77 # koa 中间件的应用

调用 next() 表示执行下一个中间件

const Koa = require("koa");const app = new Koa();app.use(async (ctx, next) => {console.log(1);next();console.log(2);
});app.use(async (ctx, next) => {console.log(3);next();console.log(4);
});app.use(async (ctx, next) => {console.log(5);next();console.log(6);
});app.listen(3000);

洋葱模型：

输出：135642

添加异步等待

const Koa = require("koa");const app = new Koa();const log = () => {return new Promise((resolve, reject) => {setTimeout(() => {console.log("kaimo313");resolve();}, 3000);});
};app.use(async (ctx, next) => {console.log(1);next();console.log(2);
});app.use(async (ctx, next) => {console.log(3);await log();next();console.log(4);
});app.use(async (ctx, next) => {console.log(5);next();console.log(6);
});app.listen(3000);

输出：132 kaimo313 564

koa 中要求每个 next 方法前面都必须增加 await 否则不存在等待效果

const Koa = require("koa");const app = new Koa();const log = () => {return new Promise((resolve, reject) => {setTimeout(() => {console.log("kaimo313");resolve();}, 3000);});
};app.use(async (ctx, next) => {console.log(1);next();console.log(2);ctx.body = "hello 1";
});app.use(async (ctx, next) => {console.log(3);await log();ctx.body = "hello 2";next();console.log(4);
});app.use(async (ctx, next) => {console.log(5);ctx.body = "hello 3";next();console.log(6);
});app.listen(3000);

会取中间件第一个执行完的结果

koa 的中间件原理：会将所有的中间件组合成一个大的 promise，当这个 promise 执行完毕后，会采用当前的 ctx.body 进行结果的响应（next 前面必须要有 await 或者 return 否则执行顺序可能达不到预期）

如果都是同步执行，加不加 await 都无所谓，由于不知道后续是否有异步逻辑，写的时候都要加上 await

next():

1. 可以把多个模块通过 next 方法来链接起来
2. 可以决定是否向下执行（可以用来实现后台的权限）
3. 可以封装一些方法，在中间件中，封装向下执行

实现中间件计时器

const Koa = require("koa");const app = new Koa();const log = () => {return new Promise((resolve, reject) => {setTimeout(() => {console.log("kaimo313");resolve();}, 3000);});
};app.use(async (ctx, next) => {console.time("kaimo");await next();ctx.body = "hello 1";console.timeEnd("kaimo");
});app.use(async (ctx, next) => {await log();ctx.body = "hello 2";return next();
});app.use(async (ctx, next) => {ctx.body = "hello 3";return next();
});app.listen(3000);