a drinking on the DOM node to get the virtual DOM tree pointing to the last rendering.
  // This property points to null by default, which means that we are equipping a new tree for the first time // So at the beginning oldVNode is null (regardless of the value of isHydrating), but if you call render on this node repeatedly, oldVNode will have a value let oldVNode = isHydrating
  ? null
  : (replaceNode && replaceNode._children) || parentDom._children;

 // Wrap vnode with Fragment and assign vnode to replaceNode and parentDom's _children = (
  (!isHydrating && replaceNode) ||
  parentDom
 )._children = createElement(Fragment, null, [vnode]);

 // List of effects that need to be called after diffing.
  // Used to place components that need to undergo various lifecycle processing after diff, such as cdm and cdu; componentWillUnmount is executed in the unmount function of diffChildren and is not executed when commitRoot is used. let commitQueue = [];
 diff(
  parentDom, // This uses the _children property of parentDom to point to [vnode] // Determine the new vnode tree and store it on the DOM element on
  // our custom `_children` property.
  vnode,
  oldVNode || EMPTY_OBJ, // Old tree EMPTY_OBJ,
  parentDom.ownerSVGElement !== undefined,
    // excessDomChildren, this parameter is used for dom reuse! isHydrating && replaceNode
   ? [replaceNode]
   : oldVNode
   ? null
   : parentDom.firstChild // If parentDom has child nodes, the entire child node will be used as a node to be reused? EMPTY_ARR.slice.call(parentDom.childNodes)
   : null,
  commitQueue,
    // oldDom, used to mark the insertion position in subsequent methods!isHydrating && replaceNode
   ? replaceNode
   : oldVNode
   ? oldVNode._dom
   : parentDom.firstChild,
  isHydrating
 );

 // Flush all queued effects
  // Call the method commitRoot(commitQueue, vnode) in all nodes_renderCallbacks in commitQueue;
}

/**
 * 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++; // Generate a unique 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) { // Perform some initialization operations when called for the first time let subs = [];
    let ctx = {};
    ctx[contextId] = this;
       
       // In diff operation, if a component is determined to be in Consumer, sub will be called to subscribe;
       // At the same time, all subsequent diffs of this node will carry this context, calling the sub method to call // context has hierarchical priority, and the component will be added to the nearest context first 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 will eventually enter the renderComponent function to perform operations such as diff, commitRoot, updateParentDomPointers, etc. subs.some(enqueueRender);
     }
    };

    this.sub = c => {
     subs.push(c);// Enter the subscription array,
     let old = c.componentWillUnmount;
     c.componentWillUnmount = () => { // Override 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 ultimately returns a context object with two functions, Provider and Consumer. // At the same time, the contextType property of the Consumer function and the _contextRef property of the Provider function both point to 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 supposed 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);
}