'use strict'
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// like a promise
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// if a value is added, it is queued until the first then()
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// if then() is called before a value is available,
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// the returned promise is deferred
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// and queued up for later resolution eg when the next value is available or the
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// Result is completed.
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const CLOSED = Promise.resolve()
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module.exports = class Result {
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constructor () {
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this._isClosed = false
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this._pendingPromises = []
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this._pendingVals = []
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// TODO Nice to have ...array argument -> add()
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}
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add (val) {
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if (this.isClosed) {
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throw new Error('cannot add() to a closed Result')
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}
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if (this._pendingPromises.length > 0) {
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this._finishPromises(val)
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} else {
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this._pendingVals.push(val)
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}
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}
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get isClosed () {
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return this._isClosed
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}
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get hasValues () {
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return this._pendingVals.length > 0
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}
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/**
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* Requests the next value, wrapped in a promise.
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* If no more values are available and the Result
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* is closed, a Promise resolved to undefined is returned.
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* Else, the returned promise will resolve or reject
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* when the next value is added.
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*
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* @param {Function} res
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* @param {Function} rej
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* @return {Promise}
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*/
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then (res, rej) {
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if (this.hasValues) {
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const val = this._pendingVals.shift()
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const method = val instanceof Error ? 'reject' : 'resolve'
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return Promise[method](val).then(res, rej)
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}
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if (this.isClosed) {
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// no more values will ever be available
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return CLOSED.then(res, rej)
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}
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const p = new Promise((resolve, reject) => {
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this._pendingPromises.push({ resolve, reject })
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})
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return p.then(res, rej)
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}
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close (val) {
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this.add(val)
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this._isClosed = true
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}
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_finishPromises (val) {
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const method = val instanceof Error ? 'reject' : 'resolve'
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let promise
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while ((promise = this._pendingPromises.shift())) {
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promise[method](val)
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}
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}
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}
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