等待循环中调用的所有承诺完成

问题描述

我正在使用 axios 承诺库，但我认为我的问题更适用.现在我正在遍历一些数据并在每次迭代中进行一次 REST 调用.
当每次调用完成时，我需要将返回值添加到一个对象中.在高层次上，它看起来像这样:

I'm using the axios promise library, but my question applies more generally I think. Right now I'm looping over some data and making a single REST call per iteration.
As each call completes I need to add the return value to an object. At a high level, it looks like this:

var mainObject = {};

myArrayOfData.forEach(function(singleElement){
  myUrl = singleElement.webAddress;
  axios.get(myUrl)
  .then(function(response) {
    mainObject[response.identifier] = response.value;
   });
});

console.log(convertToStringValue(mainObject));

当然发生的事情是当我调用 console.log 时 mainObject 中还没有任何数据，因为 axios 仍在伸出援手.处理这种情况的好方法是什么?

What's happening of course is when I call console.log the mainObject doesn't have any data in it yet, since axios is still reaching out. What's a good way of dealing with this situation?

Axios 确实有一个 all 方法以及一个姊妹 spread 方法，但是如果您提前知道将要进行多少次调用，它们似乎很有用制作，而在我的情况下，我不知道会有多少循环迭代.

Axios does have an all method along with a sister spread one, but they appear to be of use if you know ahead of time how many calls you'll be making, whereas in my case I don't know how many loop iterations there will be.

推荐答案

你需要将所有的 promise 收集到一个数组中，然后使用 Promise.all:

You need to collect all of your promises in an array and then use Promise.all:

// Example of gathering latest Stack Exchange questions across multiple sites
// Helpers for example
const apiUrl = 'https://api.stackexchange.com/2.2/questions?pagesize=1&order=desc&sort=activity&site=',
    sites = ['stackoverflow', 'ubuntu', 'superuser'],
    myArrayOfData = sites.map(function (site) {
        return {webAddress: apiUrl + site};
    });

function convertToStringValue(obj) {
    return JSON.stringify(obj, null, '	');
}

// Original question code
let mainObject = {},
    promises = [];

myArrayOfData.forEach(function (singleElement) {
    const myUrl = singleElement.webAddress;
    promises.push(axios.get(myUrl));
});

Promise.all(promises).then(function (results) {
    results.forEach(function (response) {
        const question = response.data.items[0];
        mainObject[question.question_id] = {
            title: question.title,
            link: question.link
        };
    });

    console.log(convertToStringValue(mainObject));
});

<script src="https://unpkg.com/axios@0.19.2/dist/axios.min.js"></script>

它在 axios 文档(执行多个并发请求)中有描述部分).

It's described in axios docs (Performing multiple concurrent requests section).

在 2020 年 5 月之前，可以使用 axios.all()，现已弃用.

Before May 2020 it was possible to do with axios.all(), which is now deprecated.

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