前言

webpack提供了webpack-dev-server模块来启动一个简单的web服务器，为了更大的自由度我们可以自己配置一个服务器，下面介绍如何用koa2来实现。

wepack-dev-middleware

wepack-dev-middleware把webpack处理后的配置文件传递给服务器，不过我们在使用它之前，需要把它改造成koa中间件。

安装wepack-dev-middleware：

npm install wepack-dev-middleware -D

封装成koa中间件。devMiddleware.js：

// 改造成koa中间件
const webpackDev  = require('webpack-dev-middleware');

const devMiddleware = (compiler, opts) => {
  const middleware = webpackDev(compiler, opts);
  return async (ctx, next) => {
    await middleware(ctx.req, {
      end: (content) => {
        ctx.body = content;
      },
      setHeader: (name, value) => {
        ctx.set(name, value);
      }
    }, next);
  };
};

module.exports=devMiddleware;

webpack-hot-middleware

webpack-hot-middleware模块主要用来实现热替换，也就是说我们在修改文件后只需刷客户端页面就能看到效果了。因为通过webpack-hot-middleware生成的模块是写入到内存中的，所以我们需要修改开发环境中的output配置项：

// webpack.dev.conf.js
output: {
  filename: '[name].[hash].js',
  path: '/' // 修改此配置项
}

安装webpack-hot-middleware：

npm install webpack-hot-middleware -D

封装成koa中间件。hotMiddleware.js：

// 改造成koa中间件

const webpackHot = require('webpack-hot-middleware')
const PassThrough = require('stream').PassThrough;

const hotMiddleware = (compiler, opts) => {
  const middleware = webpackHot(compiler, opts);
  return async (ctx, next) => {
    let stream = new PassThrough();
    ctx.body = stream;
    await middleware(ctx.req, {
        write: stream.write.bind(stream),
        writeHead: (status, headers) => {
          ctx.status = status;
          ctx.set(headers);
        }
      }, next);
  };
};


module.exports = hotMiddleware;

koa2实现服务器

安装koa：

npm install koa -D

server.js：

const Koa = require('koa');
const webpack = require('webpack'); // webpack模块

const config = require('./webpack.dev.conf'); // 开发环境模块
// 中间件容器，把webpack处理后的文件传递给一个服务器
const devMiddleware = require('./devMiddleware');
// 在内存中编译的插件，不写入磁盘来提高性能
const hotMiddleware = require('./hotMiddleware');
const compiler = webpack(config);

const app = new Koa();

app.use(devMiddleware(compiler, {
  publicPath: config.output.publicPath // '/'
}));
app.use(hotMiddleware(compiler));
app.listen(3000);

console.log('lostening on port 3000');

配置package.json:

"scripts": {
  "server": "node server.js --mode development"
}

启动服务器：

npm run server

通过koa2中间件配置

koa2社区也提供了封装好的中间件，例如koa-webpack和koa-webpack-middleware,不过后者已经好久没有维护了，下面我们用koa-webpack来简化配置。

安装依赖：

npm install koa-webpack -D

修改server.js:

const Koa = require('koa');
const middleware = require('koa-webpack');
const webpack = require('webpack');
const config = require ('./webpack.dev.conf.js');

const compiler = webpack(config);

const app = new Koa();

app.use(middleware({
  compiler: compiler
}));
app.listen(3000);

console.log('lostening on port 3000');

模块版本：

"devDependencies": {
  "koa": "^2.5.0",
  "koa-webpack": "^3.0.0",
  "webpack": "^4.0.1",
  "webpack-cli": "^2.0.9",
  "webpack-dev-middleware": "^3.0.0",
  "webpack-hot-middleware": "^2.21.1",
}