hubs/src/courier.js

``` Caryl ------ courier3 / \ / \ courier1 courier2

const Context = require('./context')
const EventEmitter = require('events')
const promiseTree = require('./promiseTree')


/**
 * Courier
 * @desc lifecycle: 一个请求配置有一个实例， 存在于应用的整个生命周期 
 * @class Courier
 * @extends EventEmitter
 */
class Courier extends EventEmitter {
    /**
     * Creates an instance of Courier.
     * 
     * @memberof Courier
     */
    constructor(items, protocol) {
        super()
        this.items = Context.preprocess(items, protocol)
    }

    applyPlugin(plugin){
        plugin.call(this)
    }

    /**
     * 发送信息
     * 
     * @param {any} context 
     *  @member uri
     *  @member params
     *  @member shoudStop
     *  @member model
     * @memberof Courier
     */
    async send(context, key) {
        try {
            var cur = context.reqItems[key]
            this.emit('courierBeforeParamReady', context, key)
            cur.params = cur.prepareParams && cur.prepareParams(cur, key,context)
            this.emit('courierParamsReady', context, key)

            this.emit('courierBeforeRequest', context, key)

            var payload = await context.reqItems[key].requestor.call(context, cur)
            context.payloads[key] = payload

            this.emit('courierAfterRequest', context, key)
            context.payloads[key] = cur.processResult && cur.processResult(context.payloads[key], key,context)
            this.emit('courierResultReady', context, key)

        } catch (error) {
            context.errs[key] = error
            this.emit('courierError', context, error)
        }

        return context
    }

    async deliver(state) {
        var context = new Context(this.items, state)

        this.emit('contextReady', context)

        this.emit('beforeRequest', context)

        await promiseTree(this.items, ({key})=>{
            return this.send(context, key)
        })

        this.emit('afterRequest', context)

        this.emit('resultReady', context)

        return context
    }
}

module.exports = Courier