Question

以下函数startScript包含需要一些时间才能运行的阻塞代码。在我的setInterval循环中，我想访问它返回的weight值。但是，当我运行此Node程序时，我收到一条错误消息，指出new_weight未定义。

我尝试在循环中运行startScript.on('close', ...)，但这会引发错误，指出on没有方法startScript

我在这里做错了什么？

var spawn = require('child_process').spawn
var path = require('path');
var split = require('split');
var pythonData = [];
var weight = null;

function startScript(){
  var pyScript = spawn('python', [ path.join(__dirname, 'script.py') ]);
  pyScript.stdout.on('data', function(lineChunk){
    pythonData = lineChunk.toString().replace(/[\S|\n|\[|\]]/,"").split(',');
  });

  pyScript.on('close', function(code){
    var sum = 0;
    for(var i=0; i < pythonData.length; i++){
      sum += parseFloat(pythonData[i]);
    }
    var weight = sum / pythonData.length;
    console.log("weight: " + weight);
    return weight;
  });
}

setInterval(function(){
  if (some event that occurs infrequently){
    startScript();
    var new_weight = weight + 100
    console.log(new_weight);
  }
}, 1000);

Answer 1

你不能＆＃39;从pyScript.on()返回，它是异步的。父函数在其他返回发生之前已经很久就返回了。相反，您必须使用回调。

function startScript(callback){ // ******
  var pyScript = spawn('python', [ path.join(__dirname, 'script.py') ]);
  pyScript.stdout.on('data', function(lineChunk){
    pythonData = lineChunk.toString().replace(/[\S|\n|\[|\]]/,"").split(',');
  });

  pyScript.on('close', function(code){
    var sum = 0;
    for(var i=0; i < pythonData.length; i++){
      sum += parseFloat(pythonData[i]);
    }
    var weight = sum / pythonData.length;
    console.log("weight: " + weight);
    //return weight; // you can't return weight. Instead, execute the callback.
    callback(weight); // ******
  });
}

setInterval(function(){
  if (some event that occurs infrequently){
    startScript(function(weight){ // ******
      var new_weight = weight + 100
      console.log(new_weight);
    }); // ******
  }
}, 1000);

通过阻止Node中的Javascript函数访问值

1 个答案: