广义中间件

前言

• 不直接提供中间件
• 通过间接方式提供了中间件，最常见的是间接中间件和子中间件
• 间接被 app.use() 加载
• 其他方式接入Koa切面

间接中间件

const Koa = require('koa');
let app = new Koa();
function indirectMiddleware(path, middleware) {
  return async function(ctx, next) {
    console.log(ctx.path === path, middleware);
    if (ctx.path === path) {
      await middleware(ctx, next);
    } else {
      await next();
    }
  };
}
const index = async function(ctx, next) {
  ctx.body = 'this is index page';
};
const hello = async function(ctx, next) {
  ctx.body = 'this is hello page';
};
const world = async function(ctx, next) {
  ctx.body = 'this is world page';
};
app.use(indirectMiddleware('/', index));
app.use(indirectMiddleware('/hello', hello));
app.use(indirectMiddleware('/world', world));
app.listen(3001, () => {
  console.log('the demo is start at port 3001');
});

子中间件

子中间件是广义中间件的一个最有代表场景，主要的特点有

• 初始化中间件时，内置子中间件列表
• 子中间件列表添加子中间件元素
• 子中间件列表封装成间接中间件，让后被app.use()加载

const Koa = require('koa');
let app = new Koa();
class Middleware{
  constructor() {
    this.stack = [];
  }
  get(path, childMiddleware) {
    this.stack.push({ path, middleware: childMiddleware })
  }
  middlewares() {
    let stack = this.stack;
    return async function(ctx, next) {
      let path = ctx.path;
      for( let i=0; i<stack.length; i++ ) {
        const child = stack[i];
        if( child && child.path === path && child.middleware ) {
          await child.middleware(ctx, next);
        }
      }
      await next();
    }
  }
}
const middleware = new Middleware();
middleware.get('/page/001', async(ctx, next) => { ctx.body = 'page 001' })
middleware.get('/page/002', async(ctx, next) => { ctx.body = 'page 002' })
middleware.get('/page/003', async(ctx, next) => { ctx.body = 'page 003' })
app.use(middleware.middlewares());
app.listen(3001, function(){
  console.log('the demo is start at port 3001');
})