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);