HTML5 : drawing round corner rectangle as animation

Question

I'm able to draw a rectangle with rounded corner with following code. What I'm looking is I want to form this as an animation - starting from one point and draw the line which ends at the beginning point. (like drawing with pencil) Any ideas?

ctx.beginPath(); 
  ctx.moveTo(x,y+radius);
  ctx.lineTo(x,y+height-radius); 

  ctx.quadraticCurveTo(x,y+height,x+radius,y+height);  
  ctx.lineTo(x+width-radius,y+height);  
  ctx.quadraticCurveTo(x+width,y+height,x+width,y+height-radius);  
  ctx.lineTo(x+width,y+radius);  
  ctx.quadraticCurveTo(x+width,y,x+width-radius,y);  
  ctx.lineTo(x+radius,y);  
  ctx.quadraticCurveTo(x,y,x,y+radius);  
  ctx.stroke(); 

Answer 1

Canvases are double-buffered. You'll need to defer each step of the animation using setTimeout() to give the canvas a chance to draw your changes. Update: See requestAnimationFrame as an alternative to setTimeout().

I've created one example for you that draws each of your rectangle's segments by calling the context method and pausing. I think I know the particular animation you're looking for and this isn't it, but hopefully it gives you a good start.

Code below and a demo is here: http://jsfiddle.net/q8GcR/

function animateRoundRect(ctx, x, y, width, height, radius, delay) {
    commands = [
        ['moveTo', x,y+radius],
        ['lineTo', x,y+height-radius], 
        ['quadraticCurveTo', x,y+height,x+radius,y+height],  
        ['lineTo', x+width-radius,y+height],  
        ['quadraticCurveTo', x+width,y+height,x+width,y+height-radius],  
        ['lineTo', x+width,y+radius],  
        ['quadraticCurveTo', x+width,y,x+width-radius,y],  
        ['lineTo', x+radius,y],  
        ['quadraticCurveTo', x,y,x,y+radius]
    ];  

    function draw() {
        var args = commands.shift();
        var method = args.shift();
        ctx[method].apply(ctx, args);
        ctx.stroke();
        if (commands.length) {
            setTimeout(draw, delay);
        }
    }

    ctx.beginPath();
    draw();
}

var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
ctx.lineWidth = 3;
ctx.strokeStyle = '#f00';
animateRoundRect(ctx, 20, 20, 250, 100, 10, 500);