手把手教您实现react异步加载高阶组件

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(
/* webpackChunkName: 'componentA' */
'@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(
/* webpackChunkName: 'componentB' */
'@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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