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：

 1 const Router = require('koa-router')
 2     , koa = new Router({ prefix: '/koa' })
 3 
 4     , fs = require('fs')
 5 ;
 6 /* 中间件执行顺序 与异步中间件 */
 7 const one = (ctx, next) => {
 8     console.log('>> one');
 9     next();
10     console.log('<< one');
11 };
12 
13 const two = (ctx, next) => {
14     console.log('>> two');
15     next();
16     console.log('<< two');
17 };
18 
19 const three = (ctx, next) => {
20     console.log('>> three');
21     next();
22     console.log('<< three');
23 };
24 
25 koa['get']('/middleStack', one, two, three, async (ctx) => {
26     console.log('>> four');
27     ctx.response.type = 'html';
28     let aaa = 'aa';
29     console.log('<< four');
30     ctx.response.body = aaa;
31 });

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：

1 koa['get']('/middleStack', one, two, three, async (ctx) => {
2     console.log('>> four');
3     ctx.response.type = 'html';
4     let aaa = await fs.openSync('./public/index.html','r');
5     console.log('<< four');
6     ctx.response.body = aaa;
7 });

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：

 1 /* 中间件执行顺序 与异步中间件 */
 2 const one = async (ctx, next) => {
 3     console.log('>> one');
 4     await next();
 5     console.log('<< one');
 6 };
 7 
 8 const two = async (ctx, next) => {
 9     console.log('>> two');
10     await next();
11     console.log('<< two');
12 };
13 
14 const three = async (ctx, next) => {
15     console.log('>> three');
16     await next();
17     console.log('<< three');
18 };
19 
20 koa['get']('/middleStack', one, two, three, async (ctx) => {
21     console.log('>> four');
22     ctx.response.type = 'html';
23     let aaa = await fs.openSync('./public/index.html','r');
24     console.log('<< four');
25     ctx.response.body = aaa;
26 });

>> 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。

 

posted @ 2018-08-27 13:55  nDos  阅读(856)  评论(0)  编辑  收藏  举报