使用JavaScript和HTML5画布显示绘图过程

Question

I am currently trying to make a little pythagoras tree application using JavaScript/JQuery and a HTML5 canvas.

The algorithm works fine but sadly I am not able to see the process of drawing. The UI just freezes and shows the whole drawing after having finished.

Current behaviour: http://www.kappelcation.com/index.php?contentid=4 This is the behaviour i would like to have: http://www.jjam.de/Java/Applets/Fraktale/Pythagoras_Baum.html (you need allow the Java-applet and click a few times to see the progress of drawing).

$(document).ready(function(){ //just some listeners and initialization
initCanvas();

$(autOrManSelect).change(function(){
    if(($(this).val()==1)) //changed from manual to automatic
    {
        $(pythagorasCanvas).click();
    }
});
$(startButton).click(function(event){
    $(this).unbind(event);
    $(this).html('Add Leaves');
    $(restartButton).css("visibility", "visible");
    startTree();
    $(this).click(function(){
        var next = parseInt($(depthInput).val())+1;
        $(depthInput).val(next);
        $(pythagorasCanvas).click();
    });
});
$(restartButton).click(function(){
    startTree();
});
});

function initCanvas() //adjust size, clear the canvas and draw a boarder
{
var canvas = document.getElementById("pythagorasCanvas");
canvas.height=600;
canvas.width=$(mainDiv).width()-40;
$(pythagorasCanvas).unbind("click");
if (canvas.getContext) 
{
    var context = canvas.getContext("2d");
    //clear canvas
    drawRectangle(context, new Point(0,canvas.height), new Point(canvas.width,canvas.height), new Point(canvas.width,0), new Point(0,0), "#FFFFFF");
    //draw border for canvas
    drawRectangle(context,new Point(0,canvas.height),new Point(canvas.width,canvas.height), new Point(canvas.width, 0),new Point(0,0), '#000000', true);
}
}

function startTree() //start drawing process
{   
initCanvas();
var canvas = document.getElementById("pythagorasCanvas");
if (canvas.getContext) 
{
    var context = canvas.getContext("2d");
    var rectLength = parseInt($(firstSquareInput).val());
    var startWidth = canvas.width/2-rectLength/2;
    var startHeight = canvas.height-canvas.height/4;
    var startA = new Point(startWidth, startHeight);
    var startB = new Point(startWidth+rectLength, startHeight);     
    drawBranch(context, startA, startB, 0); 
}
}

function Point(x, y) {
this.x = x;
this.y = y;
}

function drawBranch(context, a, b, depth) //gets called recursively 
{
var maxDepth = $(depthInput).val();
if(depth<maxDepth)
{
    depth++;
    //calculate positions of current rectangle
    var dx = b.x - a.x;
    var dy = a.y - b.y;
    var c = new Point(b.x-dy, b.y-dx);
    var d = new Point(a.x-dy, a.y-dx);
    //draw current rectangle
    drawRectangle(context, a, b, c, d);

    //calculate new position
    var offSetX;
    if($(triangleOffsetInput).val().toLowerCase()=="random")
    {
        offSetX = Math.random();
    }
    else
    {
        offSetX = parseFloat($(triangleOffsetInput).val())/100; //first offset in x-direction (in relation to a square length of 1)
    }
    var offSetY = -Math.sqrt(Math.pow(0.5,2)-Math.pow((0.5-offSetX),2)); //Pythagoras to get the offset in y (negative sign necessary because the y-values get smaller upwards)
    var e = new Point( d.x + offSetX*(c.x-d.x) + offSetY*(a.y-b.y), d.y + offSetX*(c.y-d.y) + offSetY*(b.x-a.x));

    if($(fillTriangleBox).prop("checked"))
    {
        drawTriangle(context, c, d, e);
    }   

    var autOrMan = $(autOrManSelect).val();
    if(autOrMan==1)
    {
        //draw new positions
        drawBranch(context, d, e, depth);
        drawBranch(context, e ,c, depth);       
    }
    else
    {
        $(pythagorasCanvas).click(function(event) 
        {
            $(this).unbind(event);
            drawBranch(context, d, e, depth);
            drawBranch(context, e ,c, depth);       
        });
    }
}
else
{
    $(pythagorasCanvas).click(function(event) 
    {
        $(this).unbind(event);
        drawBranch(context, a, b, depth);
    });
}   
}

function drawTriangle(context, c, d, e, color)
{
if(typeof(color)==='undefined')
{
    color='#'+$(triangleColorInput).val();
} 
context.strokeStyle = color;
context.beginPath();
context.moveTo(c.x, c.y);
context.lineTo(d.x, d.y);
context.lineTo(e.x, e.y);
context.closePath();
context.fillStyle = color;
context.fill();
context.stroke();
}

function drawRectangle(context, a, b, c, d, color, ignoreFill)
{
if(typeof(color)==='undefined')
{
    color='#'+$(rectColorInput).val();
} 
if(typeof(ignoreFill)==='undefined')
{
    ignoreFill=false;
}
context.strokeStyle = color;
context.beginPath();
context.moveTo(a.x, a.y);
context.lineTo(b.x, b.y)
context.lineTo(c.x, c.y);
context.lineTo(d.x, d.y);
context.closePath();
if($(fillRectBox).prop("checked") && !ignoreFill)
{
    context.fillStyle = color;
    context.fill();
}
context.stroke();
}

I have tried to use the setTimeout() function to make the drawing process asynchronous to achieve the desired behaviour but I either failed horribly at implementing it or setTimeout() does not help in this situation.

I hope you can give me a hint. Thanks in advance!

Answer 1

I think the following (untested) modification to your drawBranch function using setTimeout should work

if(autOrMan==1)
{
    //draw new positions
    setTimeout(function() {
        drawBranch(context, d, e, depth);
        drawBranch(context, e ,c, depth);       
    }, 0);
}
else
{
    $(pythagorasCanvas).click(function(event) 
    {
        $(this).unbind(event);
        drawBranch(context, d, e, depth);
        drawBranch(context, e ,c, depth);     
    });
}

Answer 2

It's freezing up because JavaScript is single-threaded . Instead of calling drawBranch recursively, schedule it recursively via setTimeout and then fall off the function. That will give the browser time to redraw the canvas.