等待循环中调用的所有诺言完成 [英] Waiting for all promises called in a loop to finish

问题描述

我正在使用 axios 承诺库，但我认为我的问题更普遍.现在，我正在遍历一些数据，并且每次迭代都进行一次REST调用.
每次调用完成后，我需要将返回值添加到对象.从高层次看，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));

当然发生了什么事，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方法以及一个姐妹spread方法，但是如果您提前知道要拨打多少电话，它们似乎很有用，而在我的情况下，我却没有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

您需要将所有的诺言收集到一个数组中，然后使用

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, '\t');
}

// 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>

在 axios文档中进行了描述(执行多个并发请求部分).

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

在2020年5月之前，可以使用axios.all()，

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

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