PReact10.5.13源码理解

React源码看过几次,每次都没有坚持下来,索性学习一下PReact部分,网上讲解源码的不少,但是基本已经过时,所以自己来梳理下

render.js部分

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);
}

 

create-context.js部分

Context的使用:
Provider的props中有value属性
Consumer中直接获取传值
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')
);

看一下源码:

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);
}
所以对于Provider组件,在渲染时会判断有没有getChildContext方法,如果有的话调用得到globalContext并一直向下传递下去
                    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
            );
当渲染遇到Consumer时,即遇到contextType属性,先从Context中拿到provider,然后拿到provider的props的value值,作为组件要获取的上下文信息。
 
同时这时候会调用provider的sub方法,进行订阅,当调用到Provider的shouldComponentUpdate中发现value发生变化时就会将所有的订阅者进入enqueueRender函数。
 

 

所以源码中,globalContext对象的每一个key指向一个Context.Provider;componentContext代表组件所在的Consumer传递的上下文信息即配对的Provider的props的value;

同时Provider的shouldComponentUpdate方法中用到了 ·this.props.value !== _props.value· 那么这里的this.props是哪来的?Provider中并没有相关属性。

主要是下面这个地方,当判断没有render方法时,会先用Compoent来实例化一个对象,并将render方法设置为doRender,并将constructor指向newType(当前函数),在doRender中调用this.constructor方法

// 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);
}

 

diff部分

diff部分比较复杂,整体整理了一张大图

 

真是不得不吐槽,博客园的编辑器bug太多了,尤其是mac上使用,比如第二次上传代码提交不了;赋值粘贴用不了。。。

只有情怀让我继续在这里更新

 

 
 
 

您可以考虑给树发个小额微信红包以资鼓励
posted @ 2021-04-02 23:26  木的树  阅读(161)  评论(0编辑  收藏  举报