JavaScript 等待所有异步调用完成

Question

我需要一些帮助来处理 JavaScript 中的异步调用。 我有一个 for 循环，每个循环调用一个异步 HttpRequest，并将其响应添加到一个数组中。 我希望程序在没有 jQuery（仅用于 DOM 操作）的情况下继续之前等待所有异步调用完成。 我已经搜索了相当多的解决方案，但没有一个真正有效的没有大量改变我的代码或依赖 jQuery。

function httpGet(theUrl, callback) {
    var xmlRequest = new XMLHttpRequest();
    xmlRequest.onreadystatechange = function() {
        if (xmlRequest.readyState == 4 && xmlRequest.status == 200) {
            callback(xmlRequest.responseText);
        }
    }
    xmlRequest.open("GET", theUrl, true);
    xmlRequest.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    xmlRequest.setRequestHeader("Accept", "application/json");
    xmlRequest.send(null);
}
$(document).ready(function() {    
    var channels = ["freecodecamp", "storbeck", "terakilobyte", "habathcx","RobotCaleb","thomasballinger","noobs2ninjas","beohoff"];
    var urls = channels.map((x) => "https://api.twitch.tv/kraken/channels/" + x);
    var data = [];
    (function(urls, data) {
        urls.forEach(function(url) {  
            function(resolve, reject) {
                httpGet(url, function(response) {
                    data.push(JSON.parse(response));
                })
            };
        })
    })(urls, data);

    // Continue after all async calls are finished
})

更新：使用 Promise 进行编辑，但仍然无法正常工作，也许我做错了什么。

function httpGet(theUrl, callback) {
    return new Promise(function(resolve, reject) {
        var xmlRequest = new XMLHttpRequest();
        xmlRequest.onreadystatechange = function() {
            if (xmlRequest.readyState == 4 && xmlRequest.status == 200) {
                callback(xmlRequest.responseText);
            }
        }
        xmlRequest.open("GET", theUrl, true);
        xmlRequest.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
        xmlRequest.setRequestHeader("Accept", "application/json");
        xmlRequest.send(null);
    })
}
$(document).ready(function() {    
    var channels = ["freecodecamp", "storbeck", "terakilobyte", "habathcx","RobotCaleb","thomasballinger","noobs2ninjas","beohoff"];
    var urls = channels.map((x) => "https://api.twitch.tv/kraken/channels/" + x);
    var data = [];
    var promises = [];
    (function(urls, data) {
        urls.forEach(function(url) {  
            var promise = httpGet(url, function(response) {
                data.push(JSON.parse(response));
            });
            promises.push(promise);
        })

        Promise.all(promises).then(function() {
            console.log(data);
        })
    })(urls, data);
})

Answer 1

对于 Promise，您不应该使用callback参数。 改为从reject调用resolve / reject函数。

不要将回调传递给调用，而是将您想要做的事情与.then处理程序中的承诺结果链接起来。

function httpGet(theUrl) {
    return new Promise(function(resolve, reject) {
        var xmlRequest = new XMLHttpRequest();
        xmlRequest.onreadystatechange = function() {
            if (xmlRequest.readyState == 4) {
                if (xmlRequest.status == 200) 
                    resolve(xmlRequest.responseText);
    //              ^^^^^^^
                else
                    reject(new Error(xmlRequest.statusText)); // or something
            }
        }
        xmlRequest.open("GET", theUrl, true);
        xmlRequest.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
        xmlRequest.setRequestHeader("Accept", "application/json");
        xmlRequest.send(null);
    });
}
$(document).ready(function() {    
    var channels = ["freecodecamp", "storbeck", "terakilobyte", "habathcx", "RobotCaleb", "thomasballinger", "noobs2ninjas", "beohoff"];
    var urls = channels.map((x) => "https://api.twitch.tv/kraken/channels/" + x);
    var promises = urls.map(function(url) {
//                      ^^^ simpler than forEach+push
        var promise = httpGet(url); // <-- no callback
        return promise.then(JSON.parse);
    });

    Promise.all(promises).then(function(data) {
//                                      ^^^^
        console.log(data);
    });
})

Answer 2

由于您使用的是 jQuery，您可以使用延迟对象来链接承诺。

收集所有承诺并使用$.when和 spread 运算符来等待所有承诺解决。 您可以使用then在解决所有 ajax 请求后运行一个函数。

ES5 示例

 $(document).ready(function () { var channels = ["freecodecamp", "storbeck", "terakilobyte", "habathcx", "RobotCaleb", "thomasballinger", "noobs2ninjas", "beohoff"]; var urls = channels.map(function (x) { return "https://api.twitch.tv/kraken/channels/" + x; }); var data = []; var promises = urls.map(function (url) { return $.get(url).then(function (response) { data.push(response); }); }); $.when.apply($, promises).then(function () { console.log('done', data); }); });

 <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

ES6 示例

 $(document).ready(function() { var channels = ["freecodecamp", "storbeck", "terakilobyte", "habathcx","RobotCaleb","thomasballinger","noobs2ninjas","beohoff"]; var urls = channels.map((x) => "https://api.twitch.tv/kraken/channels/" + x); var data = []; var promises = urls.map((url) => $.get(url).then((response) => { data.push(response); })); $.when(...promises).then(function() { console.log('done', data); }); });

 <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

Answer 3

难道不能通过将 ajax 请求的计数作为一个变量来完成：

var urls_count, data_count = 0;
function httpGet(theUrl, callback, onComplete) {
    var xmlRequest = new XMLHttpRequest();
    xmlRequest.onreadystatechange = function() {
        if (xmlRequest.readyState == 4 && xmlRequest.status == 200) {
            callback(xmlRequest.responseText);
        }
        if(xmlRequest.readyState == 4){
            data_count += 1
            if(urls_count == data_count){
                //this is called when all ajax calls complete
                onComplete();
            }
        }
    }
    xmlRequest.open("GET", theUrl, true);
    xmlRequest.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    xmlRequest.setRequestHeader("Accept", "application/json");
    xmlRequest.send(null);
}
$(document).ready(function() {    
    var channels = ["freecodecamp", "storbeck", "terakilobyte", "habathcx","RobotCaleb","thomasballinger","noobs2ninjas","beohoff"];
    var urls = channels.map((x) => "https://api.twitch.tv/kraken/channels/" + x);
    var data = [];
    urls_count = urls.length;
    var onComplete = function(){
        //your code after all ajax completes.
    }
    (function(urls, data) {
        urls.forEach(function(url) {  
            function(resolve, reject) {
                httpGet(url, function(response) {
                    data.push(JSON.parse(response));
                }, onComplete)
            };
        })
    })(urls, data);
})