javascript promise 详解 – Records And Memorys

A Promise is an object representing the eventual completion or failure of an asynchronous operation

promise 代表一个异步事件的完成或失败。

Essentially, a promise is a returned object to which you attach callbacks, instead of passing callbacks into a function.

本质上， promise 是一个被返回的object , 在这个object 内可以附加回调函数，以代替原来的在函数中指定回调函数。

Imagine a function, createAudioFileAsync(), which asynchronously generates a sound file given a configuration record and two callback functions, one called if the audio file is successfully created, and the other called if an error occurs.

Here’s some code that uses createAudioFileAsync():

在原先的写法，当调用一个异步函数的时候，需要在参数位置指定它的回调函数，比如异步生成一个语音文件时，指定它的成功和失败的回调，代码如下：

function successCallback(result) {
  console.log("Audio file ready at URL: " + result);
}

function failureCallback(error) {
  console.error("Error generating audio file: " + error);
}

createAudioFileAsync(audioSettings, successCallback, failureCallback);

function successCallback(result) {

console.log("Audio file ready at URL: " + result);

}

function failureCallback(error) {

console.error("Error generating audio file: " + error);

}

createAudioFileAsync(audioSettings, successCallback, failureCallback);

在现代的javascript 当中，是返回一个promise

createAudioFileAsync(audioSettings).then(successCallback, failureCallback);

1	createAudioFileAsync(audioSettings).then(successCallback, failureCallback);

上一句代码是以下2句的简写形式：

const promise = createAudioFileAsync(audioSettings); 
promise.then(successCallback, failureCallback);

1 2	const promise = createAudioFileAsync(audioSettings); promise.then(successCallback, failureCallback);

.then 代表promise 确实返回了什么东西，即使是 null ,

以 opentok-rtc 为例，它从永久设备取数的方法是调用 swagger-boilerplate的 getKey 方法

在 swagger-boilerplate的 serverPersistence.js 文件

    getKey(aKeyName, aAsObject) {
      if (!aKeyName) {
        return Promise.resolve(null);
      }

getKey(aKeyName, aAsObject) {

if (!aKeyName) {

return Promise.resolve(null);

}

也就是说，如果没有这个key , 返回 null , 但这并不会触发 promise 的 catch , 而是触发 resolve, 因为确实返回了什么

promise 和事件(event) 有点像，但是， promise 只能成功或者失败一次， event可以发生多次。

A promise can only succeed or fail once. It cannot succeed or fail twice, neither can it switch from success to failure or vice versa.

—

If a promise has succeeded or failed and you later add a success/failure callback, the correct callback will be called, even though the event took place earlier.

promise 可以在发生之后很久被调用， event不可以

A promise can be:

fulfilled – The action relating to the promise succeeded
rejected – The action relating to the promise failed
pending – Hasn’t fulfilled or rejected yet
settled – Has fulfilled or rejected

promise可以有以上四种状态

如何处理 promise 的成功和失败

promise.then(function(result) {
  console.log(result); // "Stuff worked!"
}, function(err) {
  console.log(err); // Error: "It broke"
});

promise.then(function(result) {

console.log(result); // "Stuff worked!"

}, function(err) {

console.log(err); // Error: "It broke"

});

怎么理解这几句：

  var setPublisherReady;
  var publisherReady = new Promise(function (resolve) {
    setPublisherReady = resolve;
  });

var setPublisherReady;

var publisherReady = new Promise(function (resolve) {

setPublisherReady = resolve;

});

变量= resolve是什么意思？

参考文章：

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_promises —– how to use promise

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise — introduce promise

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/resolve

专门讲 resolve