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递归异步延迟承诺重叠

closures promise scope recursion javascript

-1

igorludi · 技术社区 · 7 年前

用例如下:我想每隔x毫秒查询一个URL,直到得到一个肯定的答复。

我正试图在JS(节点)中实现这一点,并且对于多个这样的请求有一个问题。如果我只启动其中一个“链调用”,一切都会正常工作。

但如果我试着平行地运行一些程序,“最后一个程序获胜”(第一个程序永远不会解决)。我似乎在使用对同一个对象或某物的引用,但我无法理解。

注意“mastercounter”和“id”变量是如何被覆盖的,尽管它们是作为参数按值传递的。

以下是工作代码(其中“第二”胜于“第一”):

(如果安装请求包,则可以与节点一起运行 https://www.npmjs.com/package/request )

var req = require('request');

const ASK_EVERY_MS = 500;
const AT_MOST_MS = 5000;

var childRecursive = (t, resolve, reject, t0, id, masterCounter) => {    
    var duration = process.hrtime(t0);    
    duration = (duration[0] * Math.pow(10, 9) + duration[1]) / Math.pow(10, 6)
    if (duration + t > AT_MOST_MS) {
        reject('timeout');
    } else {
        var options = {
            url: 'http://www.google.com',
            json: false,
            headers: {
                timeout: 1000
            }
        };
        callback = function (error, response, body) {
            if (!error) {
                console.log('body', body.substring(0, 100));
                if (masterCounter % 5 == 0) {
                    return resolve("success!!!");
                } else {
                    return childRecursive(ASK_EVERY_MS, resolve, reject, t0, id, masterCounter + 1, req);
                }
            } else {
                console.log('problem with submission GET request: ' + error);
                return reject(error);
            }
        }
        console.log(id + ' masterCounter is ' + masterCounter);
        setTimeout(() => {
            console.log(id + ' requesting...');
            req.get(options, callback);
        }, t);
    }
};

var t0 = process.hrtime();
var first = new Promise((resolve, reject) => childRecursive(0, resolve, reject, t0, 'FIRST', 101));
first.then(() => {
    console.log('FIRST done');
});
var scnd = new Promise((resolve, reject) => childRecursive(0, resolve, reject, t0, 'SECOND', 201));
scnd.then(() => {
   console.log('SECOND done');
});

输出:

FIRST masterCounter is 101
SECOND masterCounter is 201
FIRST requesting...
SECOND requesting...
body <!doctype html><html itemscope="" itemtype="http://schema.org/WebPage" lang="hr"><head><meta content
SECOND masterCounter is 202
body <!doctype html><html itemscope="" itemtype="http://schema.org/WebPage" lang="hr"><head><meta content
SECOND masterCounter is 202
SECOND requesting...
SECOND requesting...
body <!doctype html><html itemscope="" itemtype="http://schema.org/WebPage" lang="hr"><head><meta content
SECOND masterCounter is 203
body <!doctype html><html itemscope="" itemtype="http://schema.org/WebPage" lang="hr"><head><meta content
SECOND masterCounter is 203
SECOND requesting...
SECOND requesting...
body <!doctype html><html itemscope="" itemtype="http://schema.org/WebPage" lang="hr"><head><meta content
SECOND masterCounter is 204
body <!doctype html><html itemscope="" itemtype="http://schema.org/WebPage" lang="hr"><head><meta content
SECOND masterCounter is 204
SECOND requesting...
SECOND requesting...
body <!doctype html><html itemscope="" itemtype="http://schema.org/WebPage" lang="hr"><head><meta content
SECOND masterCounter is 205
body <!doctype html><html itemscope="" itemtype="http://schema.org/WebPage" lang="hr"><head><meta content
SECOND masterCounter is 205
SECOND requesting...
SECOND requesting...
body <!doctype html><html itemscope="" itemtype="http://schema.org/WebPage" lang="hr"><head><meta content
SECOND done
body <!doctype html><html itemscope="" itemtype="http://schema.org/WebPage" lang="hr"><head><meta content

1 回复 | 直到 7 年前

Bergi 7 年前

你的问题是 implicitly global callback 变量。它应该是

var callback = function (error, response, body) { /*
^^^ */

或

function callback(error, response, body) {

总是 use strict mode 为了避免这样的错误!