在for循环中同步promises？

Question

我坚持使用angularjs / javascript的这种异步执行。请帮忙

笔记

虚拟API用于示例目的。

那些Dummy API实际上是一个AngularJS服务。

我使用XmlHttpRequest进行演示，所以不需要讨论第三个参数是假还是真。

问题

在for循环中，调用calculateFormulaValue函数并基于参数，它将调用一些API并在承诺的对象中获取值。但是对于循环在promise对象返回之前完成，我无法在rowCollection中保存最终对象。

我们如何重构代码以获得所需的结果？

代码在这里 - > https://jsbin.com/xocisivuro/edit?js,console

码

var rowCollection = [];
var headerCollection = ["Formula 1", "Formula 2", "Formula 3", "Formula 4", "Formula 5", "Formula 6", "Formula 7"];
var currentFormulaValues = {};
//var finalCollection = 
function generate(){

    for(var i=0;i<headerCollection.length;i++){

        calculateFormulaValue(i,headerCollection[i]);
        console.log("current index" +i + " -->" +headerCollection[i]);


    }
}

function calculateFormulaValue(j,currentFormula){

    //Some common code which need to run ..

    if(currentFormula == "Formula 1"){
        var promise = new Promise(function(resolve, reject) {
        var request = new XMLHttpRequest();

        request.open('GET', 'https://api.icndb.com/jokes/random');
        request.onload = function() {
          if (request.status == 200) {
            resolve(request.response);
          } else {

            reject(Error(request.statusText));
          }
        };

        request.onerror = function() {reject(Error('Error fetching data.')); 
        };

        request.send();
      });

      promise.then(function(data) {

        currentFormulaValues[currentFormula] = JSON.parse(data).value.id;
        //rowCollection[j] = JSON.parse(data).value.id;

        console.log("j - " + j +" ->" + (JSON.stringify(currentFormulaValues)));
      }, function(error) {
        console.log('Promise rejected.');
      });
    }

    else if(currentFormula == "Formula 2"){
        var promise = new Promise(function(resolve, reject) {
        var request = new XMLHttpRequest();

        request.open('GET', 'https://api.icndb.com/jokes/random');
        request.onload = function() {
          if (request.status == 200) {
            resolve(request.response);
          } else {

            reject(Error(request.statusText));
          }
        };

        request.onerror = function() {reject(Error('Error fetching data.')); 
        };

        request.send();
      });

      promise.then(function(data) {
        currentFormulaValues[currentFormula] = JSON.parse(data).value.id;
        //rowCollection[j] = JSON.parse(data).value.id;
        console.log("j - " + j +" ->" + (JSON.stringify(currentFormulaValues)));

      }, function(error) {
        console.log('Promise rejected.');
      });
    }

    // for all other formulas

    else{
        var promise = new Promise(function(resolve, reject) {
        var request = new XMLHttpRequest();

        request.open('GET', 'https://api.icndb.com/jokes/random');
        request.onload = function() {
          if (request.status == 200) {
            resolve(request.response);
          } else {

            reject(Error(request.statusText));
          }
        };

        request.onerror = function() {reject(Error('Error fetching data.')); 
        };

        request.send();
      });

      promise.then(function(data) {

        currentFormulaValues[currentFormula] = JSON.parse(data).value.id;
        //rowCollection[j] = JSON.parse(data).value.id;
        console.log("j - " + j +" ->" + (JSON.stringify(currentFormulaValues)));

      }, function(error) {
        console.log('Promise rejected.');
      });

    }

     if(j == headerCollection.length-1){
        console.log("SAVE FINAL")
        rowCollection.push(currentFormulaValues);
        console.log(JSON.stringify(currentFormulaValues))
      } 
}

当前输出

"current index0 -->Formula 1"
"current index1 -->Formula 2"
"current index2 -->Formula 3"
"current index3 -->Formula 4"
"current index4 -->Formula 5"
"current index5 -->Formula 6"
"SAVE FINAL"
"{}"
"current index6 -->Formula 7"
"j - 0 ->{\"Formula 1\":98}"
"j - 1 ->{\"Formula 1\":98,\"Formula 2\":175}"
"j - 2 ->{\"Formula 1\":98,\"Formula 2\":175,\"Formula 3\":523}"
"j - 3 ->{\"Formula 1\":98,\"Formula 2\":175,\"Formula 3\":523,\"Formula 4\":399}"
"j - 4 ->{\"Formula 1\":98,\"Formula 2\":175,\"Formula 3\":523,\"Formula 4\":399,\"Formula 5\":119}"
"j - 5 ->{\"Formula 1\":98,\"Formula 2\":175,\"Formula 3\":523,\"Formula 4\":399,\"Formula 5\":119,\"Formula 6\":261}"
"j - 6 ->{\"Formula 1\":98,\"Formula 2\":175,\"Formula 3\":523,\"Formula 4\":399,\"Formula 5\":119,\"Formula 6\":261,\"Formula 7\":164}"

预期输出

"current index0 -->Formula 1"
"j - 0 ->{\"Formula 1\":98}"

"current index1 -->Formula 2"
"j - 1 ->{\"Formula 1\":98,\"Formula 2\":175}"

"current index2 -->Formula 3"
"j - 2 ->{\"Formula 1\":98,\"Formula 2\":175,\"Formula 3\":523}"

"current index3 -->Formula 4"
"j - 3 ->{\"Formula 1\":98,\"Formula 2\":175,\"Formula 3\":523,\"Formula 4\":399}"

"current index4 -->Formula 5"
"j - 4 ->{\"Formula 1\":98,\"Formula 2\":175,\"Formula 3\":523,\"Formula 4\":399,\"Formula 5\":119}"

"current index5 -->Formula 6"
"j - 5 ->{\"Formula 1\":98,\"Formula 2\":175,\"Formula 3\":523,\"Formula 4\":399,\"Formula 5\":119,\"Formula 6\":261}"

"current index6 -->Formula 7"
"j - 6 ->{\"Formula 1\":98,\"Formula 2\":175,\"Formula 3\":523,\"Formula 4\":399,\"Formula 5\":119,\"Formula 6\":261,\"Formula 7\":164}"


//if j == headerCollection.length-1, then..

"SAVE FINAL"  // then do ... rowCollection.push(currentFormulaValues);

任何有助于实现预期产出的帮助都会很好。提前致谢。

Answer 1

您可以通过在此处输入return关键字来返回您在calculateFormulaValue中创建的承诺：

return promise.then(function(data) { // ... etc

在你有这个结构的所有地方都这样做，以确保函数总是返回promise（甚至更好：尝试重用每个公式共同的代码 - 你有很多代码重复）。

然后你的主循环可以构建一个新数组，其中每个元素都是返回的promise：

var promises = headerCollection.map(function (collection, i) {
    console.log("current index" +i);
    // return(!) the promise you get from each call. This will become 
    // an element in a new array, returned by *map*.
    return calculateFormulaValue(i, collection);
});

现在你可以等待所有的承诺与Promise.all完成：

Promise.all(promises).then(function () {
    // now your object is available.
});

注意：您应该进一步改进代码以避免使用全局变量，例如结果对象currentFormulaValues。