承诺只发射一次？

问题描述

以下承诺是否只有一次触发是否有任何理由？

Is there any reason why the promise below is only firing once?

console.log('start')

var Promise = require('bluebird')
var onoff = require('onoff')
var Gpio = onoff.Gpio
var button = new Gpio(4, 'in', 'both')

var buttonWatchAsync = function (button, desiredValue) {
  return new Promise (function (resolve, reject) {
    return button.watch(function(err, value) {
      if (err) return reject(err)
      if (typeof desiredValue === 'undefined') return resolve(value)
      if (desiredValue === value) return resolve()
    })
  })
}

buttonWatchAsync(button)
  .then(function (value) {
    console.log('fired promise')
    console.log(value)
  })
  .catch(function (err) {
    throw err
  })

推荐答案

因为承诺只会触发一次。承诺被创建/初始化，然后结算，一旦结算，永远不会解决或重新解决。调用解决或拒绝第二个（第三个，第四个......）时间是无操作。 （有些人认为它应该是一个错误，但事实并非如此。）承诺不是事件，它们不能再发生。因此，对于该代码正在做什么，承诺不是正确的工具。

Because promises only fire once. A promise is created/initialized, and then settled, and once settled can never be un-settled or re-settled. Calling resolve or reject a second (third, fourth, ...) time is a no-op. (Some believe it should be an error, but it isn't.) Promises are not events, they cannot recur. So for what that code is doing, a promise isn't the right tool.

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