详解webpack + react + react-router 如何实现懒加载

在 Webpack 1 中主要是由bundle-loader进行懒加载，而 Webpack 2 中引入了类似于 SystemJS 的System.import语法，首先我们对于System.import的执行流程进行简单阐述：

1. Webpack 会在编译过程中扫描代码库时将发现的System.import调用引入的文件及其相关依赖进行单独打包，注意，Webpack 会保证这些独立模块及其依赖不会与主应用的包体相冲突。
2. 当我们访问到这些独立打包的组件模块时，Webpack 会发起 JSONP 请求来抓取相关的包体。
3. System.import 同样也是 Promise，在请求完成之后System.import会将抓取到的模块作为参数传入then中的回调函数。
4. 如果我们重复访问已经加载完毕的模块，Webpack 不会重复执行抓取与解析的过程。

而 React Router 路由的懒加载实际上分为动态路由与与懒加载两步，典型的所谓动态路由配置如下：

/**
 * <Route path="/" component={Core}>
 *  <IndexRoute component={Home}/>
 *  <Route path="about" component={About}/>
 *  <Route path="users" component={Users}>
 *  <Route path="*" component={Home}/>
 * </Route>
 */
export default {
 path: '/', 
 component: Core,
 indexRoute: { 
  getComponent(location, cb) {
    ...
  },
 },
 childRoutes: [
  {
   path: 'about', 
   getComponent(location, cb) {
    ...
   },
  },
  {
   path: 'users', 
   getComponent(location, cb) {
    ...
   },
  },
  {
   path: '*', 
   getComponent(location, cb) {
    ...
   },
  },
 ],
};

正常打包

import IndexPage from './views/app.jsx'
import AboutPage from './views/about.jsx'
export default function({history}) {
  return (
    <Router history={history}>
      <Route path="/" component={IndexPage} />
      <Route path="/about" component={AboutPage} />
    </Router>
  )
}

这是一个正常打包的路由写法, 如果需要分割代码, 我们需要改造下路由, 借助getComponent和require.ensure

webpack 代码分割

export default function({history}) {
  return (
    <Router history={history}>
      <Route path="/" getComponent={(location, callback) => {
        require.ensure([], function(require) {
          callback(null, require('./HomePage.jsx'))
        })
      }} />
      <Route path="/about" getComponent={(location, callback) => {
        require.ensure([], function(require) {
          callback(null, require('./AboutPage.jsx'))
        })
      }} />
    </Router>
  )
}

这样看来代码有点累, 我们稍微改造下

const home = (location, callback) => {
 require.ensure([], require => {
  callback(null, require('./HomePage.jsx'))
 }, 'home')
}

const about = (location, callback) => {
 require.ensure([], require => {
  callback(null, require('./AboutPage.jsx'))
 }, 'about')
}
export default function({history}) {
  return (
    <Router history={history}>
      <Route path="/" getComponent={home}></Route>
      <Route path="/about" getComponent={about}></Route>
    </Router>
  )
}

这样看起来是不是简洁了很多

注意: 由于webpack的原因, 如果直接require('./AboutPage.jsx')不能正常加载, 请尝试require('./AboutPage.jsx').default

webpack2 代码分割

上面的代码看起来好像都是webpack1的写法, 那么webpack2呢?

webpac2就需要借助System.import了

export default function({history}) {
  return (
    <Router history={history}>
      <Route path="/" getComponent={(location, callback) => {
        System.import('./HomePage.jsx').then(component => {
          callback(null, component.default || component)
        })
      }} />
      <Route path="/about" getComponent={(location, callback) => {
        System.import('./AboutPage.jsx').then(component => {
          callback(null, component.default || component)
        })
      }} />
    </Router>
  )
}

我们一样可以把上面的代码优化一下

const home = (location, callback) => {
  System.import('./HomePage.jsx').then(component => {
    callback(null, component.default || component)
  })
}
const about = (location, callback) => {
  System.import('./AboutPage.jsx').then(component => {
    callback(null, component.default || component)
  })
}

export default ({ history }) => {
  return (
    <Router history={history}>
      <Route name="home" path="/" getComponent={home} />
      <Route name="about" path="/about" getComponent={about} />
    </Router>
  )
}

webpack2 + dva 实现路由和 models 懒加载

const routerThen = (app, callback, [component, model]) => {
  app.model(model.default || model)
  callback(null, component.default || component)
}

export default ({ history, app }) => {
  return (
    <Router history={history}>
      <Route name="home" path="/" getComponent={(location, callback) => {
        Promise.all([
          System.import('./views/app.jsx'),
          System.import('./models/topics')
        ]).then(routerThen.bind(null, app, callback))
      }} />
      <Route name="article" path="/article/:id" getComponent={(location, callback) => {
        Promise.all([
          System.import('./views/article.jsx'),
          System.import('./models/topic')
        ]).then(routerThen.bind(null, app, callback))
      }} />
    </Router>
  )
}