javascript for循环内的异步函数 [英] Asynchronous function inside a javascript for loop

问题描述

我正在尝试将从数据库中获取的数据插入json变量，然后将其发送到客户端，问题是当我异步地从数据库获取所有数据时，我不知道json是何时正确填充.

I am trying to insert data that I get from the database to a json variable and then send it to the client, the problem is that as I get all the data from the database asynchronously I don't know when the json is correctly filled.

var geojson={ "type": "FeatureCollection",
              "features": []
    };

var routeObjects=JSON.parse(route.route);
for(var i=0;i<routeObjects.length;i++){
        hostelery.getInfo(routeObjects[i].ID, function(err, hostelery){
            if(!err) geojson.features.push(hostelery);
        });
}

因此，当所有数据都在geojson中时，我想将其发送回客户端...

So when all the data is in the geojson I would like to send it back to the client...

任何帮助将不胜感激...

Any help would be appreciated...

非常感谢您.

推荐答案

如果您实际上只是想知道何时完成一堆异步操作，则有多种方法可以解决此问题.

If what you're really just trying to do is to know when a bunch of async operations are done, there are multiple ways to approach the problem.

一种方法是简单地对所有异步操作何时完成进行计数，然后在该计数达到其最终值时执行您想要执行的任何操作:

One way is to simply keep a count for when all the async operations have completed and then carry out whatever operation you want to when that count reaches its terminal value:

var geojson = {
    "type": "FeatureCollection",
    "features": []
};

var doneCount = 0;
var routeObjects = JSON.parse(route.route);
for (var i = 0; i < routeObjects.length; i++) {
    hostelery.getInfo(routeObjects[i].ID, function (err, hostelery) {
        if (!err) geojson.features.push(hostelery);
        ++doneCount;
        if (doneCount === routeObjects.length) {
            // all async operations are done now
            // all data is in geojson.features
            // call whatever function you want here and pass it the finished data
        }
    });
}

---

如果您的API支持承诺，或者您可以承诺化" API以使其支持承诺，则

---

If your API supports promises or you can "promisify" the API to make it support promises, then promises are a more modern way to get notified when one or more async operations are complete. Here's a promise implementation:

首先，承诺异步操作:

hostelery.getInfoAsync = function(id) {
    return new Promise(function(resolve, reject) {
        hostelery.getInfo(id, function(err, data) {
            if (err) return reject(err);
            resolve(data);
        });
    });
}

然后，您可以使用Promise.all():

var geojson = {
    "type": "FeatureCollection",
    "features": []
};

var routeObjects = JSON.parse(route.route);
Promise.all(routeObjects.map(function(item) {
    return hostelery.getInfoAsync(item.ID).then(function(value) {
        geojson.features.push(value);
    }).catch(function(err) {
        // catch and ignore errors so processing continues
        console.err(err);
        return null;
    });
})).then(function() {
    // all done here
});

由于您似乎正在使用node.js，因此还有许多异步库提供了用于管理异步操作的各种功能. Async.js 是这样的一个库.

Since it looks like you're using node.js, there are also numerous async libraries that offer various features for managing async operations. Async.js is one such library.

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