手写Koa2

Koa2源码实现

Koa2使用

const Koa = require('koa');
// const Koa = require('./my-koa');
const app = new Koa();

const mid1 = async (ctx, next)=> {
  console.log('mid1');
  ctx.body = 'mid1Before-';
  await next();
  ctx.body += 'mid1After';
}

const mid2 = async (ctx, next)=> {
  console.log('mid2');
  ctx.body += 'mid2Before-';
  await next();
  ctx.body += 'mid2After-';
}

const mid3 = async (ctx, next)=> {
  console.log('mid3');
  ctx.body += 'mid3Before-';
  await next();
  ctx.body += 'mid3After-';
}

app.use(mid1).use(mid2).use(mid3);
app.listen(3005, ()=> {
  console.log('web server is on at port 3005');
});

实现自己的Koa

const http = require('http');
const context = {
  _body: null,
  get body() {
    return this._body;
  },
  set body(val) {
    this._body = val;
  }
}

class MyKoa {
  constructor() {
    this.middlewares = [];
  }
  use(middleware) {
    middleware&&this.middlewares.push(middleware);
    return this;
  }
  listen(...args) {
    const server = http.createServer(async (req, res)=> {
      const ctx = this.createContext(req, res);
      const fn = this.compose(this.middlewares);
      await fn(ctx);
      res.end(ctx.body);
    });
    server.listen(...args);
  }
  createContext(req, res) {
    const ctx = Object.create(context);
    ctx.req = req;  
    ctx.res = res;
    return ctx;  
  }
  compose(middlewares) {
    return async (ctx)=> {
      return dispatch(0);
      function dispatch(index) {
        const fn = middlewares[index];
        if(!fn) {
          return Promise.resolve();
        }
        return Promise.resolve(fn(ctx, ()=> {
          return dispatch(index + 1);
        }));
      }
    }
  }
}

module.exports = MyKoa;

总结

Koa2的核心代码不多，但是功能无比强大，主要是它的中间件特别多，利用中间件来完成很多工作。