Koa-Compose 函数解析
1 'use strict'
2
3 /**
4 * Expose compositor.
5 */
6
7 module.exports = compose
8
9 /**
10 * Compose `middleware` returning
11 * a fully valid middleware comprised
12 * of all those which are passed.
13 *
14 * @param {Array} middleware
15 * @return {Function}
16 * @api public
17 */
18
19 function compose (middleware) {
20 if (!Array.isArray(middleware)) throw new TypeError('Middleware stack must be an array!')
21 for (const fn of middleware) {
22 if (typeof fn !== 'function') throw new TypeError('Middleware must be composed of functions!')
23 }
24
25 /**
26 * @param {Object} context
27 * @return {Promise}
28 * @api public
29 */
30
31 return function (context, next) {
32 // last called middleware #
33 let index = -1
34 return dispatch(0)
35 function dispatch (i) {
// next 不允许调用多次
36 if (i <= index) return Promise.reject(new Error('next() called multiple times'))
37 index = i
38 let fn = middleware[i]
// fn 为最后一个表示没有其余中间件 next为最后一个
39 if (i === middleware.length) fn = next
40 if (!fn) return Promise.resolve()
41 try {
// 自增
42 return Promise.resolve(fn(context, dispatch.bind(null, i + 1)))
43 } catch (err) {
44 return Promise.reject(err)
45 }
46 }
47 }
48 }
