Question

因此，我一直在研究Web抓取，这主要是在工作，但是当我尝试输出数组元素时，它们都是未定义的（请参见底部的输出）。我觉得这可能与计时问题有关，我说这是因为价格从来不会像请求一样花费相同的时间来回答。如果是问题所在，如何使它们同步？谢谢！

func startUpdating() {
    timer = DispatchSource.makeTimerSource(flags: [], queue: DispatchQueue.main)
    timer?.schedule(deadline: DispatchTime.now(),
                    repeating: DispatchTimeInterval.milliseconds(16),
                             leeway: .milliseconds(5))
    timer?.setEventHandler(qos: .userInitiated, flags: [], handler: self.update)
    timer?.resume()
}

func update() {
    coordinateX.title = String(planeNode.position.x)
    coordinateY.title = String(planeNode.position.y)
}

输出

const request = require('request');
const cheerio = require('cheerio');
const fs = require('fs');
const writeStream = fs.createWriteStream('post.csv');

var numbers = ["2202917",
"2205112",
"3514318",
"3514561",
"3585503",
"3585704",
"3610075",
"5132753",
"5247359",
"5247360"];

var y =0;
var partNumber1 ="";
var price1 ="";

writeStream.write('PartNumber,Price \n');

for (y=0; y < numbers.length; y++){
  request(url + numbers[y], function(error, response, html) {
      if (!error && response.statusCode == 200) {
      const $ = cheerio.load(html);
      price1 =$('.price').text().replace(/\s\s+/g,'');
      partNumber1 =numbers[y];
      console.log(partNumber1,' ',price1);
      writeStream.write(`${partNumber1}, ${price1} \n`);
    }
  });
}

Answer 1

您正在执行request（）调用，这是一个异步调用。因此，这将是不正常的。这是一个更简单的示例，我将用它来解释替代方案。

使用不同的时间间隔来模拟请求调用。

var numbers = ["2202917",
"2205112",
"3514318",
"3514561",
"3585503",
"3585704",
"3610075",
"5132753",
"5247359",
"5247360"];

/*
for (var y=0; y < numbers.length; y++) {
  setTimeout(function(error, response, html) {

    // the problem here is that it is referring a closure variable. The value of y keeps changing outside, and is not the same as the one you called it with.
    // In this particular case, the value of y = numbers.length, and hence the undefined output

    console.log(numbers[y]);

  }, y % 100);
}
*/

// Alternative 1
// A better way would be like this
for (var y=0; y < numbers.length; y++) {
  // doRequest(numbers[y]);
}

function doRequest(partNumber) {
  setTimeout(function(error, response, html) {
    console.log(partNumber);    
  }, partNumber % 100); 
}

// end: Alternative 1

// Alternative 2
// in case you want it to be in sync
async function doSomething() {
  for (var y = 0; y < numbers.length; y++) {
    await doPromiseRequest(numbers[y]);
  }
}

function doPromiseRequest(partNumber) {
  return new Promise((resolve, reject) => {
    setTimeout(function(error, response, html) {

      console.log(partNumber);
      resolve();

    }, partNumber % 100); 
  });
}

doSomething();

// end Alternative 2

/*

another alternative is not calling a separate function, but executing an anonymous function in the loop itself. It is not clean at all and not recommended.

for (var y = 0; y < numbers.length; y++) {
  (function (y) {
    setTimeout(function(error, response, html) {

      // the problem here is that it is referring a closure variable. The value of y keeps changing outside, and is not the same as the one you called it with
      console.log(numbers[y]);

    }, y % 100);
  })(y);
}

*/

数组元素未定义时序问题？

1 个答案: