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():
- 可以把多个模块通过 next 方法来链接起来
- 可以决定是否向下执行(可以用来实现后台的权限)
- 可以封装一些方法,在中间件中,封装向下执行
实现中间件计时器文章来源:https://www.toymoban.com/news/detail-708575.html
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);
文章来源地址https://www.toymoban.com/news/detail-708575.html
到了这里,关于77 # koa 中间件的应用的文章就介绍完了。如果您还想了解更多内容,请在右上角搜索TOY模板网以前的文章或继续浏览下面的相关文章,希望大家以后多多支持TOY模板网!