從Node中阻止Javascript函數訪問值

Question

下面的函數startScript包含阻塞代碼，需要一些時間才能運行。 在我的setInterval循環中，我想訪問它返回的weight值。 但是，當我運行此Node程序時，收到一條錯誤消息， new_weight未定義new_weight 。

我嘗試在循環中運行startScript.on('close', ...) ，但這引發了一個錯誤，指出startScript on沒有方法

我在這里做錯了什么？

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

您無法從pyScript.on()返回，它是異步的。 父函數已經很久才返回另一個返回。 相反，您必須使用回調。

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);