koa 中，中间件异步与同步的相关问题

同步中间件很容易理解，如以下代码：

const Router = require('koa-router')
    , koa = new Router({ prefix: '/koa' })

    , fs = require('fs')
;
/* 中间件执行顺序 与异步中间件 */
const one = (ctx, next) => {
    console.log('>> one');
    next();
    console.log('<< one');
};

const two = (ctx, next) => {
    console.log('>> two');
    next();
    console.log('<< two');
};

const three = (ctx, next) => {
    console.log('>> three');
    next();
    console.log('<< three');
};

koa['get']('/middleStack', one, two, three, async (ctx) => {
    console.log('>> four');
    ctx.response.type = 'html';
    let aaa = 'aa';
    console.log('<< four');
    ctx.response.body = aaa;
});

以上代码在控制台一次打印：

>> one
>> two
>> three
>> four
<< four
<< three
<< two
<< one

如果将其中一个中间件改为异步中间件：

koa['get']('/middleStack', one, two, three, async (ctx) => {
    console.log('>> four');
    ctx.response.type = 'html';
    let aaa = await fs.openSync('./public/index.html','r');
    console.log('<< four');
    ctx.response.body = aaa;
});

则控制台打印顺序为：

>> one
>> two
>> three
>> four
<< three
<< two
<< one
<< four

同步中间件并不会等待异步中间件返回结果才继续 next 之后的代码，若将所有的中间件都改为异步中间件：

/* 中间件执行顺序 与异步中间件 */
const one = async (ctx, next) => {
    console.log('>> one');
    await next();
    console.log('<< one');
};

const two = async (ctx, next) => {
    console.log('>> two');
    await next();
    console.log('<< two');
};

const three = async (ctx, next) => {
    console.log('>> three');
    await next();
    console.log('<< three');
};

koa['get']('/middleStack', one, two, three, async (ctx) => {
    console.log('>> four');
    ctx.response.type = 'html';
    let aaa = await fs.openSync('./public/index.html','r');
    console.log('<< four');
    ctx.response.body = aaa;
});

>> one
>> two
>> three
>> four
<< four
<< three
<< two
<< one

打印顺序又回到所有的中间件为同步中间件的情况。

总结：在 koa 变成中，尽量将所有的中间件都些为异步中间件，使得执行顺序是可控可理解的。