text: const fs = require('fs')
  const path = require('path')
  const webpack = require('webpack')
  
  const MFS = require('memory-fs')
  const chokidar = require('chokidar')
  
  const webpackDevMiddleware = require('webpack-dev-middleware')
  const webpackHotMiddleware = require('webpack-hot-middleware')
  
  const clientConfig = require('./webpack.client.config')
  const serverConfig = require('./webpack.server.config')
  
  const readFile = (fs, file) => {
    try {
      return fs.readFileSync(path.join(clientConfig.output.path, file), 'utf-8')
    } catch (e) {}
  }
  
  module.exports = function setupDevServer(app, templatePath, cb) {
    let bundle
    let template
    let clientManifest
  
    let ready
    const readyPromise = new Promise(resolve => {
      ready = resolve
    })
  
    const update = () => {
      if (bundle && clientManifest) {
        ready()
        cb(bundle, {
          template,
          clientManifest
        })
      }
    }
    // read template from disk and watch
    template = fs.readFileSync(templatePath, 'utf-8')
    chokidar.watch(templatePath).on('change', () => {
      template = fs.readFileSync(templatePath, 'utf-8')
      console.log('index.html template updated.')
      update()
    })
  
    // modify client config to work with hot middleware
    clientConfig.entry.app = [
      'webpack-hot-middleware/client',
      clientConfig.entry.app
    ]
    clientConfig.output.filename = '[name].js'
    clientConfig.plugins.push(
      new webpack.HotModuleReplacementPlugin(),
      new webpack.NoEmitOnErrorsPlugin()
    )
  
    // dev middleware
    const clientCompiler = webpack(clientConfig)
    const devMiddleware = webpackDevMiddleware(clientCompiler, {
      publicPath: clientConfig.output.publicPath,
      noInfo: true
    })
    app.use(devMiddleware)
  
    clientCompiler.plugin('done', stats => {
      stats = stats.toJson()
      stats.errors.forEach(err => console.error(err))
      stats.warnings.forEach(err => console.warn(err))
      if (stats.errors.length) return
      clientManifest = JSON.parse(
        readFile(devMiddleware.fileSystem, 'vue-ssr-client-manifest.json')
      )
      update()
    })
  
    // 应用热更新
    app.use(webpackHotMiddleware(clientCompiler, { heartbeat: 5000 }))
  
    // watch and update server renderer
    const serverCompiler = webpack(serverConfig)
    const mfs = new MFS()
    serverCompiler.outputFileSystem = mfs
    serverCompiler.watch({}, (err, stats) => {
      if (err) throw err
      stats = stats.toJson()
      if (stats.errors.length) return
  
      // read bundle generated by vue-ssr-webpack-plugin
      bundle = JSON.parse(readFile(mfs, 'vue-ssr-server-bundle.json'))
      update()
    })
  
    return readyPromise
  }
  