如何在“for”循环中显示每个循环的输出？答案

【问题标题】：How can I display the output of each loop on a "for" loop?如何在“for”循环中显示每个循环的输出？
【发布时间】：2014-04-15 12:55:53
【问题描述】：

我是个菜鸟，我正在使用 HTML5 Canvas 和 JS 制作“素数”绘图/动画。我很难使用“for”循环，因为浏览器仅在“for”循环完成后才显示绘图，而不是在它工作时（显示每个循环的输出，如动画）。

HTML/css：

<!doctype html>


<html>
<head>
</head>
    <style>

    body
        {
        background: #999999
        }

    #canvas 
        {
        background: white;
        border: 19px solid black;
        }

    </style>

<body>

    <canvas id="canvas" width="4000" height="4000">

    Bájate un nuevo navegador!!

    </canvas>
    <script src="jscode.js"></script>

</body>


</html>

JS (jscode.js)：

var canvas1 = document.getElementById('canvas');
var context = canvas.getContext ('2d');

// Text properties 
context.font = '32pt Arial';
context.fillStyle = 'DeepSkyBlue';
context.StrokeStyle = 'DarkSlateGray';

// Line properties
context.StrokeStyle = "rgb (200,0,0)";
context.lineWidth = 1;

// Each Natural number is represented by a segment
// "t" corresponds to the length of each segment.
var t = 3;

// Starting position within the canvas.
var x = 500;
var y = 500;


//Array stores every movement's direction.
//(We need to know the last segment direction in order "continue" the drawing) 
var Dmemory = new Array();

// Drawing using a Path function.  
// Drawing "manually".
        context.beginPath();
        //0 
        context.moveTo(x,y);
        Dmemory [0] = "A";
        //1
        /*UP*/ var y = y - t;
        context.lineTo(x,y);
        Dmemory [1] = "A";

// Continue using a "for" loop.         
// Variable "num" is the LIMIT. The program will run until the loop gets to this number.            
    var num = 10000;

// The program begins with 1 and starts counting adding 1+.
    for (i=2 ; i < num ; i++ )
    {

//Creating a "Divisors tracking variable".  
//We are going to divide each number between all the natural numbers below to verify if its a prime.
        var contador = 0;
        for ( div = i ; div > 0 ; div--)
        {   

            if (i%div == 0)
            {
            contador = contador + 1; 
            }
            //Breaking condition: More than 2 divisors.
            if (contador>2)
            {
            break;
            }

        }
        //Primes Test: Primes have ONLY two divisors: Themselves and number One
        if (contador == 2) 
            {   
                //If Prime: check the last movements direction and proceed.
                switch (Dmemory[i - 1])
                {

                case "A":
                    var y = y - t;
                    context.lineTo(x,y);
                    context.stroke();
                    Dmemory[i]="D";
                    break;

                case "B":
                    var y = y + t;
                    context.lineTo(x,y);
                    context.stroke();
                    Dmemory[i]="I";
                    break;

                case "D":
                    var x = x + t;
                    context.lineTo(x,y);
                    context.stroke();
                    Dmemory[i]="B";
                    break;

                case "I":
                    var x = x - t;
                    context.lineTo(x,y); 
                    context.stroke();
                    Dmemory[i]="A";

                }


            }
        else
            {
                //If not Prime: check the last movement direction and proceed.
                switch (Dmemory[i - 1])
                {

                case "A":
                    var y = y - t;
                    context.lineTo(x,y);
                    context.stroke();
                    Dmemory[i]="A";
                    break;

                case "B":
                    var y = y + t;
                    context.lineTo(x,y);
                    context.stroke();
                    Dmemory[i]="B";
                    break;

                case "D":
                    var x = x + t;
                    context.lineTo(x,y);
                    context.stroke();
                    Dmemory[i]="D";
                    break;

                case "I":
                    var x = x - t;
                    context.lineTo(x,y); 
                    context.stroke();
                    Dmemory[i]="I";
                    break;

                }

            }


    }

我一直在阅读其他答案，现在我明白了主要原因：网络浏览器中的 JavaScript 是单线程的。

但是，有什么方法可以完成我需要的吗？（我需要的是：随着循环的进行在屏幕上显示绘图）

提前谢谢你，

来自利马的问候 - 秘鲁

【问题讨论】：

标签： html loops canvas for-loop

【解决方案1】：

为了能够报告进度，您必须在给定的时间间隔内进行计算，以便系统有足够的时间来“呼吸”。
下面是一个定期调用的假间隔计算。我希望它可以帮助您找到解决方案。

http://jsbin.com/qarivina/1/edit?js,output

var canvas = document.getElementById('canvas');
var context = canvas.getContext ('2d');
setupNow();

function computeFromTo(i1, i2) {
  var st = now();
  for (var i=i1; i<i2; i++) computeForIndex(i);
  var ed = now();
  var timeTaken = (ed-st);
}

function computeForIndex(i) {
  var res = Math.pow(i,2.5)/Math.abs(1 + Math.sin(i));
}

function beginCallCompute() {
  var startIndex    = 0  ;
  var indexIncrease = 500 ;
  var lastIndex     = 500000;

  var computeInterval = setInterval(callCompute, 5);

  function callCompute() {
    computeFromTo(startIndex, startIndex+indexIncrease);
    startIndex += indexIncrease;
    context.clearRect(0,0,canvas.width, canvas.height);
    context.fillText(startIndex, 20, 20);
    if (startIndex>=lastIndex) {
       clearInterval(computeInterval);
    }
  }

}

beginCallCompute();


// ---------------------------

function setupNow() {
  window.now = ( performance && performance.now.bind(performance) )|| Date.now ;
}

【讨论】：