使用node.js的递归模式循环

时间:2012-04-15 12:10:12

标签: node.js for-loop design-patterns

我一直在尝试使用node.js迭代一系列城市,并发出一个迭代请求来谷歌搜索每个城市的方向(然后是JSON.parse来抽象驱动时间)。我需要找到一种同步执行此操作的方法,否则我将立即从每个城市的谷歌请求所有信息。我找到了一个在http://tech.richardrodger.com/2011/04/21/node-js-%E2%80%93-how-to-write-a-for-loop-with-callbacks/使用的好模式,但无法使回调起作用。正如您所看到的,我使用'show'函数来测试它。我的代码如下:

var request = require('request');
var fs = require('fs');
var arr = ['glasgow','preston','blackpool','chorley','newcastle','bolton','paris','york','doncaster'];
//the function I want to call on each city from [arr]
function getTravelTime(a, b,callback){
 request('https://maps.googleapis.com/maps/api/directions/json?origin='+a+'&destination='+b+'&region=en&sensor=false',function(err,res,data){
 var foo = JSON.parse(data);
 var duration = foo.routes[0].legs[0].duration.text;
 console.log(duration);
 });
};

function show(b){
 fs.writeFile('testing.txt',b);
};


function uploader(i){
 if( i < arr.length ){
   show( arr[i],function(){
   uploader(i+1);
   });
 }
}
uploader(0)

我遇到的问题是只输出数组中的第一个城市,并且回调/迭代永远不会继续。有什么想法,我出错了吗?

2 个答案:

答案 0 :(得分:15)

感谢指点,显然是由于我对javascript中回调的理解不足。只需阅读O'Reilly的JavaScript模式并点击'回调模式'部分 - doh!

对于任何不知道的人,这是代码的工作方式:

var arr = ['glasgow','preston','blackpool','chorley','newcastle','bolton','paris','york','doncaster'];

function show(a,callback){
    console.log(a);
    callback();
}

function uploader(i){
  if( i < arr.length ){
    show(arr[i],
         function(){
          uploader(i+1)
         });
   };
}
uploader(0) 

答案 1 :(得分:11)

我也面临这样的问题,所以我编写了一个递归回调函数,它将作为for循环,但你可以控制何时递增。以下是该模块,名称为syncFor.js,并将其包含在您的程序中

module.exports = function syncFor(index, len, status, func) {
    func(index, status, function (res) {
        if (res == "next") {
            index++;
            if (index < len) {
                syncFor(index, len, "r", func);
            } else {
                return func(index, "done", function () {
                })
            }
        }
    });
}

//this will be your program if u include this module
var request = require('request');
var fs = require('fs');
var arr = ['glasgow', 'preston', 'blackpool', 'chorley', 'newcastle', 'bolton', 'paris', 'york', 'doncaster'];
var syncFor = require('./syncFor'); //syncFor.js is stored in same directory
//the following is how u implement it

syncFor(0, arr.length, "start", function (i, status, call) {
    if (status === "done")
        console.log("array iteration is done")
    else
        getTravelTime(arr[i], "whatever", function () {
            call('next') // this acts as increment (i++)
        })
})


function getTravelTime(a, b, callback) {
    request('https://maps.googleapis.com/maps/api/directions/json?origin=' + a + '&destination=' + b + '&region=en&sensor=false', function (err, res, data) {
        var foo = JSON.parse(data);
        var duration = foo.routes[0].legs[0].duration.text;
        callback(); // call the callback when u get answer
        console.log(duration);
    });
};