本篇文章通过分析react-loadable包的源码，手把手教你实现一个react的异步加载高阶组件

1. 首先我们想象中的react异步加载组件应该如何入参以及暴露哪些API？

// 组件应用import * as React from 'react';import ReactDOM from 'react-dom';import Loadable from '@component/test/Loadable';import Loading from '@component/test/loading';const ComponentA = Loadable({ loader: () => import( '@component/test/componentA.js'), loading: Loading, //异步组件未加载之前loading组件 delay: 1000, //异步延迟多久再渲染 timeout: 1000, //异步组件加载超时})ComponentA.preload(); //预加载异步组件的方式const ComponentB = Loadable({ loader: () => import( '@component/test/componentB.js'), loading: Loading, //异步组件未加载之前loading组件})Loadable.preloadAll().then(() => { //}).catch(err => { //}); //预加载所有的异步组件const App = (props) => { const [isDisplay, setIsDisplay] = React.useState(false); if(isDisplay){ return <React.Fragment> <ComponentA /> <ComponentB /> </React.Fragment> }else{ return <input type='button' value='点我' onClick={()=>{setIsDisplay(true)}}/> }}ReactDOM.render(<App />, document.getElementById('app'));// loading组件import * as React from 'react';export default (props) => { const {error, pastDelay, isLoading, timedOut, retry} = props; if (props.error) { return <div>Error! <button onClick={ retry }>Retry</button></div>; } else if (timedOut) { return <div>Taking a long time... <button onClick={ retry }>Retry</button></div>; } else if (props.pastDelay) { return <div>Loading...</div>; } else { return null; }}

通过示例可以看到我们需要入参loaded、loading、delay、timeout,同时暴露单个预加载和全部预加载的API,接下来就让我们试着去一步步实现Loadable高阶组件

2.组件实现过程

整个Loaded函数大体如下

// 收集所有需要异步加载的组件 用于预加载const ALL_INITIALIZERS = [];function Loadable(opts){ return createLoadableComponent(load, opts);}// 静态方法 预加载所有组件Loadable.preloadAll = function(){}

接下来实现createLoadableComponent以及load函数

// 预加载单个异步组件function load(loader){ let promise = loader(); let state = { loading: true, loaded: null, error: null, } state.promise = promise.then(loaded => { state.loading = false; state.loaded = loaded; return loaded; }).catch(err => { state.loading = false; state.error = err; throw err; }) return state;}// 创建异步加载高阶组件function createLoadableComponent(loadFn, options){ if (!options.loading) { throw new Error("react-loadable requires a `loading` component"); } let opts = Object.assign({ loader: null, loading: null, delay: 200, timeout: null, }, options); let res = null; function init(){ if(!res){ res = loadFn(options.loader); return res.promise; } } ALL_INITIALIZERS.push(init); return class LoadableComponent extends React{}}

我们可以看到createLoadableComponent主要功能包括合并默认配置，将异步组件推入预加载数组，并返回LoadableComponent组件;load函数用于加载单个组件并返回该组件的初始加载状态

接着我们实现核心部分LoadableComponent组件

class LoadableComponent extends React.Component{ constructor(props){ super(props); //组件初始化之前调用init方法下载异步组件 init(); this.state = { error: res.error, postDelay: false, timedOut: false, loading: res.loading, loaded: res.loaded } this._delay = null; this._timeout = null; } componentWillMount(){ //设置开关保证不多次去重新请求异步组件 this._mounted = true; this._loadModule(); } _loadModule(){ if(!res.loading) return; if(typeof opts.delay === 'number'){ if(opts.delay === 0){ this.setState({pastDelay: true}); }else{ this._delay = setTimeout(()=>{ this.setState({pastDelay: true}); }, opts.delay) } } if(typeof opts.timeout === 'number'){ this._timeout = setTimeout(()=>{ this.setState({timedOut: true}); }, opts.timeout) } let update = () => { if(!this._mounted) return; this.setState({ error: res.error, loaded: res.loaded, loading: res.loading, }); } // 接收异步组件的下载结果并重新setState来render res.promise.then(()=>{ update() }).catch(err => { update() }) } // 重新加载异步组件 retry(){ this.setState({ error: null, timedOut: false, loading: false, }); res = loadFn(opts.loader); this._loadModule(); } // 静态方法 单个组件预加载 static preload(){ init() } componentWillUnmount(){ this._mounted = false; clearTimeout(this._delay); clearTimeout(this._timeout); } render(){ const {loading, error, pastDelay, timedOut, loaded} = this.state; if(loading || error){ //异步组件还未下载完成的时候渲染loading组件 return React.createElement(opts.loading, { isLoading: loading, pastDelay: pastDelay, timedOut: timedOut, error: error, retry: this.retry.bind(this), }) }else if(loaded){ // 为何此处不直接用React.createElement？ return opts.render(loaded, this.props); }else{ return null; } } }

可以看到，初始的时候调用init方法启动异步组件的下载，并在_loadModule方法里面接收异步组件的pending结果，待到异步组件下载完毕，重新setState启动render

接下来还有个细节，异步组件并没有直接启动React.createElement去渲染，而是采用opts.render方法，这是因为webpack打包生成的单独异步组件chunk暴露的是一个对象，其default才是对应的组件

实现如下

function resolve(obj) { return obj && obj.__esModule ? obj.default : obj;} function render(loaded, props) { return React.createElement(resolve(loaded), props);}

最后实现全部预加载方法

Loadable.preloadAll = function(){ let promises = []; while(initializers.length){ const init = initializers.pop(); promises.push(init()) } return Promise.all(promises);}

整个代码实现如下

const React = require("react");// 收集所有需要异步加载的组件const ALL_INITIALIZERS = [];// 预加载单个异步组件function load(loader){ let promise = loader(); let state = { loading: true, loaded: null, error: null, } state.promise = promise.then(loaded => { state.loading = false; state.loaded = loaded; return loaded; }).catch(err => { state.loading = false; state.error = err; throw err; }) return state;}function resolve(obj) { return obj && obj.__esModule ? obj.default : obj;} function render(loaded, props) { return React.createElement(resolve(loaded), props);}// 创建异步加载高阶组件function createLoadableComponent(loadFn, options){ if (!options.loading) { throw new Error("react-loadable requires a `loading` component"); } let opts = Object.assign({ loader: null, loading: null, delay: 200, timeout: null, render, }, options); let res = null; function init(){ if(!res){ res = loadFn(options.loader); return res.promise; } } ALL_INITIALIZERS.push(init); class LoadableComponent extends React.Component{ constructor(props){ super(props); init(); this.state = { error: res.error, postDelay: false, timedOut: false, loading: res.loading, loaded: res.loaded } this._delay = null; this._timeout = null; } componentWillMount(){ this._mounted = true; this._loadModule(); } _loadModule(){ if(!res.loading) return; if(typeof opts.delay === 'number'){ if(opts.delay === 0){ this.setState({pastDelay: true}); }else{ this._delay = setTimeout(()=>{ this.setState({pastDelay: true}); }, opts.delay) } } if(typeof opts.timeout === 'number'){ this._timeout = setTimeout(()=>{ this.setState({timedOut: true}); }, opts.timeout) } let update = () => { if(!this._mounted) return; this.setState({ error: res.error, loaded: res.loaded, loading: res.loading, }); } res.promise.then(()=>{ update() }).catch(err => { update() }) } // 重新加载异步组件 retry(){ this.setState({ error: null, timedOut: false, loading: false, }); res = loadFn(opts.loader); this._loadModule(); } static preload(){ init() } componentWillUnmount(){ this._mounted = false; clearTimeout(this._delay); clearTimeout(this._timeout); } render(){ const {loading, error, pastDelay, timedOut, loaded} = this.state; if(loading || error){ return React.createElement(opts.loading, { isLoading: loading, pastDelay: pastDelay, timedOut: timedOut, error: error, retry: this.retry.bind(this), }) }else if(loaded){ return opts.render(loaded, this.props); }else{ return null; } } } return LoadableComponent;}function Loadable(opts){ return createLoadableComponent(load, opts);}function flushInitializers(initializers){ }Loadable.preloadAll = function(){ let promises = []; while(initializers.length){ const init = initializers.pop(); promises.push(init()) } return Promise.all(promises);}export default Loadable;

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