koa源代码解析

koa不愧为小而美，主要代码很少。简单来说，
1，koa封装了node的http.createServer（(req,res)=>{})的入参req,res到ctx同名属性（一个自定义对象）中，
并且额外提供了ctx.request,ctx.request提供一些快捷的操作。

const app = new Koa()

app.use(middlewareFn)

2.方法use函数接收参数fn，放入中间件middleware[]数组里面。

  use(fn) {

    this.middleware.push(fn);

    return this;

  }

通过用户调用listen(port)调用this.handleRequest函数：

  listen(...args) {

    const server = http.createServer(this.callback());

    return server.listen(...args);

  }

callback() {

    const fn = compose(this.middleware);

    const handleRequest = (req, res) => {

      const ctx = this.createContext(req, res);

      return this.handleRequest(ctx, fn);

    };

    return handleRequest;

  }

调用处理过的middleware函数:

  handleRequest(ctx, fnMiddleware) {

    const res = ctx.res;

    const handleResponse = () => respond(ctx);

    return fnMiddleware(ctx).then(handleResponse).catch(onerror);

  }

即处理过的middleware数组：

function compose (middleware) {

  return function (context, next) {

    // last called middleware #

    let index = -1

    return dispatch(0)

    function dispatch (i) {

      if (i <= index) return Promise.reject(new Error('next() called multiple times'))

      index = i

      let fn = middleware[i]

      if (i === middleware.length) fn = next

      if (!fn) return Promise.resolve()

      try {

        return Promise.resolve(fn(context, dispatch.bind(null, i + 1)));

      } catch (err) {

        return Promise.reject(err)

      }

    }

  }

}

递归的展开就是洋葱模型了：

const [fn1, fn2, fn3] = this.middleware;

const fnMiddleware = function(ctx){

    return Promise.resolve(

      fn1(context, function next(){

        return Promise.resolve(

          fn2(context, function next(){

              return Promise.resolve(

                  fn3(context, function next(){

                    return Promise.resolve();

                  })

              )

          })

        )

    })

  );

};

fnMiddleware(ctx).then(handleResponse).catch(onerror);

ctx:

createContext(req, res) {

    const context = Object.create(this.context);

    const request = context.request = Object.create(this.request);

    const response = context.response = Object.create(this.response);

    context.app = request.app = response.app = this;

    context.req = request.req = response.req = req;

    context.res = request.res = response.res = res;

    request.ctx = response.ctx = context;

    request.response = response;

    response.request = request;

    context.originalUrl = request.originalUrl = req.url;

    context.state = {};

    return context;

  }

巴特西

koa源代码解析

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